The Crystalline Quartz-rich Raw Material from Olduvai Gorge (Tanzania): Why is it called quartzite when it should be called quartz?

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Antonio Tarriño, Benito Ábalos, Pablo Puelles, Luis Eguiluz, Audax Mabulla, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2347339/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 11 May, 2023 Read the published version in Archaeological and Anthropological Sciences → Version 1 posted 4 You are reading this latest preprint version Abstract The major raw material documented in the archaeological sites of Olduvai Gorge (Tanzania) is a geological material with crystalline appearance, white or colorless, foliated or seemingly massive only at the outcrop scale, with a very high quartz-rich composition, and apparently bearing a metamorphic origin (CQRM). Since the early days of research in Olduvai Gorge, a long-lasting terminological imprecision has allowed defining this material in a confused way as quartz or quartzite. Stubbornness in terminological imprecision reflects the complexity and specificity of CQRM related to a protracted and complex geological history composed by quartz-bearing metamorphic rocks of varied types and origins from recycling and/or tectonic reworking of much older Precambrian orogens and cratons. Currently the term quartzite is preferred by most researchers, despite being materials that have an appearance macro and microscopic similar to quartz and show a response to fracture mechanics and cutting-edge functional response is closer to quartz. In our view it is crucial to undertake a comprehensive analysis of the CQRM from the structural, metamorphic and petrological perspectives. Bearing this in mind, the main objective of the present study is to build a robust and conclusive background that will enable an accurate identification and classification of this quartz-rich mineral resource. This geological material should be identified as “Quartz” and he most diagnostic features supporting this interpretation can be summarized as: some of the microstructural relics identified concur undoubtedly with a hydrothermal origin, and the recognition of special deformational structures/microstructures point to tectono-metamorphic processes under granulite-facies conditions. Quartz Quartzite Olduvai Raw Materials Mineral Resoures Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Raw material studies carried out in the paleoanthropological complex of Olduvai Gorge (Tanzania) have led to the identification of about ten different rock types that were used by early humans in the numerous lithic assemblages recovered there (Blumenschine and Peters 1998; Blumenschine et al. 2012a ; Díez-Martín et al. 2009a ; Egeland et al. 2020; Favreau et al. 2020 ; Hay 1976 ; Kyara 1999 ; Leakey 1967 ; McHenry and de la Torre 2018 ; Stiles 1991 , 1998 ; Tactikos 2005 ). Source areas for all these rock types have also been easily identified (Flébot-Augustins 1997 ; Hay 1971 ; Jones 1994 ; Kyara 1999 ). Among these rocks, one in particular stands out as the most characteristic and conspicuous raw material. It consists of a light-colored, homogeneous and anisotropic siliceous crystalline material. It usually exhibits a penetrative foliation with a well-defined mineral/stretching lineation at macroscopic scale and a microscopic fabric revealing high-temperature deformation. These macro and microstructural features support its identification as “metamorphic quartz” . Given that this quartz-rich material has a relatively homogeneous cm-scale crystal grain size, with SiO 2 contents > 96%, this paper will label it “Crystalline Quartz-rich Raw Material”, or CQRM, for intended interdisciplinary descriptive purposes. This raw material has been alternatively referred to, with no clear reasons, either as "quartz", "quartz/quartzite" or "quartzite". Currently, as will be seen later, the term quartzite has been winning the terminological game. The microstructure (already photographed by some authors but not specifically identified from the geological/petrostructural viewpoint; e.g, Favreau et al. 2020 ; Sánchez-Yustos et al. 2012 ; Soto et al. 2020a , b ) is well known in geological literature on quartz-bearing metamorphic tectonites (Law 2014 ). This microstructure is identifiable in rock thin sections normal to the macroscopic foliation and parallel to the lineation and consists of recrystallized quartz grains showing rectangular subgrains. It is known as “chessboard” microstructure and is intrinsically associated with a quartz lattice preferred orientation in which the c-axes are parallel to the macroscopic lineation. This crystallographic arrangement is due to the activation of the prism-[c] intracrystalline slip system under temperatures > 600 ºC and hydrous conditions (Bouchez et al. 1984 , 1985 ; Kruhl 1996 ; Mainprice et al. 1986 ; Okudaira et al. 1995 ). In the Olduvai Region, the closest bedrock outcrops of CQRM lithologies occur in the Naibor Soit inselberg. These raw materials were increasingly selected for use by hominins between 1.85 and 1.3 Ma (de la Torre and Mora 2005; Kimura 2002 ; Kyara 1999 ; Leakey 1971 ): a) they represent ≥ 65% of the lithic assemblages identified in the Bed I "Zinj" floor (Díez-Martín et al. 2021 , 2022 ); b) they predominate (74.5%) in the early Acheulean at FLK West, in lower Bed II (Díez-Martín et al. 2016 ); c) they reach their highest proportion (≥ 93%) in uppermost Bed II, in sites such as TK and BK (Díez-Martín et al. 2009a ; Santonja et al. 2014 ). The terminological vagueness related to this particular rock type is a long-running trait of raw material and lithic studies in Olduvai Gorge. After the discovery of the paleontological sites carried out in 1911 by the expedition of the entomologist W. Kattwinkel, the volcanologist and paleontologist H. Reck completed the comprehensive paleontological and geological study of the region discovering the first Olduvai hominin skeleton. This author already identified the CQRM lithologies and labeled them with the dual term “quartz and quartzite" ( Quarz und Quarzitrücken , in the original publication) (Reck 1914 , p. 84). Subsequently, Louis Leakey’s publications on paleoanthropological remains in Olduvai did not specify the raw materials used for production of stone tools (Leakey 1932 ; Leakey et al. 1933 ). After the break imposed by World War II, researchers used the terms "quartz", "quartz/quartzite" or “quartzite” equally. The terminological ambiguity related to the CQRM in Olduvai Gorge bears an inescapable archaeological implication. Most of the researchers involved in the characterization of this raw material and determination of its sources are not specialists in the field of metamorphic tectonite rocks. In fact, they have based their studies heavily on nonspecific bibliographic references. In our view it is crucial to undertake a comprehensive analysis of the CQRM from the structural, metamorphic and petrological perspectives. Bearing this in mind, the main objective of the present study is to build a robust and conclusive background that will enable an accurate identification and classification of the materials under study. Historiographical Contextualization An extensive bibliographic analysis of about one hundred and twenty scientific studies has been carried out in order to trace and understand the terminological evolution of the CQRM within the paleoanthropological research undertaken in Olduvai Gorge. This review has allowed us not only to describe and systematize the different phases of the research devoted to these materials, but also to understand the origin of the recurrent mistakes that persist in the current state of the art. The following periods can be identified (Table 1). Table 1 Number of research articles using the terms: “Quartz”, “Quartz/Quartzite” and “Quartzite” at the Olduvai Gorge sites. Period Years Terminology (articles nr.) Quartz Quartz/ Quartzite Quartzite 1 Descriptive 1914 - 1975 - 10 - 2 Early identifying 1976 - 1998 8 6 8 3 Late identifying 1999 - 2010 14 1 7 4 Impositive 2011 - 2019 17 - 33 5 Latest 2020 - 2022 1 - 15 1. Descriptive period (1914-1975). The first studies carried out barely discussed raw material identification. The first reference addressing this issue can be found in Louis Leakey’s interpretation of the archaeological sequence in Olduvai, where CQRM was generically described as “irregular lumps of quartz and quartzite” (Leakey 1951, p. 34). Subsequent works always referred to this rock type through the terminological dichotomy “quartz/quartzite” (Cole 1954; Evernden et al. 1965; Kleindienst 1959; Leakey 1967). The same situation applied to Mary Leakey’s benchmark monograph, volume 3, “Olduvai Gorge. Excavations in Beds I&II, 1960-1963” (Hay 1971; Leakey 1971) and other relevant works published afterwards (Leakey 1975; Leakey et al. 1972; Stiles et al. 1974) where CQRM was indistinctly referred to as “quartz” , “quartz/quartzite” , “quartz and quartzite” , and “quartz or quartzite” . 2. Early identifying period (1976-1998). In 1976 Richard L. Hay, a specialist in sedimentary petrology, published his referential monograph on the geology of Olduvai Gorge. Although his work preferentially focused on establishing the normative stratigraphic sequence in Olduvai, he was also interested in the description and identification of lithic raw material rock sources. Hay (1976, p. 9) was the first author who described these CQRM specifically as: “The quartzite is extremely coarse-grained and commonly micaceous in the northern and eastern parts of the Olduvai region, including Naibor Soit and the hills and highlands to the north of the gorge. Individual crystals of quartz are generally 1 to 2 cm in diameter in these rocks. Most of the quartzite exposed to the south of the gorge is medium-grained and exhibits primary sandstone textures …” . Hay’s definition already shows a number of contradictions. Firstly, the grain size in the Olduvai CQRM is disproportionately larger (one order of magnitude) than the upper limit admitted for classification of a sedimentary siliciclastic rock as sandstone (2 mm should be classified as conglomerates or rudites in a broader sense. Secondly, the key argument for the identification of these rocks as quartzites is based on the recognition in them of a fine lamination parallel to the compositional bed layering at a larger scale ("laminated quartzites" of some authors), interpreted by R. L. Hay as a primary sedimentary rock texture. This interpretation is contentious because it is based upon incomplete observations that failed to identify (1) the presence of a penetrative lineation contained in the planar structure and (2) their complete recrystallization that resulted in a crystalloblastic texture (as already noticed by Saggerson 1966). If these two features had been recognized the above-mentioned lamination would have been interpreted correctly as a metamorphic foliation containing a mineral/stretching lineation, that is, a secondary metamorphic texture and not a primary sedimentary texture. Hay’s determination never prompted a definitive terminological consensus. During the years following Hay's work, a number of authors referred to the Olduvai CQRM exclusively as "quartzite" (Blumenschine et al. 1998; Jones 1979, 1980, 1981, 1994; Potts 1988). Even, Mary Leaky herself does not speak for the first time of quartzite until the monograph publication on the upper beds (Leakey 1994). Meanwhile, other scholars continued to choose the term quartz (Bower 1977; Ludwig 1999; Mehlman 1977; Perlès 1991; Sanhouni et al. 1997; Stiles 1979; Willoughby 1985) or quartz/quartzite (Flébot-Augustins 1990, 1997; Schick 1987; Stiles 1977, 1991, 1998) as alternatives. In this regard, Kimura (1997, p. 33) justified the use of the term quartz for the Olduvai CQRM exclusively based on fracture mechanics reasons, since this rock type “… is extremely coarse-grained and does not form conchoidal fracture ”. Summarizing, during this period a large number of authors were reluctant to use the term "quartzite" originally tagged by Hay (1976), emphasizing the lack of terminological consensus among specialists. 3. Late identifying period (1999-2010). This period was inaugurated with O. Kyara’s monographic study, the first one specifically devoted to the raw materials in the lithic assemblages of the Olduvai Gorge sites. In this work the term "quartzite" was chosen so that it “… replaces the indeterminate dual terms ‘quartz or quartzite’, and ‘quartz/quartzite’ used interchangeably by Leakey M.D. (1971) ” (Kyara 1999, p. 176). This decision was meant to provide a terminological unification that, however, was not based on scientific arguments. Kyara’s work did not achieve the intended standardization, and scholars kept using the terms "quartz" and "quartz/quartzite" (Toth and Schick 2007). Moreover, even more intense polarization began between advocates of the term "quartz" (de la Torre 2004; de la Torre et al. 2004, 2008; de la Torre and Mora 2005a, b, 2010; Díez-Martín et al. 2008, 2009a, b, 2010; Egeland 2008; Kimura 1999, 2002; Prendergast et al. 2007) and supporters of "quartzite" (Blumenschine et al. 2008; Bunn et al. 2010; Mourre 2003; Plummer 2004; Sharon 2008; Tactikos 2005) (Table 1). This dichotomy caused much confusion. For example, de la Torre (2004, p. 21), also in search of terminological unification, suggested just the opposite to that proposed previously by Kyara (1999): “ considering the petrological similarity between quartz and quartzite at Olduvai (Hay 1976), both will be included in the generic term quartz, the most used in the literature on the region ”. However, soon after, the author established raw material categories according to macroscopic criteria that included "quartz" in an intended petrological sense that was ill-defined: “… quartzs (in fact quartzites and metaquartzites in petrological terms) ” (de la Torre et al 2004, p. 24). At odds with the previous strategy, Tacktikos (2005, p. 75) stressed that identification of CQRM as quartzite is “… based on careful visual examination of the color, the groundmass or matrix, and the presence/absence of conspicuous crystals or phenocrystals ”. These criteria present a number of inappropriate geological terms as the basis for recognition: (1) color is secondary or meaningless as a criterion for classification in sedimentary rocks, (2) quartz-rich siliciclastics (quartz-arenites) essentially lack a matrix, and (3) phenocrystals are characteristic of igneous rocks. 4. Imposition period (2011-2019). The second decade of the 21st century saw an increasing number of new research teams working in Olduvai Gorge and, consequently, a growing published output. At this time the term “quartzite” gained popularity and was used by the overwhelming majority of authors (Abtosway 2018; Arroyo and de la Torre 2016, 2018, 2020; Bello-Alonso et al. 2019, 2021; Benito-Calvo and de la Torre 2011; Blumenschine et al. 2012a, b; Byrne et al. 2016; Courtenay et al. 2019; de la Torre and Mora 2014, 2018a, b, 2020; de la Torre et al. 2012, 2013, 2018a, b, c, 2021; Eren et al. 2014; Favreau et al. 2019, 2020; Fujioka et al. 2022.;Gurtov et al. 2015; Key et al. 2020; Macdonald et al. 2022; Maté-González et al. 2018; McHenry and de la Torre 2018; Panera et al. 2019; Pante and de la Torre 2018; Proffitt 2018; Reti 2013, 2016; Rubio-Jara et al. 2017; Santonja et al. 2014, 2018; Soto et al. 2020a, b; Stollhofen et al. 2021; Toth and Schick 2018; Uribelarrea et al. 2014; Yravedra et al. 2017, 2019). Meanwhile, the use of the term "quartz" declined (Díez-Martín et al. 2011, 2012, 2014a, b, 2016, 2017, 2018; Egeland et al. 2020; Eren et al. 2013, 2014; Goldman-Newman et al. 2012; Gurtov and Eren 2014; Sánchez-Yustos et al. 2012, 2015, 2016, 2017a, b, 2018). During the last years of the decade some of the most resilient advocates of the term "quartz" got swept away by imposition of the terminological mainstream (Díez-Martín et al. 2021, 2022; Sánchez-Yustos 2021; Sánchez-Yustos et al. 2019). At present, Egeland et al. (2020) appear to be the only authors that still refer to the Olduvai CQRM as quartz. The evolution of terminological trends in the identification of the CQRM at Olduvai Gorge presented in Table 1 reveals three turning points. The first is related to Hay’s definition, for the first time, of this raw material as "quartzite". Hay’s work triggered a polarization in the use of the terms “quartz” and “quartzite” until then used interchangeably and since then as contenders, although some authors maintained the use of both terms. Kyara's contribution marked the second turning point defined by a clear confrontation between both terms. It was during this period that authors opted for one term or the other. Finally, a third break occurred in 2011 with a notable increase in publications produced by the incorporation of new research teams in Olduvai. From this moment on, most researchers accepted the postulates defended by Hay (1976) and Kyara (1999), favoring the use of “quartzite” over “quartz”. However, as made clear above, these postulates were not based on accurate petrological criteria. Currently, researchers massively accept the term “quartzite” to define this rock type. Geological Context Geological framework Olduvai Gorge (Tanzania) is located in the Great East African Rift Valley, more specifically adjacent to the Oldoinyo Ogol highlands (south of the Oliondo Mountains and Loita Hills). The mountainous area to the north of Olduvai Gorge is geomorphologically constrained by hard lithologies with a significant presence of quartz-rich rocks that were affected by high-grade regional metamorphism and concomitant deformation during the Neoproterozoic. These rocks and the deformational structures developed in these materials form part of the so-called East African Orogen (EAO; Stern 1994 ), formerly known as the “Mozambique Belt” (Holmes 1951 ). This orogen extends over 8,000 km from the Sinai Peninsula to South Africa and beyond (current Antarctica) forming a 250–350 km wide belt, though locally it can approach 1,000 km. This is one of the largest ancient orogenic belts on Earth, formed by the closure of the “Mozambique Ocean” between 650 and 500 Ma ago (Ediacaran to early Paleozoic times; Thomas et al. 2013 ) and the subsequent collision of the Eastern and Western Gondwana subcontinents and their magmatic arcs (Fig. 1 ). Notably in Kenya and eastern Tanzania two major crustal units can be distinguished in the EAO (Fig. 2 ): the “Eastern Granulites" and the "Western Granulites” (Hepworth 1972 ). The area of interest here has usually been ascribed to the "Western Granulite" unit, although so far it has not been studied in detail. As a result, studies of neighboring mountain domains several tens to a few hundreds of km apart (e.g, the Pare and Usambara Mountains or the Taita and Loita Hills) usually consider the Oliondo highlands a part of the Western Granulites (e.g, Cutten et al. 2006 ; Fritz et al. 2013 ), whereas others regard them as inliers of low-grade metamorphic rocks (different from granulites) of the Western Granulites (e.g, Fritz et al. 2009 , Fig. 2 ). The Western and Eastern Granulites have different lithological composition, age range of the protoliths, metamorphic grade, age of metamorphism, structural style and igneous rock inclusions (Fritz et al. 2013 , and references therein). The Eastern Granulites terrane is tectonically emplaced onto the Western Granulites, which themselves are tectonically emplaced over the Archean Tanzania Craton, made of much older though lower-grade metamorphic rocks (Holmes 1951 ). The age of tectonic stacking (diachronous along the orogenic belt) is ascribed to the Neoproterozoic Era, between 1000 and 538.8 Ma (Cohen et al. 2013 ). Deformation and metamorphism peaks (the so-called Kuunga Orogeny) occurred ca. 640 Ma ago in the Western Granulite Belt (composed of psammitic and pelitic metasediments and their migmatized equivalents), and the final crustal consolidation somewhat later at 580 − 500 Ma (Tenczer et al. 2013 ). Abundance of metamorphosed sandstones ( “quartzites” ) among the psammitic and pelitic metasediments suggests derivation of detrital quartz grains from source areas dominated by granitic and gneissic rocks, such as those cropping out so far in the Tanzania Craton (Manya et al. 2006 ; Schlüter 1997 ; Thomas et al. 2016 ). A Geological Guide To Identify Quartz-rich Rocks In Pleistocene Lithic Industry As Raw Material Sources Quartz (and its polymorphic varieties, all compositionally being SiO2) is one of the most frequent minerals in the Earth's crust. High-purity quartz-rich rocks are important raw materials in the current high-tech industry (Götze and Möckel 2012 ). The label “quartz-rich rock” is used here to encompass those rocks dominated by the quartz mineral (without reference to their origin). The suitability to conchoidal fracture, hardness and resistance of flaked cutting-edges in quartz-rich rocks made them sought-after resources by the genus Homo since the early stages of technological behavior, from beginning of the Pleistocene. As explained in detail in a previous section, geological terms such as “quartz” and “quartzite” are used to label the raw materials of those lithic artifacts and usually its correct detectionserve as guides to search for their source areas. Although several specialists in non-geological disciplines have made considerable efforts to incorporate these geological constraints to their research, there exists considerable confusion regarding the terms (mis)used and their actual meaning. For example, “quartz” is a mineral in the geological sense that has formed as a result of geologic processes (cf. Deer et al. 1966 ; Neuendorf et al. 2005 ). However, the term quartz has sometimes been reported, implicitly or explicitly, as if it were a rock (e.g, Mourre 1996 , p. 207; Sánchez Yustos et al. 2012, p. 7, etc.). In fact, a “rock” is in its correct geological sense an aggregate of one or more minerals (Neuendorf et al. 2005 ). Petrographic observations (with the help of polarizing microscopes) have also been used in Paleoanthropology and Archaeology to describe and constrain quartz-rich rock characteristics. However, studies on the raw materials at the Olduvai sites usually failed to recognize and/or misinterpret diagnostic rock microstructures recorded in classical reference books (e.g, Passchier and Trouw 1996 ; Tucker 2013 ) and their mechanical significance (Spry 1969 ; Nicolas and Poirier 1976 ; Vernon 2004 ). The microstructural approach used in these works to discriminate the origin of quartz-rich rocks has also been taken into account to explain/infer mechanical properties that might be of archaeological interest, such as rock strength/fragility, isotropic/anisotropic character and the predictability of fracture geometry during knapping. However, in several instances they failed in the recognition of the mono- or polycrystalline character of the aggregates as well as in the identification of widespread solid-state metamorphic recrystallization fabrics imposed on pre-existing quartz rocks whatever their type. These types of quartz crystals are usually sourced (or derived) from hydrothermal veins that can attain large dimensions (several m-wide and km-long; e.g, Hippert and Massucato 1998; Lemarchand et al. 2012 ) and, as a rule, they exhibit microstructures that isolate seemingly homogeneous and relatively intact lattice domains not to be confused with the detrital “grains” of rocks with a sedimentary primary origin (i.e. pure-quartz sandstones or quartzites). The dimensions admitted for some quartz-rich rock deposits/units may also be considered so large that they might no longer be interpreted as veins, but as “sedimentary quartzite” formations, the large dimensions of which are familiar to most researchers. In spite of the above, also