Hydrogen defects in feldspars: Alkali-supported dehydrogenation of sanidine

Hydrogen defects in feldspars: Alkali-supported dehydrogenation of sanidine | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Hydrogen defects in feldspars: Alkali-supported dehydrogenation of sanidine Harald Behrens This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2572968/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 23 Jun, 2023 Read the published version in Physics and Chemistry of Minerals → Version 1 posted 9 You are reading this latest preprint version Abstract In the first two papers of this series (Behrens 2022a,b), incorporation of hydrogen in the feldspar structure, partitioning of hydrogen between feldspars and gases/fluids and self diffusion of hydrogen in feldspars has been discussed, with particular focus on sanidine. Here, the results of reactions between sanidine containing strongly bonded hydrogen defects and (Na,K)Cl are presented. Experiments were performed at ambient pressure at temperatures of 605–1000°C and hydrogen profiles were measured by IR microspectroscopy. Profiles can be interpreted by an incomplete dehydrogenation at the crystal surface or a strong concentration dependence of hydrogen diffusivity. Both is consistent with hydrogen located on interstitial sites and difficult to substitute by the larger alkali ions. Chemical diffusivities of hydrogen derived from fitting of the profiles or Boltzmann-Matano analysis are similar to self diffusivities determined by D/H exchange experiments. Activation energies are also comparable. Comparison to sodium and potassium diffusion data for sanidine (Wilangowski et al 2015 , Hergemöller et al. 2017 ) support a mechanism of proton diffusion charge-compensated by Na + diffusion for hydrogen removal in the sanidines under dry conditions. igneous feldspar hydrogen defects infrared spectroscopy dehydrogenation diffusion Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Hydrogen is a common impurity in nominally anhydrous minerals such as olivine, pyroxene, feldspar or quartz (e.g., Beran and Libowitzki 2006, Johnson 2006 , Skogby 2006 ). The incorporated hydrogen can have major impact on the properties of the minerals. In the case of feldspars, diffusion of network-forming elements such as silicon, aluminum and oxygen is strongly enhanced by dissolved hydrogen species (e.g., Yund and Anderson 1974 , Giletti et al. 1978 , Grove et al. 1984 , Yund 1986 , Elphick et al. 1988 , Farver and Yund 1990 , Baschek und Johannes 1995 , Cherniak 2010 , Farver 2010 ). The enhancement of intracrystalline diffusion is important not only on the local scale, but affects also long-range processes in rocks, i.e. the tectonic response to accumulating stress induced by plate movement or the mobilization of elements in rocks as a prerequisite for the formation of ore deposits. Thus, there is large interest in better understanding the properties of dissolved hydrogen in feldspars. In feldspars formed or recrystallized under metamorphic conditions, most of the hydrogen is typically present in inclusions. These can be fluid inclusions as well as hydrous glasses or minerals. Regarding the kinetic properties of feldspars, such localized hydrogen pools have little impact. More important are hydrogen species incorporated as point defects in the feldspar structure. Using polarized IR spectroscopy, Johnson and Rossman ( 2003 , 2004 ) identified different types of structural hydrogen defects in feldspar. According to their studies, volcanic feldspars contain only structural OH groups while plutonic and pegmatitic feldspars may host also other hydrogen species, i.e. H 2 O molecules and ammonium cations. A common OH defect in alkali feldspars and plagioclase, denoted as type IIa OH bands by Johnson and Rossman ( 2004 ), is characterized by a broad IR band system centered around 3300 cm − 1 with the maximum intensity closely aligned with the crystallographic a axis. A different IR absorption spectrum is observed for Eifel sanidine with a band at 3050 cm − 1 being most intense in the b direction, while a band near 3400 cm − 1 being most intense in the a direction. Johnson and Rossman ( 2004 ) classified the corresponding defect in Eifel sanidines as type IIb OH . However, results of the experimental studies of Behrens ( 2021a , 2021b ) suggest, that hydrogen incorporation in Eifel sanidines is more complex and at least two different OH defects with different mobilities coexist in the structure. This paper is the third part of a series of studies on hydrogen defects in feldspar. In the first part, the thermal stability of hydrogen defects in feldspars and the partitioning of water between a gas phase and feldspars (plagioclase and K-rich alkali feldspar) was investigated at ambient and elevated pressure (Behrens 2021a ). Diffusion of hydrogen species and hydrogen isotope exchange between coexisting fluid/gas phases was the main topic of the second part (Behrens 2021b ). Indication was found that different transport mechanism for hydrogen are dominating at wet and at dry conditions. At elevated water pressures, water molecules diffusing via interstitial sites act as transport vehicles for hydrogen. Under dry conditions, diffusion of protons resulting from dissociation of extrinsic defects such as AlOH groups is the dominant process. The latter mechanism is in the focus of the third paper of this