in the geological context, the term “quartzite” has been used loosely to name (1) metamorphic rocks formed by metamorphism of an almost pure quartz sandstone, (2) very hard sandstones, that is, sedimentary rocks composed almost exclusively of quartz grains cemented with additional quartz, (3) granular metamorphic differentiates formed by quartz dissolution in aqueous fluids and posterior re-precipitation coeval with metamorphism and (4) hydrothermal/pegmatitic quartz mineral aggregates occurring in veins and genetically related to magmatic intrusions and ore deposits. Only the first case corresponds truly with a real quartzite in the geological sense and, therefore, the usage of the term "quartzite" should be restricted to designate these rocks. This terminological confusion has brought to light the old “quartzite problem" (Skolnick 1965 ), requiring undoubtedly a microstructural analysis to identify the textural features registered in these rocks and discriminate clastic textures associated with siliciclastic rocks from metamorphic features (Howard 2005 ). Further complicating matters, additionally, prominent tectonic fabrics can be superimposed on any of the rock types reported during later syn-metamorphic solid-state deformation. Such fabrics are defined by penetrative (dominant at a given micro and meso-scale) planar features termed “foliations” and/or linear features termed “lineations” (Spry 1969 ). In the case of rocks that have undergone significant deformation and metamorphism, however, the lack of a comprehensive microstructural study may lead to the wrong interpretation of these planar penetrative tectonic foliations as sedimentary laminations, as in Hay ( 1976 ). This is not the only case where non-specialists may confuse metamorphic features with sedimentary structures/microstructures. Even the sigmoidal shear zone foliations well-known to “hardrock geologists” may be wrongly attributed to cross-bedding without truncation surfaces (climbing ripple cross stratification and aggrading beds) by “softrock geologists” (in the sense of Hall 1988 ) and others. Though geometrical similarities may exist between structures of radically different origins (sedimentary stratification/lamination versus tectonic/metamorphic pseudostratification or foliation; e.g, Turner 1941 ), careful microstructural observations are able to discriminate them. Quartz-rich Rocks In Metamorphic Environments Metamorphic processes may affect rock precursors of any type (igneous, pegmatitic, hydrothermal, sedimentary and even metamorphic), and usually redistribute in them large amounts of SiO 2 that are first mobilized (dissolved) and then precipitated (recrystallized) to form veins and lenses (Oliver 1996 ; Wagner et al. 2010 ). These are termed “metamorphic quartz mobilisates or differentiates” in the geological literature and occur along preexisting mechanical anisotropies. In regional metamorphic contexts, anisotropies are dominated by tectonic foliations formed in the host rocks during solid-state deformation accommodation and mineral growth process (Chapman 1950 ; Spry 1969 ; Yardley 1983 ). Quartz metamorphic segregations (either quartz veins or more irregular masses) are derived from the wall rocks during metamorphism and, therefore, undergo minor transportation. In these cases, quartz vein abundance is closely related to the abundance of quartz in the country rocks (higher in quartzites, lower in pelitic schists). These quartz bodies usually appear as blankets parallel to foliations and shear zones, or as saddles in minor fold hinges. Smaller quartz veins occurring parallel to foliations mainly form by diffusional transport (with a thickness of up to 10 cm, since diffusion is ruled out as a source of much larger veins). Quartz-kyanite veins in high-grade quartzo-feldspathic schists and gneisses are classic examples of metamorphic differentiation in quartzose rocks containing aluminous material (e.g, Dorr 1969 ; Müller et al. 2007 , 2012 ). Less dissolvable minerals are passively concentrated there by the solution and selective removal of the more soluble phases. The blankets can be continuous for meters or up to hundreds of meters (Guild 1957 ) and sometimes are of pegmatitic character, in coherence with the medium- to high-grade pressure and temperature conditions undergone by their metamorphic country rocks. Even veins of hydrothermal origin may be filled with fine and/or coarse-grained quartz crystals that exhibit a banded or layered structure (e.g, Fig. 1 in Fonseca et al. 2015 ) parallel to the host rock walls. These well-known arrangements, usually observed in large veins that can be traced for some meters at most, are related to progressive vein infilling processes by repeated crack-seal mechanisms (Bons et al. 2012 ; Ramsay 1980 ) and to mineralizing fluid diversion into shorter and wider cavities upon hydrofracture arrest (Bons 2001 ). The layering described, which can even be strengthened in metamorphic scenaRíos where the veins become overprinted by solid-state plastic deformation, can also be misinterpreted as sedimentary lamination/bedding. A note of caution is thus needed when this type of interpretation is suggested in metamorphic environments. The metamorphosed and strongly deformed equivalents of “hydrothermal vein quartz” and “pegmatitic quartz” may give rise to quartz-rich rock units and even mappable formations (made of poly-crystalline quartz aggregates) parallel to metamorphosed “sedimentary quartzites” and to “metamorphic quartz mobilisates” . Yet, new “hydrothermal vein quartz” can also be generated during the overprinting process. Eventually, all these rocks occur interleaved in nature, may exhibit similar aspects to the naked eye, and might be wrongly termed “quartzite” . Nevertheless, strictly speaking, they have radically different origins that might be revealed by careful study of their microstructure under the petrographic microscope (e.g, Lychagin et al. 2020 ). In particular cases, hydrothermal quartz in orogenic metamorphic environments can occur in giant veins up to 15–20 km in outcrop length and tens of m in width (e.g, Jia and Kerrick 2000; Lemarchand et al. 2012 ) or vein complexes hundreds of m thick. They are known worldwide from the Archaean (Kerrick and Feng 1992) to the Cenozoic (Fonseca et al. 2015 ), including notable examples in linear Paleoproterozoic orogens (Pati et al. 2008 ; Rout et al. 2022 ) and in Gondwanan orogens (Carvalho 1983 ; Chaves 2007 ; Chaves et al. 2003 ; Esteves and Faleiros 2021 ; Hippert and Massucatto 1998 ) coeval with the Mozambique Belt in Tanzania during the Neoproterozoic. In a nutshell, regional metamorphic terrains usually contain quartz-rich lenses and layers with thicknesses varying between cm and some hm thick, and dm to km in map extent. Metamorphic quartzites after sedimentary precursors are common among them. These may conform large quartzite units with several km 2 outcrops (fine examples exist to the N of the Olduvai Gorge Region) that co-exist with (1) quartz veins of hydrothermal origin formed after dissolution/precipitation processes during diagenesis/burial and low-grade regional metamorphism, (2) syn-tectonic metamorphic quartz mobilisates forming veins usually parallel to the host rock principal foliation during low- to high-grade metamorphism, (3) pegmatitic, hydrothermal veins or stockworks and irregular masses associated with larger plutonic rock intrusions, and (4) igneous quartz-rich rocks. Materials And Methods Materials Diverse quartz-rich lithologies, reminiscent of the raw materials used by Olduvai hominins to produce artifacts, have been collected in outcrops of Precambrian rocks (Fig. 3 ) located within a radius of about 40 km around Olduvai Gorge. The study of the variability of the samples collected, in principle, would permit us to: (1) identify the basic types of quartz-rich rocks that crop out on the surface of this territory, (2) resolve their key mineralogical composition, microstructure and fabric with a two-fold geological and material characterization application and (3) tag the correct rock name to the particular crystalline quartz-rich raw materials (CQRM). The best and most extensive Precambrian rock outcrops occur to the north of the Olduvai Gorge (Fig. 3 ). The only previous geological survey existing so far (Pickering 1958 ) categorized the outcropping rocks into two groups: the Oldoinyo Ogol and the Serengeti Groups. The lithostratigraphic and structural relationships within each group remain obscure and unknown. The layered nature of most units may suggest that those groups are dominated by thick (hm to km) successions of metamorphic rocks derived from terrigenous protoliths (sandstones, siltstones and mudstones), currently with gentle to moderate dips and an apparent structural simplicity. However, as Shackleton's observations in equivalent neighboring areas demonstrate: (1) the outcrops exhibit a profusion of tight to isoclinal folds, implying conspicuous and intricate succession reversals, (2) the deformation is intense, as proven by the development of a well-defined foliation parallel to the compositional banding, a pervasive lineation on the foliation surfaces and the presence of elongated minerals and mineral aggregates, and (3) shear zones occur parallel or at low angle to the foliation, possibly implying tectonic succession repetitions and/or discontinuities (Shackleton 1993 ). North of Olduvai Gorge these structures can be remotely perceived in aerial views (Fig. 3 ), strongly suggesting that the outcrop observations might be extrapolated with similar characteristics at much larger scales. For the purposes of the current study, the Oldoinyo Ogol Group contains two formations of interest: the Kissele and the Loipukoi Quartzites. The "Kissele Quartzite" was originally described by Pickering ( 1958 ) in his brief geological map explanation as: “ very coarse-grained, crumbly, red quartzite which generally overlies white or colourless, coarse-grained quartzites ” (Fig. 4 a). Our field observations confirm that reddish quartzites are the dominant lithology ( “host rocks” ) in the Kissele Quartzite outcrops (Fig. 4 b). They appear to be extremely brittle and, from our viewpoint, unsuitable for knapping. The “white or colorless quartzites” ( sensu Pickering 1958 ), in turn, resemble the milky quartz veins (Fig. 4 c) so common in low- to medium-grade metamorphic areas worldwide (e.g, Bons 2001 ). In the study area, the white/colorless quartz rocks would correspond to hydrothermal quartz veins and metamorphic differentiates later reworked and metamorphosed under high-grade conditions (Fig. 4 d). They occur not only in primary outcrops but also as loose fragments forming talus deposits on the hillslopes. These cobbles and boulders ( sensu Krumbein and Sloss 1951 ) display sharp edges and usually flat surfaces. Their size, tabular morphology and availability might have been appropriate for knapping by hominins in the area (Fig. 4 e). This type of quartz-rich rock is common in bedrock outcrops in the region such as the Naibor Soit inselberg, so often cited in archaeological literature (e.g, Egeland et al. 2020; Santonja et al. 2014 ), and indeed is the most conspicuous artifact raw material at archaeological sites in Olduvai Gorge. In this study white/colorless quartz samples were collected from outcrops at Kissele, Naibor Soit and Lekongi (Fig. 3 and Table 2 ). Table 2 Samples collected from Precambrian reliefs and Olduvai Gorge Nº Name Locality Description 1 KLG.Gns Kelogi Hornblendic Gneiss 2 LPK.Qtz Loipukoi Coarse Smoky Quartz 3 NBS.Afb Naibor Soit Anfibolite 4 LMT.Qtz Lemuta Kyanite Quartz 5 KSL.Afb Kissele Anfibolite 6 LMT.Qtz Lemuta Foliated Grey Quartz 7 NBS.Qtz Naibor Soit Coarse Polycristalline quartz 8 KSL.Sch Kissele Tremolitic Schist 9 LMT.Qtz Lemuta Recrystallized Quartz 10 LKG.Qst Lekongi Micaceous Quartz-Schist 11 KSL.Afb Kissele Anfibolite 12 KSL.Qtz Kissele Coarse Polycristalline Quartz 13 KSL.Qtz Kissele Fine-Grained Polycrystalline Quartz 14 KSL.Qtz Kissele Recrystallized Quartz 15 LPK.Qtz Loipukoi Coarse Smoky Quartz 16 KSL.Qtz Kissele Coarse Polystalline Quartz 17 KSL.Qtz Kissele Recrystallized Quartz 18 NBS.Qte Naibor Soit Quartzite 19 ODV.Cht Olduvai Gorge Chert 20 LMT.Qtz Lemuta Quartz Vein 21 LMT.Qtz Lemuta Recrystallized Quartz 22 NBS.Qtz Naibor Soit Coarse Polycristalline Quartz 23 ODV.Cht Olduvai Gorge Chert The second formation of interest in the Oldoinyo Ogol Group, the "Loipukoi Quartzite", was mapped by Pickering ( 1958 ) to the NW of the Kissele Quartzite. The closest outcrops to Olduvai Gorge are located ca. 35 km away and form various inselbergs with individual dimensions between 500 to 2,000 m in length aligned for a distance of ca. 25 km. These outcrops are formed by a grey (smoky)-colored polycrystalline quartz, whose color is due to the presence of large amounts of minute ferromagnesian mineral grains and microinclusions in quartz (Fig. 4 f). This rock exhibits a penetrative metamorphic foliation/lineation and is mechanically compact. Its microstructural homogeneity confers excellent quality for knapping. Additionally, the relative proximity of these outcrops to Olduvai Gorge makes this rock a likely candidate, although in a minority way, as a raw material in the lithic industry. In this study grey quartz samples were collected from the Loipukoi outcrop (Fig. 3 and Table 2 ). The Serengeti Group is poorly represented in the area of study. Pickering ( 1958 ) mapped a 6 km long inselberg (and three additional ones with lengths below 500 m) made of rocks ascribed to this group. The “Lemuta Quartzite” formation of the Serengeti Group is located to the west of those of the Oldoinyo Ogol Group, the closest of them being 20 km away from Olduvai Gorge. The “Lemuta Quartzite” consists of white, fully recrystallized, and foliated/lineated quartz-rich rocks, intercalated with quartz-schists (Fig. 4 g). Some quartz-rich rock beds contain stunning, mm-cm-sized kyanite crystals elongated parallel to the foliation/lineation and occasionally broken and stretched (Fig. 4 h). Occurrence of kyanite in the rock mineral assemblage is petrologically outstanding and points to a medium- to high-grade metamorphic overprint that may help to constrain the formation/reworking conditions of other rocks in the area (metamorphosed quartz-arenites or metamorphic differentiates). However, these rocks are not as mechanically strong as those previously described. In fact, their knapping qualities are very poor and their alteration produces weak granular products not suitable for use. This likely explains from the archaeological perspective why those rocks are not found among the Olduvai Gorge hominin artifacts. Bearing in mind all the above, we sampled this formation in the Lemuta outcrop (Fig. 3 and Table 2 ). Analytical Methods Petrographic and geochemical analysis Standard 30 µm thick polished rock sections were used for conventional petrographic and microstructural studies. The sections were cut parallel to the XZ or XY structural planes (XY defined by the orientation of the foliation plane, X defined by the orientation of the mineral and stretching lineation). Mineral analyses were performed in the Scientific-Technical Services microprobe unit at the University of Oviedo (Spain) with Cameca SX-50 and SX-100 automatic microprobes, the latter equipped with five wavelength dispersive spectrometers, a dispersive energy spectrometer, and with secondary electron, back-scattered electron and cathodoluminiscence detectors. The operating parameters included a 10 s integration time, a 10 nA beam current, and a 15 kV accelerating voltage. Mineral structural formulae were calculated by charge balance criteria following various procedures suggested in the bibliography for different phases (see Droop 1987 and Spear 1993 , for further details). Electron Back-Scattered Diffraction The Electron Back-Scattered Diffraction (EBSD) study was performed on selected thin sections cut as indicated above. These were ultra-polished with a colloidal silica suspension to remove surface damage and then carbon coated to prevent charging. A copper tape was attached surrounding the measurement area to reduce charging effects. Crystallographic preferred orientation measurements were performed at the University of the Basque Country (Electron Microscopy Facility-SGIker) with an automated Electron Back-Scattered Diffraction system attached to a JEOL JSM-7000F Field Emission Scanning Electronic Microscope (FE-SEM). Samples were mounted in this device on a stage tilted 70º, with the rock lineation parallel to the SEM X-axis. The beam working distance was 20 mm (Prior et al. 1999 ) and the detector was placed at 188 mm. An acceleration voltage of 20 kV and a beam current of ca. 3.5 nA were applied. Crystallographic orientations were obtained using the Channel5 software package after automated EBSD analysis on a predefined sampling grid with a step of 20–30 µm, covering up to 80% of the thin sections. These steps are significantly smaller than the average grain size of the minerals. The “raw” indexation percentage ranged between 89–97%. The obtained data were processed with MTex and Matlab (Bachmann et al. 2011 , 2010 ). Crystallographic orientation solutions with mean angular deviation (MAD) values between detected and simulated patterns over 1.2º were rejected to assure EBSD measurement reliability. The data were filtered so that the orientation diagrams contain one orientation per grain. The grain detection technique considered a critical misorientation threshold of 10º. To avoid errors in the grain detection related to the eventual presence of non-indexed pixels and the large grain size of the analysed minerals only those grains covering more than 5 pixels were considered. Fabric orientation distributions are presented in lower hemisphere, equal area stereographic diagrams. The projection plane always corresponds to structural XZ sections and the macroscopic foliation is represented there as the equatorial diameter (E-W). The lineation is horizontal within the same plane. The modal proportions were estimated on the basis of the fraction of grains indexed during the EBSD measurement. The strength of the fabric was expressed by the J texture index (Bunge 1982 ), representing the mean square value of the orientation distribution function (ODF). The calculations were performed with the MTex texture analysis software (Bachmann et al. 2010 ; Mainprice et al. 2011 ). Results Petrography Among the twenty-three rock specimens sampled in the Olduvai Gorge region (Table 2 ), eighteen correspond to quartz-rich raw material from the Loipukoi, Lemuta, Kissele and Naibor Soit areas (Fig. 3 ). Their source rocks are much stronger and more resistant to weathering and erosion than the surrounding/interbedded rock formations and, thus, define positive reliefs above the average altitude of the area. Those rocks always exhibit an outstanding planolinear fabric with a distinct foliation containing a clear stretching/mineral lineation, though sometimes seemingly massive (only at the outcrop scale, not at the microscope scale) cm- to m-thick layers, and lenses parallel to the country rock foliation also occur (Fig. 5 a). The latter usually correspond to white/colorless quartz. They are mechanically hard and their erosional dismantling produces variable amounts of loose rock fragments (as described in a preceding section; Fig. 5 b). The white/colorless quartz rocks (Fig. 5 a, b) contain a mineral assemblage in equilibrium composed of quartz (up to 98%) and muscovite (1.5%), accompanied by accessory phases (< 1%) such as gypsum, ilmenite and other opaque minerals, tourmaline, rutile, kyanite and zircon. Quartz grains appear as elongate large crystals with the largest dimension up to 1.5 cm. Elongate quartz grains exhibit a well-defined shape-preferred orientation that defines the rock macroscopic foliation and lineation (Fig. 5 c). Quartz grain boundaries are irregular and exhibit abundant microstructures (e.g, bulging, convex boundary segments among crystal defect-free and strained grains, complete inclusion of other minerals of moderate size) that denote active migration during recrystallization under temperature conditions high enough to permit activation of quartz deformation mechanisms dominated by grain boundary mobility (Law 2014 ). The concomitant development of quartz subgrains and new grains is remarkable, as their boundaries show diagnostic geometrical arrangements with two sets of grain-boundary segments at a high angle to each other, consistently oblique to the rock foliation. This diagnostic microstructure ("chessboard microstructure"; Gapais and Barbarin 1986 ; Fig. 5 d) denotes quartz plastic deformation under high-temperature (> 600 ºC), hydrous conditions and high strain rates (Bouchez et al. 1984 1985 ; Kruhl 1996 ; Mainprice et al. 1986 ; Okudaira et al. 1995 ; Passchier and Trow 1996). Two types of white mica inclusions in quartz can be distinguished in the white/colorless quartz. The first (1) consists of small flakes with average 400 µm length and a few tens of µm width. These can be either completely included within quartz (Fig. 5 c) or pinned at their grain boundaries (Fig. 5 d), which acquire higher curvatures in their proximity. In all cases they share a unique parallel orientation identical to the elongation direction of quartz grains and the macroscopic foliation. This microstructure attests to pervasive secondary recrystallization under a strain field by means of active grain boundary migration mechanisms. The second type of white mica inclusions (2) are minute crystals (up to a few tens of µm long and some µm thick) that can be appreciated in polarized light microscopic observations under crossed nicols, only if the host quartz crystal is taken to a position of optical extinction. These flakes occur in quartz grains showing subgrains and undulose extinction, that is, in plastically deformed quartz grains not obliterated by complete recrystallization. There, the micro-inclusions appear as bright flakes (upper red circle in Fig. 5 d) that usually exhibit identical orientations parallel to certain quartz host crystallographic directions (usually two or three; cf. Figure 5 e) that may coincide with fine orientation bands along quartz rhombohedral planes {r} and {z} (Derez et al. 2015 ). These microstructures are diagnostic features that indicate a hydrothermal vein quartz origin (Göetze 2012). During such quartz vein development, the mica flakes would have been attached to quartz crystal faces during free growth. The study of fluid inclusions in quartz can cast light upon the origin of the rocks under study. Thus, although fluid inclusions can be found enclosed in quartz grains, they are not as ubiquitous as in the case of hydrothermal quartz (Göetze 2012; Johnston and Butler 1946 ). In fact, they are rare in the interiors of recrystallized new grains. In turn, the subgrain and grain boundaries may contain some fluid inclusions, but they are also scarce and confer a very clean aspect. In our samples, grains with relic undulose extinction show a few intragranular healed microcracks, recognizable as fluid inclusion planes (Van Den Kerkhof and Hein 2002 ; Fig. 5 f-h). These can also be considered microstructural relics, with a twofold implication. On the one hand, their presence demonstrates a former brittle response of stressed quartz rocks under low to moderate temperatures followed by an incomplete recrystallization under higher temperatures. On the other hand, they exemplify recrystallization and grain boundary mobility/reorganization processes that result in a concomitant reorganization of the quartz solid and fluid inclusions, leaving clean grain interiors (Schmatz and Urai 2011 ; Vityk and Bodnar 1995 ). These microstructures also support that hydrothermal quartz protoliths, rich in aqueous fluids, facilitated the operation of ductile deformation mechanisms under moderate to high temperatures (Palazzin et al. 2018 ; Stünitz et al. 2017 ) that actually resulted in the outstanding planolinear macroscopic fabric and microstructure of this white/colorless quartz. Quartz Petrofabric Lattice Preferred Orientation (LPO) patterns of rock-forming minerals are designed in Geology to illustrate the orientation of certain significant crystallographic elements. Usually, these patterns are represented in stereographic diagrams using the macroscopic foliation and lineation recognized in the rocks (resulting from strain-induced microstructural reorganization) as an external referential. The presence of a preferred orientation can be unravelled in appropriate fabric diagrams (e.g, Fig. 6 ) by clusters or concentrations of crystal axes and/or planes around specific orientations. The “J texture index” quantifies the intensity and strength of such a preferred crystallographic orientation. In the case of quartz, LPO fabric diagrams are used to identify intracrystalline slip systems consisting of a slip plane and a slip direction. As a rule, mineral crystallographic planes with orientations approaching that of the macroscopic foliation are most likely to operate as slip planes. Moreover, mineral crystallographic directions close to the orientation of the macroscopic lineation are good candidates to depict the intracrystalline slip direction. The recognition of an LPO defined by the existence of a preferred orientation for certain crystallographic slip planes and axes can shed light on several deformation characteristics and thus provide valuable information on the active deformation mechanisms (brittle/plastic), deformation regime (coaxial/rotational), and thermobaric conditions (especially under lower or higher temperature (Law 2014 ; Passchier and Trow 1996). The samples studied here are all characterized by high J texture index values (up to 4.6). This points to the existence of a fair quartz texture, that is, the presence of a remarkable quartz preferred crystallographic orientation that might support a sound petrofabric interpretation. Figure 6 portrays the most common LPO patterns determined for quartz c-axes and -axes in a white/colorless quartz, taken as a representative example. A distinct preferred orientation maximum of c-axes is present at a position forming an angle of 17–20º with the lineation orientation (X structural direction). Simultaneously, the -axes scatter along a wide girdle perpendicular to the X direction. This LPO pattern denotes operation of the prism-[c] slip system in quartz by intracrystalline slip on prismatic planes (that contain the c-axis) along the c-axis direction (e.g, Blumenfeld et al. 1986 ; Kruhl 1996 ; Lister and Dornsiepen 1982 , Schmid and Casey 1986 ). Here and in most previous studies worldwide, it has been identified in deformed quartz mineral aggregates that additionally exhibit chessboard microstructures. The association of these fabric and microstructural features supports the tectonic interpretation that the hosting quartz underwent high-temperature deformation under temperature conditions above 600–650 ºC (Blumenfeld et al. 1986 ; Mainprice et al. 1986 ; Passchier and Trouw 1996 ), likely in the range 700–800 ºC (granulite facies of regional metamorphism) if the stable mineral assemblages present in related rocks is considered (e.g, Barth et al. 2010 ; Fernandez et al. 2003 ; Mainprice et al. 1986 ; Okudaira et al. 1995 ). The clear obliquity that exists between the LPO pattern and the external reference framework provided by the foliation and lineation implies that high-temperature deformation included non-coaxial (rotational) deformation components during strain accommodation, which further indicates that rock foliations acted as ductile flow planes, the flow direction being marked by the mineral-stretching lineation. Discussion The major lithic resource used by hominins documented in Olduvai Gorge is a geological material with crystalline appearance, white or colorless, foliated or seemingly massive at the outcrop scale (not at the microscope scale) and extremely rich in quartz. In the course of research conducted in the area, this CQRM (Crystalline Quartz-rich Raw Material) has been referred to as: (1) quartz, (2) quartz/quartzite, (3) quartz and quartzite, (4) quartz or quartzite, (5) quartz (meaning quartzite), and (6) quartzite. Past lithic studies undertaken in Stone Age archaeological sites did not usually take into consideration petrological criteria to characterize and classify the artifact resources. In the case of the Olduvai Gorge archaeo-paleontological complex, sixty-five years went by from its discovery before the artifact raw materials were identified for the first time on scientific grounds as quartzite (Hay 1976 ). A further thirty-five years had to elapse until this term reached a certain consensus among researchers. It is clear that this abstruse and long-lasting process of raw material determination was due to a number of petrographic particularities that characterize the CQRM from Olduvai. Undoubtedly, this CQRM exhibits a macroscopic aspect that does not match the acknowledged characteristics and geological significance of classic "quartzites" but rather, as explained above, its appearance is similar to quartz. Petrological observations made in this study of the Olduvai Gorge archetypal white/colorless quartz have revealed the presence of chessboard recrystallization microstructures fully overprinting rocks with a non-sedimentary primary microstructure. The scarce relics preserved concur with a hydrothermal origin of quartz layers. These include muscovite oriented microinclusions (µm-sized) inside large quartz crystals, the minor presence of euhedral/subhedral crystals of hydrothermal minerals such as tourmaline, gypsum and muscovite forming a paragenetically stable association, and the distribution of clouded domains rich in fluid inclusions in some quartz crystals. It is commonplace during metamorphism and recrystallization of vein quartz that a complete rearrangement (and/or leakage) of the fluid inclusions from grain interiors (clouded areas) and grain boundaries takes place, especially when recrystallized under high temperatures (Wheeler et al. 2004 ). Quartz veins likely formed a regional vein network developed during the prograde stages of regional metamorphism in the Mozambique orogenic belt. Some of the veins are large (several m-thick and hundreds of m long in current outcrops) and can be easily identified in aerial/satellite images of the Precambrian inselbergs currently present to the North of Olduvai along tracts exceeding 20 km (including the proximate Naibor Soit and Kissele areas). As commented above, Hay ( 1976 ) identified this rock type as quartzite and interpreted as primary sedimentary texture the macroscopic and visible mineral alignments produced by the preferred spatial arrangement of quartz mineral aggregates containing small muscovite crystals and other accessory minerals. From the petrological point of view, however, this premise is untenable because in terrains affected by high-grade metamorphism (like the area to the north of Olduvai), rock primary structures were erased due to the strong reworking associated with elevated pressures and temperatures prevalent during metamorphism. R. L. Hay relied, notwithstanding, on previous geological surveys accomplished by experienced hard-rock geologists such as the 1/125.000 geological map, sheet 37 "Moru" completed by Pickering ( 1960 ). In this study he reported: “Unmetamorphosed granites of the shield are unconformably overlain by relatively unmetamorphosed quartzites, sandstones, and shales of the late Precambrian Bukoban System only about 25 km west of Lake Ndutu ( Pickering, 1960 )” (Hay 1976 , p. 11). However, in fact, the closest outcrops are really more than 50 km away from the Olduvai archaeological sites. That is, they are located more than 25 km in a straight line towards the west of Lake Ndutu, in the headwaters of the gorge, which in turn is located another 25 km to the west of the archaeological sites. Those slightly metamorphic quartzites of the Bukoban System (actually the Bukoban Supergroup) that caught Hay’s attention are currently known as the "Kinenge quartzites". They are part of the Ikorongo group and lie unconformably on top of the Archaean craton rocks (Pickering 1960 ), in a tectonic domain different from the one in which Olduvai is located. These materials were the subject of later studies (Kasanzu 2016 ; Kasanzu and Manya 2010 ; Kasanzu et al. 2008 ) where they were identified correctly as sandstones forming part of sedimentary rock successions containing also quartzites, shales, dolomitic limestones and basalts, and regarded as quartz-arenites by Kasanzu and Manya ( 2010 , p. 365). The white/colorless quartz hosted in the Kissele quartzite formation of the Oldoinyo Ogol Group (Pickering 1958 ) and similar rocks cropping out nearby in the Olduvai area are the best fitting candidates as mineral resources for the hominin artifacts. This CQRM was the result of the high-grade metamorphic overprint of protoliths that actually lack any primary sandstone texture vestiges but, in turn, exhibit fabric, microstructural and field characteristics (at outcrop and map scales) indicative of hydrothermal quartz veins. Thus, they should not be classified as "quartzites" in a normative petrological/geological sense. However, the reddish quartz-rocks hosting the aforementioned CQRM can be classified as amphibolite- to granulite-facies metamorphic quartzites, identifiable attending to their color, mineral assemblage and microstructure. Both geological materials (white/colorless quartz representing hydrothermal veins and reddish quartz-rock originally comprising part of a sedimentary succession where the former were emplaced), although different in their genesis, they have shared a common posterior metamorphic/deformational overprint and, currently, depict comparable quartz petrofabric LPO patterns. After Hay’s work, reluctance to use the term quartzite by most authors led to prevalence of the tandem terminology for decades, using quartz and quartzite, as aforementioned, in varied forms that paid little or no attention to the geological origin of artifact raw materials. Kyara’s ( 1999 ) attempt to overcome this imprecision remarked that: “… rock types are not still strictly categorized. At that time the names of the artifacts made on metamorphic siliceous rocks are interchangeably referred to as «quartz and quartzite » or «quartz/quartzite »”. (Kyara 1999 , p. 115). This intended to find a consensual term binding together all the metamorphic raw materials identified at Olduvai: “For ease of data analysis, raw material types were grouped under three main working categories based on the three major rock types, namely; (i) Quartzites (inclusive of all metamorphic rocks: quartz, quartzite, purple quartzite, and gneiss); (ii) volcanics, in concurrence with Blumenschine and Masao ( 1991 ) instead of igneous rocks, which incorporate green phonolite, porphyritic phonolite, basalt, and trachyandesite, and, (iii) chert, which is the sole representative of sedimentary rocks in the region” (Kyara 1999 , p. 116). However, this categorization was not grounded on any geological criterion and gave priority to terminological simplification without considering petrologic aspects that might have helped. The lack of robust geological identification criteria was probably the cause of this continuity in long term terminological imprecision. The incorporation of new research teams working in Olduvai in the second decade of the 21st century has recently opened up a period in which the use of the term “quartz” lapsed into disuse without any petrological justification. In return, the term “quartzite” seems to acquire currently a widespread use (i.e. Díez-Martín et al. 2021 ; Sánchez-Yustos et al. 2019 ; de la Torre et al. 2012 ). At present, Egeland and colleagues seem to be the sole authors adhered to the term “quartz”, based on the solid and precise petrologic argument that: “geological work on the Tanzanian Craton reveals that these inselbergs are too metamorphically evolved to be quartzite and, thus, are probably best characterized as resistant, quartz-rich remnants of heavily weathered granulites ( Dawson, 2008 ; Begg et al. 2009 ), which is the label we adopt here. Nevertheless, we think it is reasonable for Olduvai lithic artifacts presumably harvested from these outcrops and flaked largely or exclusively from their quartz constituents to be referred to as «quartz» artifacts.” (Egeland et al. 2020, p. 101). In the framework of this new consensual and imposition period Santonja et al. ( 2014 ), aware of the quandary around this CQRM, specifically defined these materials by the local name of “Naibor quartzite” , stressing the exceptional particularism of this white/colorless quartz raw material. The "Naibor quartzite" is described by them as: “...a metamorphic rock whose almost exclusive constituent mineral is quartz (Table 3) with the shape of phenocrysts and a distinctive lamination that responds to knapping very differently from the other fine-grained quartzites present in Olduvai ( Jones, 1994 ), and more similarly to quartz ( Mourre, 1997 )” (Santonja et al. 2014 , p. 186). Although, the authors identify a metamorphosed rock SiO 2 -rich composition (97.97%) and differentiate a “distinctive lamination”, this does not imply that the raw material should be classified as quartzite. Furthermore, this “distinctive lamination” the authors mention, as said before, is in fact a tectonic/metamorphic foliation rather than a sedimentary bedding/lamination (Boggs 1987 ). Additionally, the term phenocryst is not used properly. This term is used in igneous petrology to identify isolated larger crystals surrounded by a finer grained groundmass of either identical or different mineral composition, which does not concur with the homogeneous grainsize distribution of Olduvai quartz with a granoblastic texture (Licker 2003 ). Finally, although they indicate that the CQRM respond to knapping very differently from the other fine-grained quartzites present in Olduvai, these authors seem to feel comfortable with this contradiction. They do not question why such fracture dynamics are at odds with what is expected in quartzites. This contradiction could be properly defined as the “quartzite paradox”, a characteristic trait of CQRM in Olduvai Gorge. The extremely relevant contradiction in breakage response emerging from the Olduvai paradox is actually simple to answer: if this raw material is composed of large quartz monocrystal grains, then it will exactly respond to knapping as quartz, which is an anisotropic material with substantial crystallographic symmetry. No classical quartzite has a grain size > 2 mm (if so, it would be quartz-rudite). On the contrary, quartzite is usually a microcrystalline mineral aggregate with conchoidal fracture that behaves in breakage in a different way from quartz, which exhibits in any CQRM homogeneous crystalline domains larger than 2 mm until 10–20 mm are reached. This oddity was already identified by Kimura ( 1997 ) when she pointed out that: “quartz fractures unevenly, while the quartzite of Olduvai is extremely coarse-grained and does not form conchoidal fracture” (Kimura 1997 , p. 33). Maté-González et al. ( 2018 ) show that the cut-marks made by CQRM artifacts in Olduvai on fossil bones exhibit similar marks to those produced by quartz. However, their final terminological choice is driven by a rather deterministic rationale when they state that “… in a strict geological definition, these materials are clearly quartzites, and not quartz ( Hay, 1976 ; Santonja et al. 2014 ), regardless of their quartz-like behaviour ” (Maté-González et al. 2018 , p. 449). Thus, Maté-González and colleagues are arguing that the Olduvai CQRM is a raw material that looks, fractures and interacts with animal tissues like quartz but is “clearly quartzite” based on the authority judgment pronounced by Hay and more recently followed by Santonja and colleagues. This is circular reasoning leading to a logical fallacy that is not supported by scientific data. Conclusions The major hominin artifact lithic raw material documented in the Olduvai Gorge region is a geological material with crystalline appearance, white or colorless, foliated or seemingly massive only at the outcrop scale, with a very high quartz-rich composition, and apparently bearing a metamorphic origin. In this work we propose the term CQRM (Crystalline Quartz-rich Raw Material) to designate these materials for intended interdisciplinary descriptive purposes and these natural materials with these petrological features support its identification as “Quartz” . Stubbornness in terminological confusion reflects the complexity and specificity of CQRM at the Olduvai Gorge sites. Such uncertainties relied on the following issues, related to a protracted and complex geological history. Firstly, those materials crop out widely in nearby inselbergs (in some cases, several km 2 in area) composed by quartz-bearing metamorphic rocks of varied types and origins. Second, these metamorphic rocks proceed in part from sedimentary recycling and/or tectonic reworking of much older orogens and cratons (Mesoproterozoic, Paleoproterozoic and even Archean, > 2.5 Ga). Third, the metamorphic rocks were transformed in the solid state by intense ductile deformation under high-grade pressure and temperature (granulitic facies), reaching temperatures in excess of 750–800ºC during the Neoproterozoic Era, 1.0–0.5 Ga. In this study we show that the terminological inaccuracy/ambiguity related to the CQRM conundrum and the related paradox originated in a complex geologic/petrologic context that had not been addressed so far from a multidisciplinary geological perspective. These materials are usually recognized as a white/colorless quartz-rich material (in a purely descriptive sense) and because they occur in areas dominated by outcrops of quartzites ( sensu Pickering, with somewhat different mineral content, microstructure and macroscopic aspect than classic quartzites), they have been classified mainly as quartzites by researchers who are not specialists in hard rock petrology, instead of having been classified as quartz. Therefore, in order to solve these problems, the concurrence of structural/metamorphic petrologists is essential to describe these materials, identify their origin and eventually discuss the implications of their presence in the regional geological context. CQRM formed as a nearly pure-quartz mineral found in hydrothermal veins and dykes, as well as in quartz metamorphic differentiates that after a strong tectonothermal overprint acquired an outstanding planolinear penetrative fabric (that, with untrained eyes, might be mistaken for sedimentary bedding). This raw material was formed as vein mineralizations made up exclusively of quartz, with the particularity that they were hosted in quartzitic country rocks also made up exclusively of quartz. Logically, they share a similar chemical and mineralogical composition. Their partial dissolution by metamorphic fluids provided the source chemical components of secondary quartz veins. After penetrative ductile deformation, both the host rocks and the hosted veins acquired a penetrative fabric (with convergent microstructure and petrofabric), but relics of their original texture can still be differentiated. The mineralogical similarity between these quartz-rich geological products (host rocks and hosted minerals) largely explains the persistence and even the imposition of the name of quartzite for these CQRM by researchers inexperienced in the study of raw materials. The most diagnostic features supporting this interpretation can be summarized as follows. First, some of the microstructural relics identified in the CQRM concur undoubtedly with a hydrothermal origin. Among them it is worth noting: (1) presence of oriented muscovite microinclusions (µm-sized) inside large quartz crystals, (2) occurrence of minor euhedral/subhedral crystals of hydrothermal minerals such as tourmaline, anhydrite (currently stabilized to gypsum) and muscovite forming a paragenetically stable association, and (3) existence of a distribution of clouded domains rich in fluid inclusions in some quartz crystals. Second, the recognition of special deformational structures/microstructures (elongate quartz grains defining the macroscopic foliation and lineation, irregular quartz grain boundaries with concave geometries, small oriented muscovite flakes pinned to quartz boundaries, chessboard recrystallization microstructures fully overprinting rocks with a non-sedimentary primary microstructure…) and intense quartz LPOs (characterized by concentrations of -axes around the lineation orientation) point to operation of tectonometamorphic processes under granulite-facies conditions. Undoubtedly, the precise characterization of the CQRM will allow us to speak with propriety about these raw materials used in the Olduvai Gorge sites and it can also help cast light on the supply and management strategies for these exceptional and unique mineral resources used by the hominin species who inhabited the Olduvai region during the Stone Age. Declarations Acknowledgments We wish to thank the Tanzanian Commission for Science and Technology (COSTECH), the Division of Antiquities and NCAA (Ngorongoro Conservation Area Authority), within the Ministry of Natural Resources and Tourism for permission to conduct research in Olduvai Gorge. Autor Declarations Funding This study was funded by the Spanish Ministry of Science and Innovation through the National Plan I+D+i (projects PID2020-118359GB-I00 and HAR2017-82463-C4-4-P) and the UPV/EHU GIU20/010 research grant. Conflicts of interest The authors declare no competing interests. Ethics approval Not applicable. Consent to participate Not applicable. Consent for publication Not applicable. Data and material availability All relevant data are within the manuscript. All geological samples are stored in the Lithoteque of the Department of Geography, Prehistory and Archaeology of the University of the Basque Country. Code availability Not applicable. 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MILA 6:1-48 Stiles D (1979) Early Acheulian and Developed Oldowan. Current Anthropology 20:126-129. https://doi.org/10.2307/2741876 Stiles D (1991) Early hominid behaviour and culture tradition: raw material studies in Bed II, Olduvai Gorge. African Archaeological Review 9:1-19. https://doi.org/10.1007/bf01117214 Stiles D (1998) Raw material as evidence for human behaviour in the Lower Pleistocene: the Olduvai case. In: Petraglia MD, Korisettar R (eds) Early Human Behaviour in Global Context. Rise and Diversity of the Lower Paleolithic Record. London, Routledge, pp 133-150 Stiles DN, Hay RL, O'Neil JR (1974) The MNK Chert factory site, Olduvai Gorge, Tanzania. World Archaeology 5:285-308. https://doi.org/10.1080/00438243.1974.9979575 Stollhofen H, Stanistreet IG, Toth N, Schick KD, Rodríguez-Cintas Á, Albert RM, Farrugia P, Njau JK, Pante MC, Herrmann EW, Ruck L, Bamford MK, Blumenschine RJ, Masao FT (2021) Olduvai's oldest Oldowan. Journal of Human Evolution 15:102910. https://doi.org/10.1016/j.jhevol.2020.102910 Stünitz H, Thust A, Heilbronner R, Behrens H, Kilian R, Tarantola A, Gerald JDF (2017) Water redistribution in experimentally deformed natural milky quartz single crystals – Implications for H 2 O-weakening processes. Journal of Geophysical Research: Solid Earth 122:866-984. https://doi.org/10.1002/2016JB013533 Tactikos JC (2005) A Landscape Perspective on the Oldowan from Olduvai Gorge, Tanzania. Dissertation, Department of Anthropology, Rutgers, the. State University of New Jersey Tenczer V, Hauzenberger Ch, Fritz H, Hoinkes G, Muhongo S, Klötzli U (2013) Crustal age domains and metamorphic reworking of the deep crust in Northern-Central Tanzania: A U/Pb zircón and monazite age study. Mineralogy and Petrology 107:679-707. https://doi.org/10.1007/s00710-012-0210-1 Thomas RJ, Roberts NMW, Jacobs J, Bushi AM, Horstwood MSA, Mruma A (2013) Structural and geochronological constraints on the evolution of the eastern margin of the Tanzania Craton in the Mpwapwa area, central Tanzania. Precambrian Research 224:671-689. https://doi.org/10.1016/j.precamres.2012.11.010 Thomas RJ, Spencer C, Bushi AM, Baglow N, Boniface N, de Kock G, Horstwood MSA, Hollick L, Jacobs J, Kajara S, Kamihanda G, Key RM, Maganga Z, Mbawala F, McCourt W, Momburi P, Moses F, Mruma A, Myambilwa Y, Roberts NMW, Saidi H, Nyanda P, Nyoka K, Millar I (2016) Geochronology of the central Tanzania Craton and its southern and eastern orogenic margins. Precambrian Research 277:47-67. https://doi.org/10.1016/j.precamres.2016.02.008 Toth N, Schick K (2007) Overview of Paleolithic Anthropology. In Henke HCW, Hardt T, Tatersall I (eds) Handbook of Paleoanthropology. 