series. Dehydrogenation experiments were performed with sanidine in contact with alkali chloride. Behrens ( 2021a ) has already demonstrated that the release of hydrogen from natural sanidine is strongly accelerated when alkali ions are available for charge compensation of out-diffusing protons. Here, this process is investigated in detail over a large temperature range. The data are compared to self- diffusion of hydrogen determined by isotope exchange experiments (Behrens 2021a ) and alkali diffusion data (Neusser et al. 2012 ; Schäffer et al. 2014 ; Wilangowski et al. 2015 ; Hergemöller et al. 2017 ) to constrain the mechanisms of dehydrogenation. Samples and experiments Cuboids of sanidine were used to probe hydrogen diffusion in three orthogonal directions. The sanidine SV has gemstone quality and is described in detail in the first paper (Behrens 2021a ). Based on cleavage planes in (010) and (001) crystals were oriented. The pleochroisms of the OH vibration bands at 3400 cm − 1 and 3050 cm − 1 in polarized IR absorption spectra gives further constraints of the crystallographic axis in the cuboid. The crystals were heated for at least 7 days in air at 900°C to remove large part of the mobile type OH IIa defects. Experiments were performed at ambient pressure in a large temperature range from 605 to 1000°C (Table 1 ). At temperatures below 800°C, the samples were embedded in solid alkali chloride. Some of these experiments were performed in a closed glass apparatus, similar to that used for D-H exchange experiments (Behrens 2021b , Fig. 1 ). A dry atmosphere was set by evacuating and filling the apparatus several times with air pre-dried over phosphorus pentoxide. In other experiments, the salt-sanidine assembly was loaded in a Pt-boat and continuously flushed with a pre-dried air stream. Table 1 Conditions and results of dehydrogenation experiments. No T t Section a b c* alkali source, comments (°C) (h) c S /c C log D H c S /c C log D H c S /c C log D H SV-M1 605 1488 (010) 0.713 -14.40 0.512 -14.43 KCl powder, closed apparatus SV-L1 650 504 perp a 0.457 -13.69 KCl powder, closed apparatus SV-H1 720 288 (001) 0.305 -13.30 0.232 -13.26 NaCl powder, flushed by dry air cracks 30 µm in a, 70 µm in b SV-P1 720 150 (010) 0.538 -13.26 0.604 -13.23 KCl powder, closed apparatus SV-K1 750 97 (010) 0.293 -13.00 KCl powder, flushed by dry air SV-I1 783 66 (010) 0.265 -12.84 NaCl powder cracks 25 µm in a, 80 µm in c* SV-Q1 900 24 (010) 0.212 -11.85 K 0.95 Na 0.05 vapor SV-S1 900 4 (010) 0.408 -11.72 K 0.95 Na 0.05 melt 0.467 -11.65 0.452 -11.65 SV-FH 1000 4.17 (010) 0.153 -11.49 0.058 -11.40 NaCl vapor cracks 220 µm in a Notes. a, b, c* refer to the crystallographic orientation of the profiles Diffusion coefficients D H determined assuming constant diffusivity (Eq. ( 1 ). Units are m 2 /s. c S /c C is the hydrogen concentration at the surface compared to the center, based on fitting by Eq. ( 1 ) At temperatures above the melting points of the alkali chlorides, closed gold capsules were used. Either the cuboid was placed directly in the melt, or it was set on a small table above the melt so that the sample is only in contact with the vapor phase. This set-up was shown in Fig. 9 of Behrens ( 2021b ). Alkali chlorides were either pure NaCl or pure KCl or K 0.95 Na 0.05 Cl. NaCl was chosen because of the high mobility of Na + in sanidine (Wilangowski et al. 2015 ) and because the smaller size of Na + compared to K + favors substitution for H + on interstitial sites (Behrens 2021a ). However, a serious disadvantage of using NaCl is that the cation exchange in the sanidine causes severe stress and cracking (Scheidl et al. 2014, Petrishcheva et al. 2019 ). Therefore, K-rich chlorides were used in most of the experiments. At the time the experiments were done, the partition coefficients between alkali chloride and alkali feldspar were not yet known. It was assumed that K 0.95 Na 0.05 Cl is close to the equilibrium composition of a melt coexisting with the sanidine. Partitioning coefficients determined by Neusser et al. (2009) imply that the equilibrium chloride composition for a sanidine with 85 mol% Or component is more sodic. However, as discussed below, this does not affect the results of this paper. Analytics After experiment, the cuboids were cut in two different orientations as shown in Fig. 1 of Behrens ( 2021b ). After grinding and polishing to a thickness of about 500 µm, hydrogen profiles were recorded from the rim to the center using an IR microscope A590 coupled with an FTIR spectrometer Bruker IFS88. A globar light source, a KBr beamsplitter and an MCT (mercury cadmium telluride) detector were applied in these measurements. 