3. Springer, Berlin-Heidelberg-New York, pp:1943-1963. https://doi.org/10.1007/978-3-540-33761-4_64 Toth N, Schick K (2018) An overview of the cognitive implications of the Oldowan Industrial Complex. Azania: Archaeological Research in Africa 53:3-39. https://doi.org/10.1080/0067270X.2018.1439558 Tucker ME (2013) Sedimentary Petrology. An introduction to the origin of sedimentary rocks, 3rd Edition. Willey & Sons Turner FJ (1941) The development of pseudostratification by metamorphic differentiation in the schists of Otago, New Zealand. American Journal of Science 239:1-16. https://doi.org/10.2475/ajs.239.1.1 Uribelarrea D, Domínguez-Rodrigo M, Pérez-González A, Vegas Salamanca J, Baquedano E, Mabulla A, Musiba C, Barboni D, Cobo-Sánchez L (2014) Geo-archaeological and geometrically corrected reconstruction of the 1.84 Ma FLK Zinj paleolandscape at Olduvai Gorge, Tanzania. Quaternary International 322-323:7-31. https://doi.org/10.1016/j.quaint.2013.12.023 Van Den Kerkhof AM, Hein UF (2002) Fluid inclusion petrography. Lithos 55:27-47. https://doi.org/10.1016/S0024-4937(00)00037-2 Vernon RH (2004) A practical guide to rock microstructure. Cambridge University Press, Cambridge. https://doi.org/10.1017/9781108654609 Vityk MO, Bodnar RJ (1995) Textural evolution of synthetic fluid inclusions in quartz during reequilibration, with applications to tectonic reconstruction. Contributions to mineralogy and Petrology 212:309-323. https://doi.org/10.1007/BF02688246 Wagner T, Boyce AJ, Erzinger J (2010) Fluid-rock interaction during formation of metamorphic quartz veins: a REE and stable isotope study from the Rhenish Massif, Germany. American Journal of Science 310:645-682. https://doi.org/10.2475/07.2010.04 Wheeler J, Jiang Z, Prior DJ, Tullis J (2004) Dynamic recrystallization of quartz. Materials Science Forum 467-470:1243-1250. https://doi.org/10.4028/www.scientific.net/MSF.467-470.1243 Willoughby Pamela R (1985) Spheroids and battered stones in the African Early Stone Age. World Archaeology 17:44-60. https://doi.org/10.1080/00438243.1985.9979949 Yardley BWD (1983) Quartz veins and devolatilization during metamorphism. Journal of the Geological Society 140:657-663. https://doi.org/10.1144/gsjgs.140.4.0657 Yravedra J, Martín-Perea DM, Díez-Martín F, Domingo MS, Arriaza MC, Organista E, Aramendi J, Barba R, Baquedano E, Domínguez-Rodrigo M (2019) Level U3.1, a new archaeological level discovered at BK (upper bed II, Olduvai Gorge) with evidence of megafaunal exploitation. Journal of African Earth Sciences 158:103545. https://doi.org/10.1016/j.jafrearsci.2019.103545 Yravedra J, Maté-González MA, Palomeque-González JF, Aramendi J, Estaca-Gómez V, San-Juan-Blazquez M, García-Vargas E, Organista E, González-Aguilera D, Arriaza MC, Cobo-Sánchez L, Gidna A, Uribelarrea D, Baquedano E, Mabulla A, Domínguez-Rodrigo M (2017) A new approach to raw material use in the exploitation of animal carcasses at BK (Upper Bed II, Olduvai Gorge, Tanzania): a micro-photogrammetric and geometric morphometric analysis of fossil cut marks. Boreas 46:860-873. https://doi.org/10.1111/bor.12224 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 11 May, 2023 Read the published version in Archaeological and Anthropological Sciences → Version 1 posted Reviewers invited by journal 11 Dec, 2022 Editor assigned by journal 09 Dec, 2022 Submission checks completed at journal 08 Dec, 2022 First submitted to journal 05 Dec, 2022 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-2347339","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":158509543,"identity":"0a38dea5-1187-4d11-b5d7-896f0edaec3d","order_by":0,"name":"Antonio Tarriño","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA8ElEQVRIiWNgGAWjYNACAyidAMT8DDyElDNDtPDAtEg2EKWFgQFhtMEBAlr4Z/cffFxQYMNgz3722YeHO+7IGR/vPSbBUHHYHpcWiTuHmY1nGKQx8PCkG89IPPPM2OzMuTQJhjOHExtw6bmRzCbNY3AY6LA0ZobEtsOJ227kmEkwth1OwKVDHq6F/xlEy+b5b4Ba/uF2mAFciwTUlg0SPEAtDYcZcTnM8EaysTGPQRoPzw2ILcYSZ3KMLRKOpeP0i9yNxIePef7YyLH3pzEz/mw7LMfffsbwxocaa5wOgwG0uEggpH4UjIJRMApGAV4AAB7RTZIAiWW7AAAAAElFTkSuQmCC","orcid":"","institution":"University of the Basque Country","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Antonio","middleName":"","lastName":"Tarriño","suffix":""},{"id":158509545,"identity":"e852f7f6-2e5d-4be3-89c7-f59cb14c4cf9","order_by":1,"name":"Benito Ábalos","email":"","orcid":"","institution":"University of the Basque Country","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Benito","middleName":"","lastName":"Ábalos","suffix":""},{"id":158509547,"identity":"a884e0d1-5e80-4c76-9f6e-d69fb4bad892","order_by":2,"name":"Pablo Puelles","email":"","orcid":"","institution":"University of the Basque Country","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Pablo","middleName":"","lastName":"Puelles","suffix":""},{"id":158509549,"identity":"09af722f-eae9-40fa-b8f3-ceca1eecc484","order_by":3,"name":"Luis Eguiluz","email":"","orcid":"","institution":"University of the Basque Country","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Luis","middleName":"","lastName":"Eguiluz","suffix":""},{"id":158509551,"identity":"0d262d53-5bdb-4e43-b985-52cae3b8e8e3","order_by":4,"name":"Audax Mabulla","email":"","orcid":"","institution":"University of Dar es Salaam","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Audax","middleName":"","lastName":"Mabulla","suffix":""},{"id":158509553,"identity":"eb4b49f6-02d3-4540-a0cf-09462e5da936","order_by":5,"name":"Enrique Baquedano","email":"","orcid":"","institution":"Regional Archaeological Museum of Madrid, Alcalá de Henares","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Enrique","middleName":"","lastName":"Baquedano","suffix":""},{"id":158509555,"identity":"d834b3aa-9d88-44e0-8fd6-9337c26b3860","order_by":6,"name":"Manuel Domínguez-Rodrigo","email":"","orcid":"","institution":"University of Alcalá","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Manuel","middleName":"","lastName":"Domínguez-Rodrigo","suffix":""},{"id":158509557,"identity":"497d3d58-fe51-43c7-9e76-cba2ec3044e7","order_by":7,"name":"Fernando Díez-Martín","email":"","orcid":"","institution":"University of Valladolid","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Fernando","middleName":"","lastName":"Díez-Martín","suffix":""}],"badges":[],"createdAt":"2022-12-05 19:29:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2347339/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2347339/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s12520-023-01774-9","type":"published","date":"2023-05-11T20:46:35+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":30210123,"identity":"63f520b5-4e1c-4885-9c55-895f15b9893f","added_by":"auto","created_at":"2022-12-12 15:31:00","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":428286,"visible":true,"origin":"","legend":"\u003cp\u003eLocation of the Olduvai Gorge (black star) in the orogenic context of the Gondwana supercontinent ca. 550 Ma ago. Paleogeographic reconstruction that shows the Mozambique belt or East African Orogen (EAO) as result of Mozambique Ocean closure during the late Ediacaran-Early Cambrian. Modified from Grey et al. (2008), Meert and Lieberman (2008), and Miller et al. (2011).\u003c/p\u003e","description":"","filename":"Figure01.png","url":"https://assets-eu.researchsquare.com/files/rs-2347339/v1/d13e6199d641fd8569043bd4.png"},{"id":30210124,"identity":"9167048d-fd5a-43a2-8589-f98d9b94ea0d","added_by":"auto","created_at":"2022-12-12 15:31:00","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":417493,"visible":true,"origin":"","legend":"\u003cp\u003eGeological sketch map of central East Africa (notably Tanzania and southern Kenya; based upon Fritz et al. (2009) and Tenczer et al. (2012) showing the distribution of its main tectonic elements and mountain areas with outcrops of the Neoproterozoic Mozambique orogenic belt. ANS: Arabian-Nubian Shield, WG: Western Granulites, EG: Eastern Granulites. See text for further details.\u003c/p\u003e","description":"","filename":"Figure02.png","url":"https://assets-eu.researchsquare.com/files/rs-2347339/v1/af31ab221de15afe38ec6b77.png"},{"id":30212130,"identity":"5ba32212-959a-4629-ae32-1cbabeba4e57","added_by":"auto","created_at":"2022-12-12 15:47:00","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2936914,"visible":true,"origin":"","legend":"\u003cp\u003eGoogle map view of the region to the North of the Olduvai Gorge showing quartz-rich sampling sites used for this study.\u003c/p\u003e","description":"","filename":"Figure03.png","url":"https://assets-eu.researchsquare.com/files/rs-2347339/v1/b7b87186b5dd3ec94ed9b648.png"},{"id":30210985,"identity":"55e12223-15a8-4471-bfff-0e4177017d82","added_by":"auto","created_at":"2022-12-12 15:39:00","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":3088658,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a)\u003c/strong\u003e Outcrop detail of “Kissele quartzites” (sensu Pickering, 1958) in Naibor Soit showing a contact between very coarse-grained, crumbly, reddish quartzite and a compact, white/colorless polycrystalline quartz vein. \u003cstrong\u003e(b)\u003c/strong\u003e Naibor Soit outcrop of very coarse-grained, reddish quartzite strongly foliated and crumbly, thus unsuitable for knapping. \u003cstrong\u003e(c)\u003c/strong\u003e Naibor Soit outcrop of foliated, white/colorless polycrystalline quartz (several m thick) reminiscent of milky quartz veins common in low- to medium-grade metamorphic siliciclastic successions. \u003cstrong\u003e(d)\u003c/strong\u003e Outcrop of white/colorless deformed quartz veins. These likely correspond to hydrothermal and/or metamorphic differentiates later reworked and metamorphosed under high-grade conditions. \u003cstrong\u003e(e)\u003c/strong\u003e Hillslope of the Naibor Soit inselberg with climbing people, showing a white quartz rock outcrop at the summit and scattered loose white/colorless quartz fallen fragments, mainly with tabular formats. The inset shows a closer view of the ground with irregular shaped cm-scale white quartz fragments. \u003cstrong\u003e(f)\u003c/strong\u003e Outcrop of the “Loipukoi quartzites” (sensu Pickering, 1958) formed by grey (smoky)-colored polycrystalline quartz rocks with a penetrative metamorphic foliation/lineation. The rock color is due mainly to the presence of large amounts of minute ilmenite and other opaque grain minerals. \u003cstrong\u003e(g)\u003c/strong\u003e Outcrop of the “Lemuta quartzite” (sensu Pickering, 1958) consisting of white, recrystallized, and foliated/lineated quartz-rich layers interbedded with quartz-schists. \u003cstrong\u003e(h)\u003c/strong\u003e Quartz-rich bed containing mm/cm-sized kyanite (Ky) crystals elongated parallel to the rock foliation/lineation. These are sometimes broken and stretched and their presence (petrologically outstanding) characterizes a medium- to high-grade metamorphism.\u003c/p\u003e","description":"","filename":"Figure04.png","url":"https://assets-eu.researchsquare.com/files/rs-2347339/v1/bc8cace1234a3baa7f88b93b.png"},{"id":30210987,"identity":"42603443-0f86-492d-9773-408fa07b7d09","added_by":"auto","created_at":"2022-12-12 15:39:00","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2402632,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a)\u003c/strong\u003e Microphotograph (crossed nicols) showing rutile (Rt) and muscovite with various habits (Mus) as accessory minerals included in a large quartz (Qtz) crystal that occupies the entire field of view. \u003cstrong\u003e(b)\u003c/strong\u003eMicrophotograph (crossed nicols) showing gypsum (Gp) and muscovite (Mus) as accessory mineral inclusions within quartz. \u003cstrong\u003e(c)\u003c/strong\u003e Microphotograph (crossed nicols) showing the shape-preferred orientation of elongated quartz grains (Qtz) in a white quartz. Elongated quartz grains exhibit highly irregular boundaries in detail, denoting their high mobility during recrystallization, and contain nearly parallel scattered muscovite flakes (Ms1, e.g. those in the red circle) a few hundreds of µm long and 2-30 µm thick that also contribute to define the foliation. \u003cstrong\u003e(d)\u003c/strong\u003e Microphotograph (crossed nicols) showing a chessboard microstructure characterized by the formation of rectangular subgrains with nearly orthogonal boundaries and formed as a result of solid-state intracrystalline deformation. Two types of muscovite inclusions can be identified (red circles): larger muscovite flakes (Ms1) with a parallel orientation that defines the foliation, and much smaller muscovite flakes (Ms2) observed as bright spots and thin traits with diverse orientations oblique with respect to Ms1. \u003cstrong\u003e(e)\u003c/strong\u003e Close view of the interior of a quartz grain taken under crossed nicols in a position close to its optical extinction. The bright spots (e.g. those seen in the red circle) correspond to µm-sized muscovite grains (Ms2) and are aligned along two directions that coincide with fine extinction bands (FEB1 and FEB2; cf. Derez et al., 2011) parallel to quartz host rhomb crystallographic planes. \u003cstrong\u003e(f)\u003c/strong\u003e Close view of the interior of a quartz grain (crossed nicols) of the type imaged in (d). The image shows irregular bands rich in fluid inclusions inside a monocrystal that confer a clouded appearance to the domain enclosed by the dashed lines (by contrast with much cleaner quartz crystal domains outside). In the interior of the closed bands several solid and fluid inclusions are elongated and/or aligned parallel to the direction of the red segments, which is controlled by host quartz prismatic crystallographic planes. \u003cstrong\u003e(g)\u003c/strong\u003eHigh magnification image (parallel plane polarized light) of a quartz crystal interior showing a curved band (delimited by white dashes) containing µm sized (spherical to ellipsoidal) and elongated-parallel fluid and solid (opaque and irregular) inclusions. The band is a curved surface tilted ca. 45º with respect to the plane of the image. It can correspond to a relic primary hydrothermal quartz growth feature (not a healed microcrack). The quartz host contains a much smaller proportion of micro- and nano-inclusions and, thus, exhibits a clean aspect. \u003cstrong\u003e(h)\u003c/strong\u003e High magnification image (crossed nicols) of a quartz crystal interior showing a slightly curved band (delimited by white dashes) defined by µm sized (spherical to ellipsoidal) elongated fluid inclusions. The fluid inclusion band is nearly orthogonal to the plane of the image and the elongation direction of the inclusions (red segments) is parallel to the orientation of fine extinction bands (FEB) of the host quartz grain, that is, parallel to a specific crystallographic direction. This concurs with these being relic primary microstructures (of hydrothermal origin), since recrystallization would lead to a complete reorganization (and even decrepitation or removal) of the fluid inclusions, leaving clean quartz grain interiors and fluid accumulation/migration along grain boundary paths. Microphotographs (e) and (f) were taken from Kissele white quartz, whereas (g) and (h) correspond to similar samples from Naibor Soit.\u003c/p\u003e","description":"","filename":"Figure05.png","url":"https://assets-eu.researchsquare.com/files/rs-2347339/v1/70b85daf8a8987c97f09a796.png"},{"id":30210127,"identity":"25170700-44ce-4b6c-9812-bdd0d89d5130","added_by":"auto","created_at":"2022-12-12 15:31:00","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":470980,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a)\u003c/strong\u003e Orientations of poles to quartz {0001}, {11-20} and {10-10} crystallographic planes presented in lower hemisphere, equal area stereographic projections of XZ structural sections (foliation is the equatorial diameter E-W and the lineation is horizontal within that plane). In the stereograms the color patterns represent multiples of mean uniform distribution. \u003cstrong\u003e(b)\u003c/strong\u003e Idealized lower hemisphere stereographic projections showing the relationships between quartz [c] and \u0026lt;a\u0026gt; crystallographic axis lattice preferred orientations and intracrystalline slip systems operating under increasing temperatures (from ca. 300 ºC at the left to \u0026gt;600 ºC at the right) in a non-coaxial deformation regime (after Schmid and Casey, 1986).\u003c/p\u003e","description":"","filename":"Figure06.png","url":"https://assets-eu.researchsquare.com/files/rs-2347339/v1/3c7ee0f2b91187e31e01e044.png"},{"id":44728909,"identity":"a5628fea-4832-4456-b7c8-3c0649af55cd","added_by":"auto","created_at":"2023-10-16 21:09:10","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8589672,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2347339/v1/1bb22b08-b2b2-4899-900c-13791fbd8c81.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"The Crystalline Quartz-rich Raw Material from Olduvai Gorge (Tanzania): Why is it called quartzite when it should be called quartz?","fulltext":[{"header":"Introduction","content":"\u003cp\u003eRaw material studies carried out in the paleoanthropological complex of Olduvai Gorge (Tanzania) have led to the identification of about ten different rock types that were used by early humans in the numerous lithic assemblages recovered there (Blumenschine and Peters 1998; Blumenschine et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2012a\u003c/span\u003e; D\u0026iacute;ez-Mart\u0026iacute;n et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2009a\u003c/span\u003e; Egeland et al. 2020; Favreau et al. \u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Hay \u003cspan citationid=\"CR99\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Kyara \u003cspan citationid=\"CR121\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Leakey \u003cspan citationid=\"CR126\" class=\"CitationRef\"\u003e1967\u003c/span\u003e; McHenry and de la Torre \u003cspan citationid=\"CR141\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Stiles \u003cspan citationid=\"CR199\" class=\"CitationRef\"\u003e1991\u003c/span\u003e, \u003cspan citationid=\"CR200\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Tactikos \u003cspan citationid=\"CR204\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Source areas for all these rock types have also been easily identified (Fl\u0026eacute;bot-Augustins \u003cspan citationid=\"CR83\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Hay \u003cspan citationid=\"CR98\" class=\"CitationRef\"\u003e1971\u003c/span\u003e; Jones \u003cspan citationid=\"CR109\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Kyara \u003cspan citationid=\"CR121\" class=\"CitationRef\"\u003e1999\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAmong these rocks, one in particular stands out as the most characteristic and conspicuous raw material. It consists of a light-colored, homogeneous and anisotropic siliceous crystalline material. It usually exhibits a penetrative foliation with a well-defined mineral/stretching lineation at macroscopic scale and a microscopic fabric revealing high-temperature deformation. These macro and microstructural features support its identification as \u003cem\u003e\u0026ldquo;metamorphic quartz\u0026rdquo;\u003c/em\u003e. Given that this quartz-rich material has a relatively homogeneous cm-scale crystal grain size, with SiO\u003csub\u003e2\u003c/sub\u003e contents\u0026thinsp;\u0026gt;\u0026thinsp;96%, this paper will label it \u0026ldquo;Crystalline Quartz-rich Raw Material\u0026rdquo;, or CQRM, for intended interdisciplinary descriptive purposes. This raw material has been alternatively referred to, with no clear reasons, either as \"quartz\", \"quartz/quartzite\" or \"quartzite\". Currently, as will be seen later, the term quartzite has been winning the terminological game.\u003c/p\u003e \u003cp\u003eThe microstructure (already photographed by some authors but not specifically identified from the geological/petrostructural viewpoint; e.g, Favreau et al. \u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; S\u0026aacute;nchez-Yustos et al. \u003cspan citationid=\"CR181\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Soto et al. \u003cspan citationid=\"CR192\" class=\"CitationRef\"\u003e2020a\u003c/span\u003e, \u003cspan citationid=\"CR193\" class=\"CitationRef\"\u003eb\u003c/span\u003e) is well known in geological literature on quartz-bearing metamorphic tectonites (Law \u003cspan citationid=\"CR122\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). This microstructure is identifiable in rock thin sections normal to the macroscopic foliation and parallel to the lineation and consists of recrystallized quartz grains showing rectangular subgrains. It is known as \u003cem\u003e\u0026ldquo;chessboard\u0026rdquo;\u003c/em\u003e microstructure and is intrinsically associated with a quartz lattice preferred orientation in which the c-axes are parallel to the macroscopic lineation. This crystallographic arrangement is due to the activation of the prism-[c] intracrystalline slip system under temperatures\u0026thinsp;\u0026gt;\u0026thinsp;600 \u0026ordm;C and hydrous conditions (Bouchez et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1984\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1985\u003c/span\u003e; Kruhl \u003cspan citationid=\"CR119\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Mainprice et al. \u003cspan citationid=\"CR137\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Okudaira et al. \u003cspan citationid=\"CR150\" class=\"CitationRef\"\u003e1995\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn the Olduvai Region, the closest bedrock outcrops of CQRM lithologies occur in the Naibor Soit inselberg. These raw materials were increasingly selected for use by hominins between 1.85 and 1.3 Ma (de la Torre and Mora 2005; Kimura \u003cspan citationid=\"CR117\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Kyara \u003cspan citationid=\"CR121\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Leakey \u003cspan citationid=\"CR127\" class=\"CitationRef\"\u003e1971\u003c/span\u003e): a) they represent\u0026thinsp;\u0026ge;\u0026thinsp;65% of the lithic assemblages identified in the Bed I \"Zinj\" floor (D\u0026iacute;ez-Mart\u0026iacute;n et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2021\u003c/span\u003e, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e); b) they predominate (74.5%) in the early Acheulean at FLK West, in lower Bed II (D\u0026iacute;ez-Mart\u0026iacute;n et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e); c) they reach their highest proportion (\u0026ge;\u0026thinsp;93%) in uppermost Bed II, in sites such as TK and BK (D\u0026iacute;ez-Mart\u0026iacute;n et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2009a\u003c/span\u003e; Santonja et al. \u003cspan citationid=\"CR183\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe terminological vagueness related to this particular rock type is a long-running trait of raw material and lithic studies in Olduvai Gorge. After the discovery of the paleontological sites carried out in 1911 by the expedition of the entomologist W. Kattwinkel, the volcanologist and paleontologist H. Reck completed the comprehensive paleontological and geological study of the region discovering the first Olduvai hominin skeleton. This author already identified the CQRM lithologies and labeled them with the dual term \u0026ldquo;quartz and quartzite\" (\u003cem\u003eQuarz und Quarzitr\u0026uuml;cken\u003c/em\u003e, in the original publication) (Reck \u003cspan citationid=\"CR166\" class=\"CitationRef\"\u003e1914\u003c/span\u003e, p. 84). Subsequently, Louis Leakey\u0026rsquo;s publications on paleoanthropological remains in Olduvai did not specify the raw materials used for production of stone tools (Leakey \u003cspan citationid=\"CR123\" class=\"CitationRef\"\u003e1932\u003c/span\u003e; Leakey et al. \u003cspan citationid=\"CR125\" class=\"CitationRef\"\u003e1933\u003c/span\u003e). After the break imposed by World War II, researchers used the terms \"quartz\", \"quartz/quartzite\" or \u0026ldquo;quartzite\u0026rdquo; equally.\u003c/p\u003e \u003cp\u003eThe terminological ambiguity related to the CQRM in Olduvai Gorge bears an inescapable archaeological implication. Most of the researchers involved in the characterization of this raw material and determination of its sources are not specialists in the field of metamorphic tectonite rocks. In fact, they have based their studies heavily on nonspecific bibliographic references. In our view it is crucial to undertake a comprehensive analysis of the CQRM from the structural, metamorphic and petrological perspectives. Bearing this in mind, the main objective of the present study is to build a robust and conclusive background that will enable an accurate identification and classification of the materials under study.\u003c/p\u003e"},{"header":"Historiographical Contextualization","content":"\u003cp\u003eAn extensive bibliographic analysis of about one hundred and twenty scientific studies has been carried out in order to trace and understand the terminological evolution of the CQRM within the paleoanthropological research undertaken in Olduvai Gorge. This review has allowed us not only to describe and systematize the different phases of the research devoted to these materials, but also to understand the origin of the recurrent mistakes that persist in the current state of the art. The following periods can be identified (Table 1).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e Number of research articles using the terms: \u0026ldquo;Quartz\u0026rdquo;, \u0026ldquo;Quartz/Quartzite\u0026rdquo; and \u0026ldquo;Quartzite\u0026rdquo; at the Olduvai Gorge sites.\u003c/p\u003e\n\u003ctable border=\"0\" cellpadding=\"0\" cellspacing=\"0\" width=\"473\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"2\" rowspan=\"3\" width=\"32.346723044397464%\"\u003e\n \u003cp\u003e\u003cstrong\u003ePeriod\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"3\" width=\"21.775898520084567%\"\u003e\n \u003cp\u003e\u003cstrong\u003eYears\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" width=\"45.87737843551797%\"\u003e\n \u003cp\u003e\u003cstrong\u003eTerminology (articles nr.)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" width=\"27.522935779816514%\"\u003e\n \u003cp\u003e\u003cstrong\u003eQuartz\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"36.23853211009175%\"\u003e\n \u003cp\u003e\u003cstrong\u003eQuartz/\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" width=\"36.23853211009175%\"\u003e\n \u003cp\u003e\u003cstrong\u003eQuartzite\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"100%\"\u003e\n \u003cp\u003e\u003cstrong\u003eQuartzite\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"4%\"\u003e\n \u003cp\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"28.42105263157895%\"\u003e\n \u003cp\u003e\u003cstrong\u003eDescriptive\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.68421052631579%\"\u003e\n \u003cp\u003e1914 - 1975\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"12.631578947368421%\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.63157894736842%\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.63157894736842%\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"4%\"\u003e\n \u003cp\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"28.42105263157895%\"\u003e\n \u003cp\u003e\u003cstrong\u003eEarly identifying\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.68421052631579%\"\u003e\n \u003cp\u003e1976 - 1998\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"12.631578947368421%\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.63157894736842%\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.63157894736842%\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"4%\"\u003e\n \u003cp\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"28.42105263157895%\"\u003e\n \u003cp\u003e\u003cstrong\u003eLate identifying\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.68421052631579%\"\u003e\n \u003cp\u003e1999 - 2010\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"12.631578947368421%\"\u003e\n \u003cp\u003e14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.63157894736842%\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.63157894736842%\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"4%\"\u003e\n \u003cp\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"28.42105263157895%\"\u003e\n \u003cp\u003e\u003cstrong\u003eImpositive\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.68421052631579%\"\u003e\n \u003cp\u003e2011 - 2019\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"12.631578947368421%\"\u003e\n \u003cp\u003e17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.63157894736842%\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.63157894736842%\"\u003e\n \u003cp\u003e33\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"4%\"\u003e\n \u003cp\u003e\u003cstrong\u003e5\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"28.42105263157895%\"\u003e\n \u003cp\u003e\u003cstrong\u003eLatest\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.68421052631579%\"\u003e\n \u003cp\u003e2020 - 2022\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"12.631578947368421%\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.63157894736842%\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.63157894736842%\"\u003e\n \u003cp\u003e15\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e1. Descriptive period (1914-1975). The first studies carried out barely discussed raw material identification. The first reference addressing this issue can be found in Louis Leakey\u0026rsquo;s interpretation of the archaeological sequence in Olduvai, where CQRM was generically described as \u003cem\u003e\u0026ldquo;irregular lumps of quartz and quartzite\u0026rdquo;\u003c/em\u003e (Leakey 1951, p. 34). Subsequent works always referred to this rock type through the terminological dichotomy \u003cem\u003e\u0026ldquo;quartz/quartzite\u0026rdquo;\u003c/em\u003e (Cole 1954; Evernden et al. 1965; Kleindienst 1959; Leakey 1967). The same situation applied to Mary Leakey\u0026rsquo;s benchmark monograph, volume 3, \u0026ldquo;Olduvai Gorge. Excavations in Beds I\u0026amp;II, 1960-1963\u0026rdquo; (Hay 1971; Leakey 1971) and other relevant works published afterwards (Leakey 1975; Leakey et al. 1972; Stiles et al. 1974) where CQRM was indistinctly referred to as \u003cem\u003e\u0026ldquo;quartz\u0026rdquo;\u003c/em\u003e, \u003cem\u003e\u0026ldquo;quartz/quartzite\u0026rdquo;\u003c/em\u003e, \u003cem\u003e\u0026ldquo;quartz and quartzite\u0026rdquo;\u003c/em\u003e, and \u003cem\u003e\u0026ldquo;quartz or quartzite\u0026rdquo;\u003c/em\u003e.\u003c/p\u003e\n\u003cp\u003e2. Early identifying period (1976-1998). In 1976 Richard L. Hay, a specialist in sedimentary petrology, published his referential monograph on the geology of Olduvai Gorge. Although his work preferentially focused on establishing the normative stratigraphic sequence in Olduvai, he was also interested in the description and identification of lithic raw material rock sources. Hay (1976, p. 9) was the first author who described these CQRM specifically as: \u003cem\u003e\u0026ldquo;The quartzite is extremely coarse-grained and commonly micaceous in the northern and eastern parts of the Olduvai region, including Naibor Soit and the hills and highlands to the north of the gorge. Individual crystals of quartz are generally 1 to 2 cm in diameter in these rocks. Most of the quartzite exposed to the south of the gorge is medium-grained and exhibits primary sandstone textures \u0026hellip;\u0026rdquo;\u003c/em\u003e. Hay\u0026rsquo;s definition already shows a number of contradictions. Firstly, the grain size in the Olduvai CQRM is disproportionately larger (one order of magnitude) than the upper limit admitted for classification of a sedimentary siliciclastic rock as sandstone (\u0026lt;2 mm) or as quartzite if the sandstone is very rich in quartz detrital grains (Folk 1974; Robertson 1999). In fact, siliciclastic rocks with grain sizes \u0026gt;2 mm should be classified as conglomerates or rudites in a broader sense. Secondly, the key argument for the identification of these rocks as quartzites is based on the recognition in them of a fine lamination parallel to the compositional bed layering at a larger scale (\u0026quot;laminated quartzites\u0026quot; of some authors), interpreted by R. L. Hay as a primary sedimentary rock texture. This interpretation is contentious because it is based upon incomplete observations that failed to identify (1) the presence of a penetrative lineation contained in the planar structure and (2) their complete recrystallization that resulted in a crystalloblastic texture (as already noticed by Saggerson 1966). If these two features had been recognized the above-mentioned lamination would have been interpreted correctly as a metamorphic foliation containing a mineral/stretching lineation, that is, a secondary metamorphic texture and not a primary sedimentary texture.\u003c/p\u003e\n\u003cp\u003eHay\u0026rsquo;s determination never prompted a definitive terminological consensus. During the years following Hay\u0026apos;s work, a number of authors referred to the Olduvai CQRM exclusively as \u0026quot;quartzite\u0026quot; (Blumenschine et al. 1998; Jones 1979, 1980, 1981, 1994; Potts 1988). Even, Mary Leaky herself does not speak for the first time of quartzite until the monograph publication on the upper beds (Leakey 1994). Meanwhile, other scholars continued to choose the term quartz (Bower 1977; Ludwig 1999; Mehlman 1977; Perl\u0026egrave;s 1991; Sanhouni et al. 1997; Stiles 1979; Willoughby 1985) or quartz/quartzite (Fl\u0026eacute;bot-Augustins 1990, 1997; Schick 1987; Stiles 1977, 1991, 1998) as alternatives. In this regard, Kimura (1997, p. 33) justified the use of the term quartz for the Olduvai CQRM exclusively based on fracture mechanics reasons, since this rock type \u0026ldquo;\u0026hellip; \u003cem\u003eis extremely coarse-grained and does not form conchoidal fracture\u003c/em\u003e\u0026rdquo;. Summarizing, during this period a large number of authors were reluctant to use the term \u0026quot;quartzite\u0026quot; originally tagged by Hay (1976), emphasizing the lack of terminological consensus among specialists.\u003c/p\u003e\n\u003cp\u003e3. Late identifying period (1999-2010). This period was inaugurated with O. Kyara\u0026rsquo;s monographic study, the first one specifically devoted to the raw materials in the lithic assemblages of the Olduvai Gorge sites. In this work the term \u0026quot;quartzite\u0026quot; was chosen so that it \u0026ldquo;\u0026hellip; \u003cem\u003ereplaces the indeterminate dual terms \u0026lsquo;quartz or quartzite\u0026rsquo;, and \u0026lsquo;quartz/quartzite\u0026rsquo; used interchangeably by Leakey M.D. (1971)\u003c/em\u003e\u0026rdquo; (Kyara 1999, p. 176). This decision was meant to provide a terminological unification that, however, was not based on scientific arguments. Kyara\u0026rsquo;s work did not achieve the intended standardization, and scholars kept using the terms \u0026quot;quartz\u0026quot; and \u0026quot;quartz/quartzite\u0026quot; (Toth and Schick 2007). Moreover, even more intense polarization began between advocates of the term \u0026quot;quartz\u0026quot; (de la Torre 2004; de la Torre et al. 2004, 2008; de la Torre and Mora 2005a, b, 2010; D\u0026iacute;ez-Mart\u0026iacute;n et al. 2008, 2009a, b, 2010; Egeland 2008; Kimura 1999, 2002; Prendergast et al. 2007) and supporters of \u0026quot;quartzite\u0026quot; (Blumenschine et al. 2008; Bunn et al. 2010; Mourre 2003; Plummer 2004; Sharon 2008; Tactikos 2005) (Table 1). This dichotomy caused much confusion. For example, de la Torre (2004, p. 21), also in search of terminological unification, suggested just the opposite to that proposed previously by Kyara (1999): \u0026ldquo;\u003cem\u003econsidering the petrological similarity between quartz and quartzite at Olduvai (Hay 1976), both will be included in the generic term quartz, the most used in the literature on the region\u003c/em\u003e\u0026rdquo;. However, soon after, the author established raw material categories according to macroscopic criteria that included \u0026quot;quartz\u0026quot; in an intended petrological sense that was ill-defined: \u0026ldquo;\u0026hellip; \u003cem\u003equartzs (in fact quartzites and metaquartzites in petrological terms)\u003c/em\u003e\u0026rdquo; (de la Torre et al 2004, p. 24). At odds with the previous strategy, Tacktikos (2005, p. 75) stressed that identification of CQRM as quartzite is \u0026ldquo;\u0026hellip; \u003cem\u003ebased on careful visual examination of the color, the groundmass or matrix, and the presence/absence of conspicuous crystals or phenocrystals\u003c/em\u003e\u0026rdquo;. These criteria present a number of inappropriate geological terms as the basis for recognition: (1) color is secondary or meaningless as a criterion for classification in sedimentary rocks, (2) quartz-rich siliciclastics (quartz-arenites) essentially lack a matrix, and (3) phenocrystals are characteristic of igneous rocks.\u003c/p\u003e\n\u003cp\u003e4. Imposition period (2011-2019). The second decade of the 21st century saw an increasing number of new research teams working in Olduvai Gorge and, consequently, a growing published output. At this time the term \u0026ldquo;quartzite\u0026rdquo; gained popularity and was used by the overwhelming majority of authors (Abtosway 2018; Arroyo and de la Torre 2016, 2018, 2020; Bello-Alonso et al. 2019, 2021; Benito-Calvo and de la Torre 2011; Blumenschine et al. 2012a, b; Byrne et al. 2016; Courtenay et al. 2019; de la Torre and Mora 2014, 2018a, b, 2020; de la Torre et al. 2012, 2013, 2018a, b, c, 2021; Eren et al. 2014; Favreau et al. 2019, 2020; Fujioka et al. 2022.;Gurtov et al. 2015; Key et al. 2020; Macdonald et al. 2022; Mat\u0026eacute;-Gonz\u0026aacute;lez et al. 2018; McHenry and de la Torre 2018; Panera et al. 2019; Pante and de la Torre 2018; Proffitt 2018; Reti 2013, 2016; Rubio-Jara et al. 2017; Santonja et al. 2014, 2018; Soto et al. 2020a, b; Stollhofen et al. 2021; Toth and Schick 2018; Uribelarrea et al. 2014; Yravedra et al. 2017, 2019). Meanwhile, the use of the term \u0026quot;quartz\u0026quot; declined (D\u0026iacute;ez-Mart\u0026iacute;n et al. 2011, 2012, 2014a, b, 2016, 2017, 2018; Egeland et al. 2020; Eren et al. 2013, 2014; Goldman-Newman et al. 2012; Gurtov and Eren 2014; S\u0026aacute;nchez-Yustos et al. 2012, 2015, 2016, 2017a, b, 2018). During the last years of the decade some of the most resilient advocates of the term \u0026quot;quartz\u0026quot; got swept away by imposition of the terminological mainstream (D\u0026iacute;ez-Mart\u0026iacute;n et al. 2021, 2022; S\u0026aacute;nchez-Yustos 2021; S\u0026aacute;nchez-Yustos et al. 2019). At present, Egeland et al. (2020) appear to be the only authors that still refer to the Olduvai CQRM as quartz.\u003c/p\u003e\n\u003cp\u003eThe evolution of terminological trends in the identification of the CQRM at Olduvai Gorge presented in Table 1 reveals three turning points. The first is related to Hay\u0026rsquo;s definition, for the first time, of this raw material as \u0026quot;quartzite\u0026quot;. Hay\u0026rsquo;s work triggered a polarization in the use of the terms \u0026ldquo;quartz\u0026rdquo; and \u0026ldquo;quartzite\u0026rdquo; until then used interchangeably and since then as contenders, although some authors maintained the use of both terms. Kyara\u0026apos;s contribution marked the second turning point defined by a clear confrontation between both terms. It was during this period that authors opted for one term or the other. Finally, a third break occurred in 2011 with a notable increase in publications produced by the incorporation of new research teams in Olduvai. From this moment on, most researchers accepted the postulates defended by Hay (1976) and Kyara (1999), favoring the use of \u0026ldquo;quartzite\u0026rdquo; over \u0026ldquo;quartz\u0026rdquo;. However, as made clear above, these postulates were not based on accurate petrological criteria. Currently, researchers massively accept the term \u0026ldquo;quartzite\u0026rdquo; to define this rock type.\u003c/p\u003e"},{"header":"Geological Context","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eGeological framework\u003c/h2\u003e \u003cp\u003eOlduvai Gorge (Tanzania) is located in the Great East African Rift Valley, more specifically adjacent to the Oldoinyo Ogol highlands (south of the Oliondo Mountains and Loita Hills). The mountainous area to the north of Olduvai Gorge is geomorphologically constrained by hard lithologies with a significant presence of quartz-rich rocks that were affected by high-grade regional metamorphism and concomitant deformation during the Neoproterozoic. These rocks and the deformational structures developed in these materials form part of the so-called East African Orogen (EAO; Stern \u003cspan citationid=\"CR196\" class=\"CitationRef\"\u003e1994\u003c/span\u003e), formerly known as the \u003cem\u003e\u0026ldquo;Mozambique Belt\u0026rdquo;\u003c/em\u003e (Holmes \u003cspan citationid=\"CR102\" class=\"CitationRef\"\u003e1951\u003c/span\u003e). This orogen extends over 8,000 km from the Sinai Peninsula to South Africa and beyond (current Antarctica) forming a 250\u0026ndash;350 km wide belt, though locally it can approach 1,000 km. This is one of the largest ancient orogenic belts on Earth, formed by the closure of the \u003cem\u003e\u0026ldquo;Mozambique Ocean\u0026rdquo;\u003c/em\u003e between 650 and 500 Ma ago (Ediacaran to early Paleozoic times; Thomas et al. \u003cspan citationid=\"CR206\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) and the subsequent collision of the Eastern and Western Gondwana subcontinents and their magmatic arcs (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eNotably in Kenya and eastern Tanzania two major crustal units can be distinguished in the EAO (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e): the \u003cem\u003e\u0026ldquo;Eastern Granulites\"\u003c/em\u003e and the \u003cem\u003e\"Western Granulites\u0026rdquo;\u003c/em\u003e (Hepworth \u003cspan citationid=\"CR100\" class=\"CitationRef\"\u003e1972\u003c/span\u003e). The area of interest here has usually been ascribed to the \"Western Granulite\" unit, although so far it has not been studied in detail. As a result, studies of neighboring mountain domains several tens to a few hundreds of km apart (e.g, the Pare and Usambara Mountains or the Taita and Loita Hills) usually consider the Oliondo highlands a part of the Western Granulites (e.g, Cutten et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Fritz et al. \u003cspan citationid=\"CR86\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), whereas others regard them as inliers of low-grade metamorphic rocks (different from granulites) of the Western Granulites (e.g, Fritz et al. \u003cspan citationid=\"CR87\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The Western and Eastern Granulites have different lithological composition, age range of the protoliths, metamorphic grade, age of metamorphism, structural style and igneous rock inclusions (Fritz et al. \u003cspan citationid=\"CR86\" class=\"CitationRef\"\u003e2013\u003c/span\u003e, and references therein).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe Eastern Granulites terrane is tectonically emplaced onto the Western Granulites, which themselves are tectonically emplaced over the Archean Tanzania Craton, made of much older though lower-grade metamorphic rocks (Holmes \u003cspan citationid=\"CR102\" class=\"CitationRef\"\u003e1951\u003c/span\u003e). The age of tectonic stacking (diachronous along the orogenic belt) is ascribed to the Neoproterozoic Era, between 1000 and 538.8 Ma (Cohen et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Deformation and metamorphism peaks (the so-called Kuunga Orogeny) occurred ca. 640 Ma ago in the Western Granulite Belt (composed of psammitic and pelitic metasediments and their migmatized equivalents), and the final crustal consolidation somewhat later at 580\u0026thinsp;\u0026minus;\u0026thinsp;500 Ma (Tenczer et al. \u003cspan citationid=\"CR205\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Abundance of metamorphosed sandstones (\u003cem\u003e\u0026ldquo;quartzites\u0026rdquo;\u003c/em\u003e) among the psammitic and pelitic metasediments suggests derivation of detrital quartz grains from source areas dominated by granitic and gneissic rocks, such as those cropping out so far in the Tanzania Craton (Manya et al. \u003cspan citationid=\"CR139\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Schl\u0026uuml;ter \u003cspan citationid=\"CR186\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Thomas et al. \u003cspan citationid=\"CR207\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eA Geological Guide To Identify Quartz-rich Rocks In Pleistocene Lithic Industry As Raw Material Sources\u003c/h3\u003e\n\u003cp\u003eQuartz (and its polymorphic varieties, all compositionally being SiO2) is one of the most frequent minerals in the Earth's crust. High-purity quartz-rich rocks are important raw materials in the current high-tech industry (G\u0026ouml;tze and M\u0026ouml;ckel \u003cspan citationid=\"CR92\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). The label \u0026ldquo;quartz-rich rock\u0026rdquo; is used here to encompass those rocks dominated by the quartz mineral (without reference to their origin). The suitability to conchoidal fracture, hardness and resistance of flaked cutting-edges in quartz-rich rocks made them sought-after resources by the genus Homo since the early stages of technological behavior, from beginning of the Pleistocene. As explained in detail in a previous section, geological terms such as \u0026ldquo;quartz\u0026rdquo; and \u0026ldquo;quartzite\u0026rdquo; are used to label the raw materials of those lithic artifacts and usually its correct detectionserve as guides to search for their source areas.\u003c/p\u003e \u003cp\u003eAlthough several specialists in non-geological disciplines have made considerable efforts to incorporate these geological constraints to their research, there exists considerable confusion regarding the terms (mis)used and their actual meaning. For example, \u0026ldquo;quartz\u0026rdquo; is a mineral in the geological sense that has formed as a result of geologic processes (cf. Deer et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1966\u003c/span\u003e; Neuendorf et al. \u003cspan citationid=\"CR148\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). However, the term quartz has sometimes been reported, implicitly or explicitly, as if it were a rock (e.g, Mourre \u003cspan citationid=\"CR143\" class=\"CitationRef\"\u003e1996\u003c/span\u003e, p. 207; S\u0026aacute;nchez Yustos et al. 2012, p. 7, etc.). In fact, a \u0026ldquo;rock\u0026rdquo; is in its correct geological sense an aggregate of one or more minerals (Neuendorf et al. \u003cspan citationid=\"CR148\" class=\"CitationRef\"\u003e2005\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePetrographic observations (with the help of polarizing microscopes) have also been used in Paleoanthropology and Archaeology to describe and constrain quartz-rich rock characteristics. However, studies on the raw materials at the Olduvai sites usually failed to recognize and/or misinterpret diagnostic rock microstructures recorded in classical reference books (e.g, Passchier and Trouw \u003cspan citationid=\"CR155\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Tucker \u003cspan citationid=\"CR210\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) and their mechanical significance (Spry \u003cspan citationid=\"CR195\" class=\"CitationRef\"\u003e1969\u003c/span\u003e; Nicolas and Poirier \u003cspan citationid=\"CR149\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Vernon \u003cspan citationid=\"CR214\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The microstructural approach used in these works to discriminate the origin of quartz-rich rocks has also been taken into account to explain/infer mechanical properties that might be of archaeological interest, such as rock strength/fragility, isotropic/anisotropic character and the predictability of fracture geometry during knapping. However, in several instances they failed in the recognition of the mono- or polycrystalline character of the aggregates as well as in the identification of widespread solid-state metamorphic recrystallization fabrics imposed on pre-existing quartz rocks whatever their type. These types of quartz crystals are usually sourced (or derived) from hydrothermal veins that can attain large dimensions (several m-wide and km-long; e.g, Hippert and Massucato 1998; Lemarchand et al. \u003cspan citationid=\"CR131\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) and, as a rule, they exhibit microstructures that isolate seemingly homogeneous and relatively intact lattice domains not to be confused with the detrital \u003cem\u003e\u0026ldquo;grains\u0026rdquo;\u003c/em\u003e of rocks with a sedimentary primary origin (i.e. pure-quartz sandstones or quartzites). The dimensions admitted for some quartz-rich rock deposits/units may also be considered so large that they might no longer be interpreted as veins, but as \u003cem\u003e\u0026ldquo;sedimentary quartzite\u0026rdquo;\u003c/em\u003e formations, the large dimensions of which are familiar to most researchers.\u003c/p\u003e \u003cp\u003eIn spite of the above, also in the geological context, the term \u003cem\u003e\u0026ldquo;quartzite\u0026rdquo;\u003c/em\u003e has been used loosely to name (1) metamorphic rocks formed by metamorphism of an almost pure quartz sandstone, (2) very hard sandstones, that is, sedimentary rocks composed almost exclusively of quartz grains cemented with additional quartz, (3) granular metamorphic differentiates formed by quartz dissolution in aqueous fluids and posterior re-precipitation coeval with metamorphism and (4) hydrothermal/pegmatitic quartz mineral aggregates occurring in veins and genetically related to magmatic intrusions and ore deposits. Only the first case corresponds truly with a real quartzite in the geological sense and, therefore, the usage of the term \"quartzite\" should be restricted to designate these rocks. This terminological confusion has brought to light the old \u0026ldquo;quartzite problem\" (Skolnick \u003cspan citationid=\"CR191\" class=\"CitationRef\"\u003e1965\u003c/span\u003e), requiring undoubtedly a microstructural analysis to identify the textural features registered in these rocks and discriminate clastic textures associated with siliciclastic rocks from metamorphic features (Howard \u003cspan citationid=\"CR103\" class=\"CitationRef\"\u003e2005\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFurther complicating matters, additionally, prominent tectonic fabrics can be superimposed on any of the rock types reported during later syn-metamorphic solid-state deformation. Such fabrics are defined by penetrative (dominant at a given micro and meso-scale) planar features termed \u003cem\u003e\u0026ldquo;foliations\u0026rdquo;\u003c/em\u003e and/or linear features termed \u003cem\u003e\u0026ldquo;lineations\u0026rdquo;\u003c/em\u003e (Spry \u003cspan citationid=\"CR195\" class=\"CitationRef\"\u003e1969\u003c/span\u003e). In the case of rocks that have undergone significant deformation and metamorphism, however, the lack of a comprehensive microstructural study may lead to the wrong interpretation of these planar penetrative tectonic foliations as sedimentary laminations, as in Hay (\u003cspan citationid=\"CR99\" class=\"CitationRef\"\u003e1976\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThis is not the only case where non-specialists may confuse metamorphic features with sedimentary structures/microstructures. Even the sigmoidal shear zone foliations well-known to \u003cem\u003e\u0026ldquo;hardrock geologists\u0026rdquo;\u003c/em\u003e may be wrongly attributed to cross-bedding without truncation surfaces (climbing ripple cross stratification and aggrading beds) by \u003cem\u003e\u0026ldquo;softrock geologists\u0026rdquo;\u003c/em\u003e (in the sense of Hall \u003cspan citationid=\"CR97\" class=\"CitationRef\"\u003e1988\u003c/span\u003e) and others. Though geometrical similarities may exist between structures of radically different origins (sedimentary stratification/lamination versus tectonic/metamorphic pseudostratification or foliation; e.g, Turner \u003cspan citationid=\"CR211\" class=\"CitationRef\"\u003e1941\u003c/span\u003e), careful microstructural observations are able to discriminate them.\u003c/p\u003e\n\u003ch3\u003eQuartz-rich Rocks In Metamorphic Environments\u003c/h3\u003e\n\u003cp\u003eMetamorphic processes may affect rock precursors of any type (igneous, pegmatitic, hydrothermal, sedimentary and even metamorphic), and usually redistribute in them large amounts of SiO\u003csub\u003e2\u003c/sub\u003e that are first mobilized (dissolved) and then precipitated (recrystallized) to form veins and lenses (Oliver \u003cspan citationid=\"CR151\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Wagner et al. \u003cspan citationid=\"CR216\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). These are termed \u003cem\u003e\u0026ldquo;metamorphic quartz mobilisates or differentiates\u0026rdquo;\u003c/em\u003e in the geological literature and occur along preexisting mechanical anisotropies. In regional metamorphic contexts, anisotropies are dominated by tectonic foliations formed in the host rocks during solid-state deformation accommodation and mineral growth process (Chapman \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e1950\u003c/span\u003e; Spry \u003cspan citationid=\"CR195\" class=\"CitationRef\"\u003e1969\u003c/span\u003e; Yardley \u003cspan citationid=\"CR219\" class=\"CitationRef\"\u003e1983\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eQuartz metamorphic segregations (either quartz veins or more irregular masses) are derived from the wall rocks during metamorphism and, therefore, undergo minor transportation. In these cases, quartz vein abundance is closely related to the abundance of quartz in the country rocks (higher in quartzites, lower in pelitic schists). These quartz bodies usually appear as blankets parallel to foliations and shear zones, or as saddles in minor fold hinges. Smaller quartz veins occurring parallel to foliations mainly form by diffusional transport (with a thickness of up to 10 cm, since diffusion is ruled out as a source of much larger veins).\u003c/p\u003e \u003cp\u003eQuartz-kyanite veins in high-grade quartzo-feldspathic schists and gneisses are classic examples of metamorphic differentiation in quartzose rocks containing aluminous material (e.g, Dorr \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e1969\u003c/span\u003e; M\u0026uuml;ller et al. \u003cspan citationid=\"CR146\" class=\"CitationRef\"\u003e2007\u003c/span\u003e, \u003cspan citationid=\"CR147\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Less dissolvable minerals are passively concentrated there by the solution and selective removal of the more soluble phases. The blankets can be continuous for meters or up to hundreds of meters (Guild \u003cspan citationid=\"CR94\" class=\"CitationRef\"\u003e1957\u003c/span\u003e) and sometimes are of pegmatitic character, in coherence with the medium- to high-grade pressure and temperature conditions undergone by their metamorphic country rocks.\u003c/p\u003e \u003cp\u003eEven veins of hydrothermal origin may be filled with fine and/or coarse-grained quartz crystals that exhibit a banded or layered structure (e.g, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e in Fonseca et al. \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) parallel to the host rock walls. These well-known arrangements, usually observed in large veins that can be traced for some meters at most, are related to progressive vein infilling processes by repeated crack-seal mechanisms (Bons et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Ramsay \u003cspan citationid=\"CR165\" class=\"CitationRef\"\u003e1980\u003c/span\u003e) and to mineralizing fluid diversion into shorter and wider cavities upon hydrofracture arrest (Bons \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). The layering described, which can even be strengthened in metamorphic scenaR\u0026iacute;os where the veins become overprinted by solid-state plastic deformation, can also be misinterpreted as sedimentary lamination/bedding. A note of caution is thus needed when this type of interpretation is suggested in metamorphic environments.\u003c/p\u003e \u003cp\u003eThe metamorphosed and strongly deformed equivalents of \u003cem\u003e\u0026ldquo;hydrothermal vein quartz\u0026rdquo;\u003c/em\u003e and \u003cem\u003e\u0026ldquo;pegmatitic quartz\u0026rdquo;\u003c/em\u003e may give rise to quartz-rich rock units and even mappable formations (made of poly-crystalline quartz aggregates) parallel to metamorphosed \u003cem\u003e\u0026ldquo;sedimentary quartzites\u0026rdquo;\u003c/em\u003e and to \u003cem\u003e\u0026ldquo;metamorphic quartz mobilisates\u0026rdquo;\u003c/em\u003e. Yet, new \u003cem\u003e\u0026ldquo;hydrothermal vein quartz\u0026rdquo;\u003c/em\u003e can also be generated during the overprinting process. Eventually, all these rocks occur interleaved in nature, may exhibit similar aspects to the naked eye, and might be wrongly termed \u003cem\u003e\u0026ldquo;quartzite\u0026rdquo;\u003c/em\u003e. Nevertheless, strictly speaking, they have radically different origins that might be revealed by careful study of their microstructure under the petrographic microscope (e.g, Lychagin et al. \u003cspan citationid=\"CR135\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn particular cases, hydrothermal quartz in orogenic metamorphic environments can occur in giant veins up to 15\u0026ndash;20 km in outcrop length and tens of m in width (e.g, Jia and Kerrick 2000; Lemarchand et al. \u003cspan citationid=\"CR131\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) or vein complexes hundreds of m thick. They are known worldwide from the Archaean (Kerrick and Feng 1992) to the Cenozoic (Fonseca et al. \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), including notable examples in linear Paleoproterozoic orogens (Pati et al. \u003cspan citationid=\"CR156\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Rout et al. \u003cspan citationid=\"CR170\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) and in Gondwanan orogens (Carvalho \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Chaves \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Chaves et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Esteves and Faleiros \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Hippert and Massucatto \u003cspan citationid=\"CR101\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) coeval with the Mozambique Belt in Tanzania during the Neoproterozoic.\u003c/p\u003e \u003cp\u003eIn a nutshell, regional metamorphic terrains usually contain quartz-rich lenses and layers with thicknesses varying between cm and some hm thick, and dm to km in map extent. Metamorphic quartzites after sedimentary precursors are common among them. These may conform large quartzite units with several km\u003csup\u003e2\u003c/sup\u003e outcrops (fine examples exist to the N of the Olduvai Gorge Region) that co-exist with (1) quartz veins of hydrothermal origin formed after dissolution/precipitation processes during diagenesis/burial and low-grade regional metamorphism, (2) syn-tectonic metamorphic quartz mobilisates forming veins usually parallel to the host rock principal foliation during low- to high-grade metamorphism, (3) pegmatitic, hydrothermal veins or stockworks and irregular masses associated with larger plutonic rock intrusions, and (4) igneous quartz-rich rocks.\u003c/p\u003e"},{"header":"Materials And Methods","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eMaterials\u003c/h2\u003e \u003cp\u003eDiverse quartz-rich lithologies, reminiscent of the raw materials used by Olduvai hominins to produce artifacts, have been collected in outcrops of Precambrian rocks (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) located within a radius of about 40 km around Olduvai Gorge. The study of the variability of the samples collected, in principle, would permit us to: (1) identify the basic types of quartz-rich rocks that crop out on the surface of this territory, (2) resolve their key mineralogical composition, microstructure and fabric with a two-fold geological and material characterization application and (3) tag the correct rock name to the particular crystalline quartz-rich raw materials (CQRM).\u003c/p\u003e \u003cp\u003eThe best and most extensive Precambrian rock outcrops occur to the north of the Olduvai Gorge (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The only previous geological survey existing so far (Pickering \u003cspan citationid=\"CR158\" class=\"CitationRef\"\u003e1958\u003c/span\u003e) categorized the outcropping rocks into two groups: the Oldoinyo Ogol and the Serengeti Groups. The lithostratigraphic and structural relationships within each group remain obscure and unknown. The layered nature of most units may suggest that those groups are dominated by thick (hm to km) successions of metamorphic rocks derived from terrigenous protoliths (sandstones, siltstones and mudstones), currently with gentle to moderate dips and an apparent structural simplicity. However, as Shackleton's observations in equivalent neighboring areas demonstrate: (1) the outcrops exhibit a profusion of tight to isoclinal folds, implying conspicuous and intricate succession reversals, (2) the deformation is intense, as proven by the development of a well-defined foliation parallel to the compositional banding, a pervasive lineation on the foliation surfaces and the presence of elongated minerals and mineral aggregates, and (3) shear zones occur parallel or at low angle to the foliation, possibly implying tectonic succession repetitions and/or discontinuities (Shackleton \u003cspan citationid=\"CR189\" class=\"CitationRef\"\u003e1993\u003c/span\u003e). North of Olduvai Gorge these structures can be remotely perceived in aerial views (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), strongly suggesting that the outcrop observations might be extrapolated with similar characteristics at much larger scales.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor the purposes of the current study, the Oldoinyo Ogol Group contains two formations of interest: the Kissele and the Loipukoi Quartzites. The \"Kissele Quartzite\" was originally described by Pickering (\u003cspan citationid=\"CR158\" class=\"CitationRef\"\u003e1958\u003c/span\u003e) in his brief geological map explanation as: \u0026ldquo;\u003cem\u003every coarse-grained, crumbly, red quartzite which generally overlies white or colourless, coarse-grained quartzites\u003c/em\u003e\u0026rdquo; (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). Our field observations confirm that reddish quartzites are the dominant lithology (\u003cem\u003e\u0026ldquo;host rocks\u0026rdquo;\u003c/em\u003e) in the Kissele Quartzite outcrops (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). They appear to be extremely brittle and, from our viewpoint, unsuitable for knapping. The \u0026ldquo;white or colorless quartzites\u0026rdquo; (\u003cem\u003esensu\u003c/em\u003e Pickering \u003cspan citationid=\"CR158\" class=\"CitationRef\"\u003e1958\u003c/span\u003e), in turn, resemble the milky quartz veins (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec) so common in low- to medium-grade metamorphic areas worldwide (e.g, Bons \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). In the study area, the white/colorless quartz rocks would correspond to hydrothermal quartz veins and metamorphic differentiates later reworked and metamorphosed under high-grade conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). They occur not only in primary outcrops but also as loose fragments forming talus deposits on the hillslopes. These cobbles and boulders (\u003cem\u003esensu\u003c/em\u003e Krumbein and Sloss \u003cspan citationid=\"CR120\" class=\"CitationRef\"\u003e1951\u003c/span\u003e) display sharp edges and usually flat surfaces. Their size, tabular morphology and availability might have been appropriate for knapping by hominins in the area (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). This type of quartz-rich rock is common in bedrock outcrops in the region such as the Naibor Soit inselberg, so often cited in archaeological literature (e.g, Egeland et al. 2020; Santonja et al. \u003cspan citationid=\"CR183\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), and indeed is the most conspicuous artifact raw material at archaeological sites in Olduvai Gorge. In this study white/colorless quartz samples were collected from outcrops at Kissele, Naibor Soit and Lekongi (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSamples collected from Precambrian reliefs and Olduvai Gorge\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eN\u0026ordm;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eName\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLocality\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eDescription\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKLG.Gns\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKelogi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eHornblendic Gneiss\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLPK.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLoipukoi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCoarse Smoky Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNBS.Afb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNaibor Soit\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAnfibolite\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLMT.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLemuta\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eKyanite Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKSL.Afb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKissele\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAnfibolite\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLMT.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLemuta\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFoliated Grey Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNBS.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNaibor Soit\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCoarse Polycristalline quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKSL.Sch\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKissele\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTremolitic Schist\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLMT.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLemuta\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRecrystallized Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLKG.Qst\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLekongi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMicaceous Quartz-Schist\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKSL.Afb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKissele\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAnfibolite\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKSL.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKissele\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCoarse Polycristalline Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKSL.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKissele\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFine-Grained Polycrystalline Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKSL.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKissele\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRecrystallized Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLPK.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLoipukoi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCoarse Smoky Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKSL.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKissele\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCoarse Polystalline Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKSL.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKissele\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRecrystallized Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNBS.Qte\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNaibor Soit\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eQuartzite\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eODV.Cht\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOlduvai Gorge\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eChert\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLMT.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLemuta\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eQuartz Vein\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLMT.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLemuta\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRecrystallized Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNBS.Qtz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNaibor Soit\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCoarse Polycristalline Quartz\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eODV.Cht\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOlduvai Gorge\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eChert\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe second formation of interest in the Oldoinyo Ogol Group, the \"Loipukoi Quartzite\", was mapped by Pickering (\u003cspan citationid=\"CR158\" class=\"CitationRef\"\u003e1958\u003c/span\u003e) to the NW of the Kissele Quartzite. The closest outcrops to Olduvai Gorge are located ca. 35 km away and form various inselbergs with individual dimensions between 500 to 2,000 m in length aligned for a distance of ca. 25 km. These outcrops are formed by a grey (smoky)-colored polycrystalline quartz, whose color is due to the presence of large amounts of minute ferromagnesian mineral grains and microinclusions in quartz (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). This rock exhibits a penetrative metamorphic foliation/lineation and is mechanically compact. Its microstructural homogeneity confers excellent quality for knapping. Additionally, the relative proximity of these outcrops to Olduvai Gorge makes this rock a likely candidate, although in a minority way, as a raw material in the lithic industry. In this study grey quartz samples were collected from the Loipukoi outcrop (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe Serengeti Group is poorly represented in the area of study. Pickering (\u003cspan citationid=\"CR158\" class=\"CitationRef\"\u003e1958\u003c/span\u003e) mapped a 6 km long inselberg (and three additional ones with lengths below 500 m) made of rocks ascribed to this group. The \u0026ldquo;Lemuta Quartzite\u0026rdquo; formation of the Serengeti Group is located to the west of those of the Oldoinyo Ogol Group, the closest of them being 20 km away from Olduvai Gorge. The \u0026ldquo;Lemuta Quartzite\u0026rdquo; consists of white, fully recrystallized, and foliated/lineated quartz-rich rocks, intercalated with quartz-schists (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg). Some quartz-rich rock beds contain stunning, mm-cm-sized kyanite crystals elongated parallel to the foliation/lineation and occasionally broken and stretched (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh). Occurrence of kyanite in the rock mineral assemblage is petrologically outstanding and points to a medium- to high-grade metamorphic overprint that may help to constrain the formation/reworking conditions of other rocks in the area (metamorphosed quartz-arenites or metamorphic differentiates). However, these rocks are not as mechanically strong as those previously described. In fact, their knapping qualities are very poor and their alteration produces weak granular products not suitable for use. This likely explains from the archaeological perspective why those rocks are not found among the Olduvai Gorge hominin artifacts. Bearing in mind all the above, we sampled this formation in the Lemuta outcrop (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eAnalytical Methods\u003c/h3\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003ePetrographic and geochemical analysis\u003c/h2\u003e \u003cp\u003eStandard 30 \u0026micro;m thick polished rock sections were used for conventional petrographic and microstructural studies. The sections were cut parallel to the XZ or XY structural planes (XY defined by the orientation of the foliation plane, X defined by the orientation of the mineral and stretching lineation).\u003c/p\u003e \u003cp\u003eMineral analyses were performed in the Scientific-Technical Services microprobe unit at the University of Oviedo (Spain) with Cameca SX-50 and SX-100 automatic microprobes, the latter equipped with five wavelength dispersive spectrometers, a dispersive energy spectrometer, and with secondary electron, back-scattered electron and cathodoluminiscence detectors. The operating parameters included a 10 s integration time, a 10 nA beam current, and a 15 kV accelerating voltage. Mineral structural formulae were calculated by charge balance criteria following various procedures suggested in the bibliography for different phases (see Droop \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e1987\u003c/span\u003e and Spear \u003cspan citationid=\"CR194\" class=\"CitationRef\"\u003e1993\u003c/span\u003e, for further details).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eElectron Back-Scattered Diffraction\u003c/h2\u003e \u003cp\u003eThe Electron Back-Scattered Diffraction (EBSD) study was performed on selected thin sections cut as indicated above. These were ultra-polished with a colloidal silica suspension to remove surface damage and then carbon coated to prevent charging. A copper tape was attached surrounding the measurement area to reduce charging effects. Crystallographic preferred orientation measurements were performed at the University of the Basque Country (Electron Microscopy Facility-SGIker) with an automated Electron Back-Scattered Diffraction system attached to a JEOL JSM-7000F Field Emission Scanning Electronic Microscope (FE-SEM). Samples were mounted in this device on a stage tilted 70\u0026ordm;, with the rock lineation parallel to the SEM X-axis. The beam working distance was 20 mm (Prior et al. \u003cspan citationid=\"CR163\" class=\"CitationRef\"\u003e1999\u003c/span\u003e) and the detector was placed at 188 mm. An acceleration voltage of 20 kV and a beam current of ca. 3.5 nA were applied. Crystallographic orientations were obtained using the Channel5 software package after automated EBSD analysis on a predefined sampling grid with a step of 20\u0026ndash;30 \u0026micro;m, covering up to 80% of the thin sections. These steps are significantly smaller than the average grain size of the minerals. The \u003cem\u003e\u0026ldquo;raw\u0026rdquo;\u003c/em\u003e indexation percentage ranged between 89\u0026ndash;97%. The obtained data were processed with MTex and Matlab (Bachmann et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2011\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Crystallographic orientation solutions with mean angular deviation (MAD) values between detected and simulated patterns over 1.2\u0026ordm; were rejected to assure EBSD measurement reliability. The data were filtered so that the orientation diagrams contain one orientation per grain. The grain detection technique considered a critical misorientation threshold of 10\u0026ordm;. To avoid errors in the grain detection related to the eventual presence of non-indexed pixels and the large grain size of the analysed minerals only those grains covering more than 5 pixels were considered. Fabric orientation distributions are presented in lower hemisphere, equal area stereographic diagrams. The projection plane always corresponds to structural XZ sections and the macroscopic foliation is represented there as the equatorial diameter (E-W). The lineation is horizontal within the same plane.\u003c/p\u003e \u003cp\u003eThe modal proportions were estimated on the basis of the fraction of grains indexed during the EBSD measurement. The strength of the fabric was expressed by the J texture index (Bunge \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1982\u003c/span\u003e), representing the mean square value of the orientation distribution function (ODF). The calculations were performed with the MTex texture analysis software (Bachmann et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Mainprice et al. \u003cspan citationid=\"CR138\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003ePetrography\u003c/h2\u003e \u003cp\u003eAmong the twenty-three rock specimens sampled in the Olduvai Gorge region (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), eighteen correspond to quartz-rich raw material from the Loipukoi, Lemuta, Kissele and Naibor Soit areas (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Their source rocks are much stronger and more resistant to weathering and erosion than the surrounding/interbedded rock formations and, thus, define positive reliefs above the average altitude of the area. Those rocks always exhibit an outstanding planolinear fabric with a distinct foliation containing a clear stretching/mineral lineation, though sometimes seemingly massive (only at the outcrop scale, not at the microscope scale) cm- to m-thick layers, and lenses parallel to the country rock foliation also occur (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). The latter usually correspond to white/colorless quartz. They are mechanically hard and their erosional dismantling produces variable amounts of loose rock fragments (as described in a preceding section; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eThe white/colorless quartz rocks (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, b) contain a mineral assemblage in equilibrium composed of quartz (up to 98%) and muscovite (1.5%), accompanied by accessory phases (\u0026lt;\u0026thinsp;1%) such as gypsum, ilmenite and other opaque minerals, tourmaline, rutile, kyanite and zircon. Quartz grains appear as elongate large crystals with the largest dimension up to 1.5 cm. Elongate quartz grains exhibit a well-defined shape-preferred orientation that defines the rock macroscopic foliation and lineation (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). Quartz grain boundaries are irregular and exhibit abundant microstructures (e.g, bulging, convex boundary segments among crystal defect-free and strained grains, complete inclusion of other minerals of moderate size) that denote active migration during recrystallization under temperature conditions high enough to permit activation of quartz deformation mechanisms dominated by grain boundary mobility (Law \u003cspan citationid=\"CR122\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The concomitant development of quartz subgrains and new grains is remarkable, as their boundaries show diagnostic geometrical arrangements with two sets of grain-boundary segments at a high angle to each other, consistently oblique to the rock foliation. This diagnostic microstructure (\"chessboard microstructure\"; Gapais and Barbarin \u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed) denotes quartz plastic deformation under high-temperature (\u0026gt;\u0026thinsp;600 \u0026ordm;C), hydrous conditions and high strain rates (Bouchez et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1984\u003c/span\u003e \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1985\u003c/span\u003e; Kruhl \u003cspan citationid=\"CR119\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Mainprice et al. \u003cspan citationid=\"CR137\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Okudaira et al. \u003cspan citationid=\"CR150\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Passchier and Trow 1996).\u003c/p\u003e \u003cp\u003eTwo types of white mica inclusions in quartz can be distinguished in the white/colorless quartz. The first (1) consists of small flakes with average 400 \u0026micro;m length and a few tens of \u0026micro;m width. These can be either completely included within quartz (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec) or pinned at their grain boundaries (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed), which acquire higher curvatures in their proximity. In all cases they share a unique parallel orientation identical to the elongation direction of quartz grains and the macroscopic foliation. This microstructure attests to pervasive secondary recrystallization under a strain field by means of active grain boundary migration mechanisms. The second type of white mica inclusions (2) are minute crystals (up to a few tens of \u0026micro;m long and some \u0026micro;m thick) that can be appreciated in polarized light microscopic observations under crossed nicols, only if the host quartz crystal is taken to a position of optical extinction. These flakes occur in quartz grains showing subgrains and undulose extinction, that is, in plastically deformed quartz grains not obliterated by complete recrystallization. There, the micro-inclusions appear as bright flakes (upper red circle in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed) that usually exhibit identical orientations parallel to certain quartz host crystallographic directions (usually two or three; cf. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee) that may coincide with fine orientation bands along quartz rhombohedral planes {r} and {z} (Derez et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). These microstructures are diagnostic features that indicate a hydrothermal vein quartz origin (G\u0026ouml;etze 2012). During such quartz vein development, the mica flakes would have been attached to quartz crystal faces during free growth.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe study of fluid inclusions in quartz can cast light upon the origin of the rocks under study. Thus, although fluid inclusions can be found enclosed in quartz grains, they are not as ubiquitous as in the case of hydrothermal quartz (G\u0026ouml;etze 2012; Johnston and Butler \u003cspan citationid=\"CR105\" class=\"CitationRef\"\u003e1946\u003c/span\u003e). In fact, they are rare in the interiors of recrystallized new grains. In turn, the subgrain and grain boundaries may contain some fluid inclusions, but they are also scarce and confer a very clean aspect. In our samples, grains with relic undulose extinction show a few intragranular healed microcracks, recognizable as fluid inclusion planes (Van Den Kerkhof and Hein \u003cspan citationid=\"CR213\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef-h). These can also be considered microstructural relics, with a twofold implication. On the one hand, their presence demonstrates a former brittle response of stressed quartz rocks under low to moderate temperatures followed by an incomplete recrystallization under higher temperatures. On the other hand, they exemplify recrystallization and grain boundary mobility/reorganization processes that result in a concomitant reorganization of the quartz solid and fluid inclusions, leaving clean grain interiors (Schmatz and Urai \u003cspan citationid=\"CR187\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Vityk and Bodnar \u003cspan citationid=\"CR215\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). These microstructures also support that hydrothermal quartz protoliths, rich in aqueous fluids, facilitated the operation of ductile deformation mechanisms under moderate to high temperatures (Palazzin et al. \u003cspan citationid=\"CR152\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; St\u0026uuml;nitz et al. \u003cspan citationid=\"CR203\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) that actually resulted in the outstanding planolinear macroscopic fabric and microstructure of this white/colorless quartz.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eQuartz Petrofabric\u003c/h3\u003e\n\u003cp\u003eLattice Preferred Orientation (LPO) patterns of rock-forming minerals are designed in Geology to illustrate the orientation of certain significant crystallographic elements. Usually, these patterns are represented in stereographic diagrams using the macroscopic foliation and lineation recognized in the rocks (resulting from strain-induced microstructural reorganization) as an external referential. The presence of a preferred orientation can be unravelled in appropriate fabric diagrams (e.g, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e) by clusters or concentrations of crystal axes and/or planes around specific orientations. The \u0026ldquo;J texture index\u0026rdquo; quantifies the intensity and strength of such a preferred crystallographic orientation. In the case of quartz, LPO fabric diagrams are used to identify intracrystalline slip systems consisting of a slip plane and a slip direction. As a rule, mineral crystallographic planes with orientations approaching that of the macroscopic foliation are most likely to operate as slip planes. Moreover, mineral crystallographic directions close to the orientation of the macroscopic lineation are good candidates to depict the intracrystalline slip direction. The recognition of an LPO defined by the existence of a preferred orientation for certain crystallographic slip planes and axes can shed light on several deformation characteristics and thus provide valuable information on the active deformation mechanisms (brittle/plastic), deformation regime (coaxial/rotational), and thermobaric conditions (especially under lower or higher temperature (Law \u003cspan citationid=\"CR122\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Passchier and Trow 1996).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe samples studied here are all characterized by high J texture index values (up to 4.6). This points to the existence of a fair quartz texture, that is, the presence of a remarkable quartz preferred crystallographic orientation that might support a sound petrofabric interpretation. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e portrays the most common LPO patterns determined for quartz c-axes and \u0026lt;\u0026thinsp;a\u0026gt;-axes in a white/colorless quartz, taken as a representative example. A distinct preferred orientation maximum of c-axes is present at a position forming an angle of 17\u0026ndash;20\u0026ordm; with the lineation orientation (X structural direction). Simultaneously, the \u0026lt;\u0026thinsp;a\u0026gt;-axes scatter along a wide girdle perpendicular to the X direction. This LPO pattern denotes operation of the prism-[c] slip system in quartz by intracrystalline slip on prismatic planes (that contain the c-axis) along the c-axis direction (e.g, Blumenfeld et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Kruhl \u003cspan citationid=\"CR119\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Lister and Dornsiepen \u003cspan citationid=\"CR133\" class=\"CitationRef\"\u003e1982\u003c/span\u003e, Schmid and Casey \u003cspan citationid=\"CR188\" class=\"CitationRef\"\u003e1986\u003c/span\u003e). Here and in most previous studies worldwide, it has been identified in deformed quartz mineral aggregates that additionally exhibit chessboard microstructures. The association of these fabric and microstructural features supports the tectonic interpretation that the hosting quartz underwent high-temperature deformation under temperature conditions above 600\u0026ndash;650 \u0026ordm;C (Blumenfeld et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Mainprice et al. \u003cspan citationid=\"CR137\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Passchier and Trouw \u003cspan citationid=\"CR155\" class=\"CitationRef\"\u003e1996\u003c/span\u003e), likely in the range 700\u0026ndash;800 \u0026ordm;C (granulite facies of regional metamorphism) if the stable mineral assemblages present in related rocks is considered (e.g, Barth et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Fernandez et al. \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Mainprice et al. \u003cspan citationid=\"CR137\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Okudaira et al. \u003cspan citationid=\"CR150\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). The clear obliquity that exists between the LPO pattern and the external reference framework provided by the foliation and lineation implies that high-temperature deformation included non-coaxial (rotational) deformation components during strain accommodation, which further indicates that rock foliations acted as ductile flow planes, the flow direction being marked by the mineral-stretching lineation.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe major lithic resource used by hominins documented in Olduvai Gorge is a geological material with crystalline appearance, white or colorless, foliated or seemingly massive at the outcrop scale (not at the microscope scale) and extremely rich in quartz. In the course of research conducted in the area, this CQRM (Crystalline Quartz-rich Raw Material) has been referred to as: (1) quartz, (2) quartz/quartzite, (3) quartz and quartzite, (4) quartz or quartzite, (5) quartz (meaning quartzite), and (6) quartzite. Past lithic studies undertaken in Stone Age archaeological sites did not usually take into consideration petrological criteria to characterize and classify the artifact resources. In the case of the Olduvai Gorge archaeo-paleontological complex, sixty-five years went by from its discovery before the artifact raw materials were identified for the first time on scientific grounds as quartzite (Hay \u003cspan citationid=\"CR99\" class=\"CitationRef\"\u003e1976\u003c/span\u003e). A further thirty-five years had to elapse until this term reached a certain consensus among researchers. It is clear that this abstruse and long-lasting process of raw material determination was due to a number of petrographic particularities that characterize the CQRM from Olduvai. Undoubtedly, this CQRM exhibits a macroscopic aspect that does not match the acknowledged characteristics and geological significance of classic \"quartzites\" but rather, as explained above, its appearance is similar to quartz.\u003c/p\u003e \u003cp\u003ePetrological observations made in this study of the Olduvai Gorge archetypal white/colorless quartz have revealed the presence of chessboard recrystallization microstructures fully overprinting rocks with a non-sedimentary primary microstructure. The scarce relics preserved concur with a hydrothermal origin of quartz layers. These include muscovite oriented microinclusions (\u0026micro;m-sized) inside large quartz crystals, the minor presence of euhedral/subhedral crystals of hydrothermal minerals such as tourmaline, gypsum and muscovite forming a paragenetically stable association, and the distribution of clouded domains rich in fluid inclusions in some quartz crystals. It is commonplace during metamorphism and recrystallization of vein quartz that a complete rearrangement (and/or leakage) of the fluid inclusions from grain interiors (clouded areas) and grain boundaries takes place, especially when recrystallized under high temperatures (Wheeler et al. \u003cspan citationid=\"CR217\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). Quartz veins likely formed a regional vein network developed during the prograde stages of regional metamorphism in the Mozambique orogenic belt. Some of the veins are large (several m-thick and hundreds of m long in current outcrops) and can be easily identified in aerial/satellite images of the Precambrian inselbergs currently present to the North of Olduvai along tracts exceeding 20 km (including the proximate Naibor Soit and Kissele areas).\u003c/p\u003e \u003cp\u003eAs commented above, Hay (\u003cspan citationid=\"CR99\" class=\"CitationRef\"\u003e1976\u003c/span\u003e) identified this rock type as quartzite and interpreted as primary sedimentary texture the macroscopic and visible mineral alignments produced by the preferred spatial arrangement of quartz mineral aggregates containing small muscovite crystals and other accessory minerals. From the petrological point of view, however, this premise is untenable because in terrains affected by high-grade metamorphism (like the area to the north of Olduvai), rock primary structures were erased due to the strong reworking associated with elevated pressures and temperatures prevalent during metamorphism.\u003c/p\u003e \u003cp\u003eR. L. Hay relied, notwithstanding, on previous geological surveys accomplished by experienced hard-rock geologists such as the 1/125.000 geological map, sheet 37 \"Moru\" completed by Pickering (\u003cspan citationid=\"CR159\" class=\"CitationRef\"\u003e1960\u003c/span\u003e). In this study he reported: \u003cem\u003e\u0026ldquo;Unmetamorphosed granites of the shield are unconformably overlain by relatively unmetamorphosed quartzites, sandstones, and shales of the late Precambrian Bukoban System only about 25 km west of Lake Ndutu (\u003c/em\u003ePickering, \u003cspan citationid=\"CR159\" class=\"CitationRef\"\u003e1960\u003c/span\u003e\u003cem\u003e)\u0026rdquo;\u003c/em\u003e (Hay \u003cspan citationid=\"CR99\" class=\"CitationRef\"\u003e1976\u003c/span\u003e, p. 11). However, in fact, the closest outcrops are really more than 50 km away from the Olduvai archaeological sites. That is, they are located more than 25 km in a straight line towards the west of Lake Ndutu, in the headwaters of the gorge, which in turn is located another 25 km to the west of the archaeological sites. Those slightly metamorphic quartzites of the Bukoban System (actually the Bukoban Supergroup) that caught Hay\u0026rsquo;s attention are currently known as the \"Kinenge quartzites\". They are part of the Ikorongo group and lie unconformably on top of the Archaean craton rocks (Pickering \u003cspan citationid=\"CR159\" class=\"CitationRef\"\u003e1960\u003c/span\u003e), in a tectonic domain different from the one in which Olduvai is located. These materials were the subject of later studies (Kasanzu \u003cspan citationid=\"CR110\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Kasanzu and Manya \u003cspan citationid=\"CR112\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Kasanzu et al. \u003cspan citationid=\"CR111\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) where they were identified correctly as sandstones forming part of sedimentary rock successions containing also quartzites, shales, dolomitic limestones and basalts, and regarded as quartz-arenites by Kasanzu and Manya (\u003cspan citationid=\"CR112\" class=\"CitationRef\"\u003e2010\u003c/span\u003e, p. 365).\u003c/p\u003e \u003cp\u003eThe white/colorless quartz hosted in the Kissele quartzite formation of the Oldoinyo Ogol Group (Pickering \u003cspan citationid=\"CR158\" class=\"CitationRef\"\u003e1958\u003c/span\u003e) and similar rocks cropping out nearby in the Olduvai area are the best fitting candidates as mineral resources for the hominin artifacts. This CQRM was the result of the high-grade metamorphic overprint of protoliths that actually lack any primary sandstone texture vestiges but, in turn, exhibit fabric, microstructural and field characteristics (at outcrop and map scales) indicative of hydrothermal quartz veins. Thus, they should not be classified as \"quartzites\" in a normative petrological/geological sense. However, the reddish quartz-rocks hosting the aforementioned CQRM can be classified as amphibolite- to granulite-facies metamorphic quartzites, identifiable attending to their color, mineral assemblage and microstructure. Both geological materials (white/colorless quartz representing hydrothermal veins and reddish quartz-rock originally comprising part of a sedimentary succession where the former were emplaced), although different in their genesis, they have shared a common posterior metamorphic/deformational overprint and, currently, depict comparable quartz petrofabric LPO patterns.\u003c/p\u003e \u003cp\u003eAfter Hay\u0026rsquo;s work, reluctance to use the term quartzite by most authors led to prevalence of the tandem terminology for decades, using quartz and quartzite, as aforementioned, in varied forms that paid little or no attention to the geological origin of artifact raw materials. Kyara\u0026rsquo;s (\u003cspan citationid=\"CR121\" class=\"CitationRef\"\u003e1999\u003c/span\u003e) attempt to overcome this imprecision remarked that: \u0026ldquo;\u0026hellip;\u003cem\u003erock types are not still strictly categorized. At that time the names of the artifacts made on metamorphic siliceous rocks are interchangeably referred to as \u0026laquo;quartz and quartzite\u003c/em\u003e\u0026raquo; \u003cem\u003eor \u0026laquo;quartz/quartzite\u003c/em\u003e\u0026raquo;\u0026rdquo;. (Kyara \u003cspan citationid=\"CR121\" class=\"CitationRef\"\u003e1999\u003c/span\u003e, p. 115). This intended to find a consensual term binding together all the metamorphic raw materials identified at Olduvai: \u003cem\u003e\u0026ldquo;For ease of data analysis, raw material types were grouped under three main working categories based on the three major rock types, namely; (i) Quartzites (inclusive of all metamorphic rocks: quartz, quartzite, purple quartzite, and gneiss); (ii) volcanics, in concurrence with\u003c/em\u003e Blumenschine and Masao (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1991\u003c/span\u003e\u003cem\u003e) instead of igneous rocks, which incorporate green phonolite, porphyritic phonolite, basalt, and trachyandesite, and, (iii) chert, which is the sole representative of sedimentary rocks in the region\u0026rdquo;\u003c/em\u003e (Kyara \u003cspan citationid=\"CR121\" class=\"CitationRef\"\u003e1999\u003c/span\u003e, p. 116). However, this categorization was not grounded on any geological criterion and gave priority to terminological simplification without considering petrologic aspects that might have helped. The lack of robust geological identification criteria was probably the cause of this continuity in long term terminological imprecision.\u003c/p\u003e \u003cp\u003eThe incorporation of new research teams working in Olduvai in the second decade of the 21st century has recently opened up a period in which the use of the term \u0026ldquo;quartz\u0026rdquo; lapsed into disuse without any petrological justification. In return, the term \u0026ldquo;quartzite\u0026rdquo; seems to acquire currently a widespread use (i.e. D\u0026iacute;ez-Mart\u0026iacute;n et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; S\u0026aacute;nchez-Yustos et al. \u003cspan citationid=\"CR176\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; de la Torre et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). At present, Egeland and colleagues seem to be the sole authors adhered to the term \u0026ldquo;quartz\u0026rdquo;, based on the solid and precise petrologic argument that: \u003cem\u003e\u0026ldquo;geological work on the Tanzanian Craton reveals that these inselbergs are too metamorphically evolved to be quartzite and, thus, are probably best characterized as resistant, quartz-rich remnants of heavily weathered granulites (\u003c/em\u003eDawson, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Begg et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2009\u003c/span\u003e), \u003cem\u003ewhich is the label we adopt here. Nevertheless, we think it is reasonable for Olduvai lithic artifacts presumably harvested from these outcrops and flaked largely or exclusively from their quartz constituents to be referred to as \u0026laquo;quartz\u0026raquo; artifacts.\u0026rdquo;\u003c/em\u003e (Egeland et al. 2020, p. 101).\u003c/p\u003e \u003cp\u003eIn the framework of this new consensual and imposition period Santonja et al. (\u003cspan citationid=\"CR183\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), aware of the quandary around this CQRM, specifically defined these materials by the local name of \u003cem\u003e\u0026ldquo;Naibor quartzite\u0026rdquo;\u003c/em\u003e, stressing the exceptional particularism of this white/colorless quartz raw material. The \"Naibor quartzite\" is described by them as: \u003cem\u003e\u0026ldquo;...a metamorphic rock whose almost exclusive constituent mineral is quartz (Table\u0026nbsp;3) with the shape of phenocrysts and a distinctive lamination that responds to knapping very differently from the other fine-grained quartzites present in Olduvai (\u003c/em\u003eJones, \u003cspan citationid=\"CR109\" class=\"CitationRef\"\u003e1994\u003c/span\u003e\u003cem\u003e), and more similarly to quartz (\u003c/em\u003eMourre, \u003cspan citationid=\"CR144\" class=\"CitationRef\"\u003e1997\u003c/span\u003e\u003cem\u003e)\u0026rdquo;\u003c/em\u003e (Santonja et al. \u003cspan citationid=\"CR183\" class=\"CitationRef\"\u003e2014\u003c/span\u003e, p. 186). Although, the authors identify a metamorphosed rock SiO\u003csub\u003e2\u003c/sub\u003e-rich composition (97.97%) and differentiate a \u0026ldquo;distinctive lamination\u0026rdquo;, this does not imply that the raw material should be classified as quartzite. Furthermore, this \u003cem\u003e\u0026ldquo;distinctive lamination\u0026rdquo;\u003c/em\u003e the authors mention, as said before, is in fact a tectonic/metamorphic foliation rather than a sedimentary bedding/lamination (Boggs \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). Additionally, the term phenocryst is not used properly. This term is used in igneous petrology to identify isolated larger crystals surrounded by a finer grained groundmass of either identical or different mineral composition, which does not concur with the homogeneous grainsize distribution of Olduvai quartz with a granoblastic texture (Licker \u003cspan citationid=\"CR132\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Finally, although they indicate that the CQRM respond to knapping very differently from the other fine-grained quartzites present in Olduvai, these authors seem to feel comfortable with this contradiction. They do not question why such fracture dynamics are at odds with what is expected in quartzites. This contradiction could be properly defined as the \u0026ldquo;quartzite paradox\u0026rdquo;, a characteristic trait of CQRM in Olduvai Gorge.\u003c/p\u003e \u003cp\u003eThe extremely relevant contradiction in breakage response emerging from the Olduvai paradox is actually simple to answer: if this raw material is composed of large quartz monocrystal grains, then it will exactly respond to knapping as quartz, which is an anisotropic material with substantial crystallographic symmetry. No classical quartzite has a grain size\u0026thinsp;\u0026gt;\u0026thinsp;2 mm (if so, it would be quartz-rudite). On the contrary, quartzite is usually a microcrystalline mineral aggregate with conchoidal fracture that behaves in breakage in a different way from quartz, which exhibits in any CQRM homogeneous crystalline domains larger than 2 mm until 10\u0026ndash;20 mm are reached. This oddity was already identified by Kimura (\u003cspan citationid=\"CR115\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) when she pointed out that: \u003cem\u003e\u0026ldquo;quartz fractures unevenly, while the quartzite of Olduvai is extremely coarse-grained and does not form conchoidal fracture\u0026rdquo;\u003c/em\u003e (Kimura \u003cspan citationid=\"CR115\" class=\"CitationRef\"\u003e1997\u003c/span\u003e, p. 33).\u003c/p\u003e \u003cp\u003eMat\u0026eacute;-Gonz\u0026aacute;lez et al. (\u003cspan citationid=\"CR140\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) show that the cut-marks made by CQRM artifacts in Olduvai on fossil bones exhibit similar marks to those produced by quartz. However, their final terminological choice is driven by a rather deterministic rationale when they state that \u0026ldquo;\u0026hellip; \u003cem\u003ein a strict geological definition, these materials are clearly quartzites, and not quartz (\u003c/em\u003eHay, \u003cspan citationid=\"CR99\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Santonja et al. \u003cspan citationid=\"CR183\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), \u003cem\u003eregardless of their quartz-like behaviour\u003c/em\u003e\u0026rdquo; (Mat\u0026eacute;-Gonz\u0026aacute;lez et al. \u003cspan citationid=\"CR140\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, p. 449). Thus, Mat\u0026eacute;-Gonz\u0026aacute;lez and colleagues are arguing that the Olduvai CQRM is a raw material that looks, fractures and interacts with animal tissues like quartz but is \u0026ldquo;clearly quartzite\u0026rdquo; based on the authority judgment pronounced by Hay and more recently followed by Santonja and colleagues. This is circular reasoning leading to a logical fallacy that is not supported by scientific data.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThe major hominin artifact lithic raw material documented in the Olduvai Gorge region is a geological material with crystalline appearance, white or colorless, foliated or seemingly massive only at the outcrop scale, with a very high quartz-rich composition, and apparently bearing a metamorphic origin. In this work we propose the term CQRM (Crystalline Quartz-rich Raw Material) to designate these materials for intended interdisciplinary descriptive purposes and these natural materials with these petrological features support its identification as \u003cem\u003e\u0026ldquo;Quartz\u0026rdquo;\u003c/em\u003e.\u003c/p\u003e \u003cp\u003eStubbornness in terminological confusion reflects the complexity and specificity of CQRM at the Olduvai Gorge sites. Such uncertainties relied on the following issues, related to a protracted and complex geological history. Firstly, those materials crop out widely in nearby inselbergs (in some cases, several km\u003csup\u003e2\u003c/sup\u003e in area) composed by quartz-bearing metamorphic rocks of varied types and origins. Second, these metamorphic rocks proceed in part from sedimentary recycling and/or tectonic reworking of much older orogens and cratons (Mesoproterozoic, Paleoproterozoic and even Archean, \u0026gt;\u0026thinsp;2.5 Ga). Third, the metamorphic rocks were transformed in the solid state by intense ductile deformation under high-grade pressure and temperature (granulitic facies), reaching temperatures in excess of 750\u0026ndash;800\u0026ordm;C during the Neoproterozoic Era, 1.0\u0026ndash;0.5 Ga.\u003c/p\u003e \u003cp\u003eIn this study we show that the terminological inaccuracy/ambiguity related to the CQRM conundrum and the related paradox originated in a complex geologic/petrologic context that had not been addressed so far from a multidisciplinary geological perspective. These materials are usually recognized as a white/colorless quartz-rich material (in a purely descriptive sense) and because they occur in areas dominated by outcrops of quartzites (\u003cem\u003esensu\u003c/em\u003e Pickering, with somewhat different mineral content, microstructure and macroscopic aspect than classic quartzites), they have been classified mainly as quartzites by researchers who are not specialists in hard rock petrology, instead of having been classified as quartz. Therefore, in order to solve these problems, the concurrence of structural/metamorphic petrologists is essential to describe these materials, identify their origin and eventually discuss the implications of their presence in the regional geological context.\u003c/p\u003e \u003cp\u003eCQRM formed as a nearly pure-quartz mineral found in hydrothermal veins and dykes, as well as in quartz metamorphic differentiates that after a strong tectonothermal overprint acquired an outstanding planolinear penetrative fabric (that, with untrained eyes, might be mistaken for sedimentary bedding). This raw material was formed as vein mineralizations made up exclusively of quartz, with the particularity that they were hosted in quartzitic country rocks also made up exclusively of quartz. Logically, they share a similar chemical and mineralogical composition. Their partial dissolution by metamorphic fluids provided the source chemical components of secondary quartz veins. After penetrative ductile deformation, both the host rocks and the hosted veins acquired a penetrative fabric (with convergent microstructure and petrofabric), but relics of their original texture can still be differentiated. The mineralogical similarity between these quartz-rich geological products (host rocks and hosted minerals) largely explains the persistence and even the imposition of the name of quartzite for these CQRM by researchers inexperienced in the study of raw materials.\u003c/p\u003e \u003cp\u003eThe most diagnostic features supporting this interpretation can be summarized as follows. First, some of the microstructural relics identified in the CQRM concur undoubtedly with a hydrothermal origin. Among them it is worth noting: (1) presence of oriented muscovite microinclusions (\u0026micro;m-sized) inside large quartz crystals, (2) occurrence of minor euhedral/subhedral crystals of hydrothermal minerals such as tourmaline, anhydrite (currently stabilized to gypsum) and muscovite forming a paragenetically stable association, and (3) existence of a distribution of clouded domains rich in fluid inclusions in some quartz crystals. Second, the recognition of special deformational structures/microstructures (elongate quartz grains defining the macroscopic foliation and lineation, irregular quartz grain boundaries with concave geometries, small oriented muscovite flakes pinned to quartz boundaries, chessboard recrystallization microstructures fully overprinting rocks with a non-sedimentary primary microstructure\u0026hellip;) and intense quartz LPOs (characterized by concentrations of \u0026lt;\u0026thinsp;c\u0026gt;-axes around the lineation orientation) point to operation of tectonometamorphic processes under granulite-facies conditions.\u003c/p\u003e \u003cp\u003eUndoubtedly, the precise characterization of the CQRM will allow us to speak with propriety about these raw materials used in the Olduvai Gorge sites and it can also help cast light on the supply and management strategies for these exceptional and unique mineral resources used by the hominin species who inhabited the Olduvai region during the Stone Age.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments \u003c/strong\u003eWe wish to thank the Tanzanian Commission for Science and Technology (COSTECH), the Division of Antiquities and NCAA (Ngorongoro Conservation Area Authority), within the Ministry of Natural Resources and Tourism for permission to conduct research in Olduvai Gorge. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAutor Declarations\u003c/strong\u003e\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eFunding \u003c/strong\u003eThis study was funded by the Spanish Ministry of Science and Innovation through the National Plan I+D+i (projects PID2020-118359GB-I00 and HAR2017-82463-C4-4-P) and the UPV/EHU GIU20/010 research grant.\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eConflicts of interest\u003c/strong\u003e The authors declare no competing interests.\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eEthics approval\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eConsent to participate\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eData and material availability\u003c/strong\u003e All relevant data are within the manuscript. All geological samples are stored in the Lithoteque of the Department of Geography, Prehistory and Archaeology of the University of the Basque Country.\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eCode availability\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eAutors\u0026rsquo; contributions\u003c/strong\u003e All authors contributed to this work. AT and FD conceptualized the work. AT, LE and FD did fieldwork and collect samples, AT and FD wrote the archaeological contextualization. BA, PP, LE and AT wrote the geological contextualization and made the figures. BA, PP, AT and LE carried out petrological analysis. Founding acquisition by FD, AT and BA. FD, AM, EB and MD and EB are responsible for project management. All authors read, reviewed and approved the final manuscript.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAbtosway M (2018) Geochemical Source Analysis: Preliminaries and Potential at Olduvai Gorge, Tanzania (Unpublished master\u0026apos;s thesis). University of Calgary. https://doi.org/10.11575/PRISM/32301\u003c/li\u003e\n\u003cli\u003eArroyo A, de la Torre I (2016) Assessing the function of pounding tools in the Early Stone Age: A microscopic approach to the analysis of percussive artefacts from Beds I and II, Olduvai Gorge (Tanzania). Journal of Archaeological Science 74:23-34. https://doi.org/10.1016/j.jas.2016.08.003\u003c/li\u003e\n\u003cli\u003eArroyo A, de la Torre I (2018) Pounding tools in HWK EE and EF-HR (Olduvai Gorge Tanzania): Percussive activities in the Oldowan-Acheulean transition. 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Boreas 46:860-873. https://doi.org/10.1111/bor.12224\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"archaeological-and-anthropological-sciences","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"aasc","sideBox":"Learn more about [Archaeological and Anthropological Sciences](http://link.springer.com/journal/12517)","snPcode":"12520","submissionUrl":"https://submission.nature.com/new-submission/12520/3","title":"Archaeological and Anthropological Sciences","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Quartz, Quartzite, Olduvai, Raw Materials, Mineral Resoures","lastPublishedDoi":"10.21203/rs.3.rs-2347339/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2347339/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe major raw material documented in the archaeological sites of Olduvai Gorge (Tanzania) is a geological material with crystalline appearance, white or colorless, foliated or seemingly massive only at the outcrop scale, with a very high quartz-rich composition, and apparently bearing a metamorphic origin (CQRM). Since the early days of research in Olduvai Gorge, a long-lasting terminological imprecision has allowed defining this material in a confused way as quartz or quartzite. Stubbornness in terminological imprecision reflects the complexity and specificity of CQRM related to a protracted and complex geological history composed by quartz-bearing metamorphic rocks of varied types and origins from recycling and/or tectonic reworking of much older Precambrian orogens and cratons. Currently the term quartzite is preferred by most researchers, despite being materials that have an appearance macro and microscopic similar to quartz and show a response to fracture mechanics and cutting-edge functional response is closer to quartz. In our view it is crucial to undertake a comprehensive analysis of the CQRM from the structural, metamorphic and petrological perspectives. Bearing this in mind, the main objective of the present study is to build a robust and conclusive background that will enable an accurate identification and classification of this quartz-rich mineral resource. This geological material should be identified as \u0026ldquo;Quartz\u0026rdquo; and he most diagnostic features supporting this interpretation can be summarized as: some of the microstructural relics identified concur undoubtedly with a hydrothermal origin, and the recognition of special deformational structures/microstructures point to tectono-metamorphic processes under granulite-facies conditions.\u003c/p\u003e","manuscriptTitle":"The Crystalline Quartz-rich Raw Material from Olduvai Gorge (Tanzania): Why is it called quartzite when it should be called quartz?","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-12-12 15:30:55","doi":"10.21203/rs.3.rs-2347339/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewersInvited","content":"","date":"2022-12-11T18:27:05+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2022-12-09T07:59:37+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2022-12-08T13:11:17+00:00","index":"","fulltext":""},{"type":"submitted","content":"Archaeological and Anthropological Sciences","date":"2022-12-05T19:17:04+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"archaeological-and-anthropological-sciences","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"aasc","sideBox":"Learn more about [Archaeological and Anthropological Sciences](http://link.springer.com/journal/12517)","snPcode":"12520","submissionUrl":"https://submission.nature.com/new-submission/12520/3","title":"Archaeological and Anthropological Sciences","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"be4f15ef-4a07-44ac-ad20-a360749f661a","owner":[],"postedDate":"December 12th, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2023-10-16T20:58:06+00:00","versionOfRecord":{"articleIdentity":"rs-2347339","link":"https://doi.org/10.1007/s12520-023-01774-9","journal":{"identity":"archaeological-and-anthropological-sciences","isVorOnly":false,"title":"Archaeological and Anthropological Sciences"},"publishedOn":"2023-05-11 20:46:35","publishedOnDateReadable":"May 11th, 2023"},"versionCreatedAt":"2022-12-12 15:30:55","video":"","vorDoi":"10.1007/s12520-023-01774-9","vorDoiUrl":"https://doi.org/10.1007/s12520-023-01774-9","workflowStages":[]},"version":"v1","identity":"rs-2347339","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2347339","identity":"rs-2347339","version":["v1"]},"buildId":"ehx78VzkSd0WSzXnipQa-","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}