50–100 scans were accumulated for each spectrum with a spectral resolution of 2 cm − 1 . The spectrometer provides a partial polarized beam which is very well suited for precise measurements of diffusion profiles. A circular aperture corresponding to a focus diameter of 50 µm was applied, corresponding to an effective diameter of the exited area of 60 µm (Behrens 2021a )). Some profiles were recently re-measured with an IR microscope Bruker IRscope II using a slit aperture to adjust an area of 20 x 100 µm 2 in the focus plane. Aligning the long side parallel to the edge of the section, a spatial resolution of ≈ 40 µm could be achieved. The profiles measured by both setups show no significant difference. Examples of IR absorption spectra of the sanidine in the range of OH stretching vibrations can be found in Behrens ( 2021a , b ). Die absorbance A of the main OH peak was determined after subtraction of a linear baseline. Hydrogen concentration was calculated by the Lambert-Beer law as described in these papers. Results Cuboids were well preserved after experiments with K-rich chlorides while pronounced crack formation in the near-surface region occurred in the runs with NaCl (SV-H1, SV-I1 and SV-FH). Cracks are aligned parallel to each other, but the orientation does not correspond to any of the feldspar cleavages. The lengths of the cracks depend on the orientation of the surface from which crack growth started, and for a given surface the cracks have all similar length (see Table 1 ). These observations agree well with those of Abart and co-workers (Neusser et al. 2021, Scheidl et al. 2014, Petrishcheva et al. 2019 ) who performed similar alkali exchange experiments with Eifel sanidines. In these papers a detailed description of the mechanisms of alkali exchange and crack formation is given. The driving force for crack formation is the coherency stress created by the replacement of the large K + ion by the smaller Na + ions at the diffusion front. Hydrogen profiles obtained from two experiments are presented in Fig. 1 a,b. The complete set of experiments can be found in the electronic supplement. A common feature is that extrapolations of the measured profiles do not pass through the origin. For one experiment using NaCl as the alkali source (SV-H1), a traverse from rim to the center was analyzed by electron microprobe CAMECA Camebax. Measurement conditions were the same as reported in Behrens ( 2021a ). The basic feldspar composition was preserved along the profile, only the Na/K ratio had changed. The alkali exchange profile was much shorter than the hydrogen profile (Fig. 2 ), and the crack tips evolved even beyond the region with measurable variation in alkali abundance. Discussion Due to the limited spatial resolution of the IR beam it is not possible to determine the concentration of hydrogen in the sanidine directly at the contact to the surrounding media. The first measurement point was typically 50 µm away from the surface and represents the average concentration over a range of 50 ± 30 µm. The measured profiles can be explained by a remaining hydrogen content in the surface area of the sanidine as well as by a very strong decrease of the concentration near the surface. Both approaches are discussed in following. Profile evaluation assuming constant diffusivity In the first approach, the absorbance-distance profiles were evaluated by the model of one-dimensional diffusion between an infinite reservoir (the gas phase) and a semi-infinite medium (the crystal) with the boundary condition of a constant surface concentration. Since the absorbance of the OH stretching vibration band is proportional to concentration of hydrogen, the measured absorbance can be directly used for the determination of the diffusion coefficient. Assuming a constant diffusion coefficient D H , the solution of Fick's second law for these boundary conditions is (Crank 1975 ) $$\frac{A-{A}_{center}}{{A}_{surface}-{A}_{center}}=1-e rf\left(\frac{x}{\sqrt{4\bullet {D}_{H}\bullet t}}\right)$$ 1 where t is the run duration, A is the absorbance at the distance x from the surface, A center and A surface are absorbances in the center and at the surface, respectively. The term D H characterizes the chemical diffusion of hydrogen during dehydrogenation. The measured profiles were fitted to Eq. 1 with A surface , A center and D H being adjustable parameter. The fitted curves are shown as solid lines in Fig. 1 a,b and fit results are also included in the electronic supplement. For all profiles, the correlation coefficient r 2 of the fit was > 0.95, in many cases even > 0.99. This could be regarded as a good confirmation of the evaluation approach. A unique feature of all experiments is that the fit curves do not pass through the origin, i.e. some hydrogen remained in the crystal at the contact to alkali chloride. The relative abundance of hydrogen at the surface, expressed by the absorbance ration A surface /A center , varies between 0.06 and 0.71. There is a rough trend of decreasing surface concentration of hydrogen with increasing temperature (Fig. 3 ). However, the scatter of the data is high, and the correlation coefficient is small (r 2 = 0.45). One reason for the variation in A surface /A center is that the water pressure ( p H2O ) in the experiments was low but it was not fixed at constant value. Another reason is some variation in the hydrogen concentration in the center of the sample between 81 and 115 ppm (see supplement). The trend of A surface /A center with temperature is consistent with the structural interpretation of the type IIb OH defects in sanidine proposed in Behrens ( 2021a , b ). Accordingly, this defect consists of a proton on an interstitial site compensating the charge deficiency of a nearby Al 3+ . Replacing the proton by a larger alkali requires energy for local expansion of the feldspar structure and, hence, substitution of hydrogen by alkali is a thermally activated process. Boltzmann-Matano (BM) analysis In the second approach it is assumed that the hydrogen content drops to zero at the surface during dehydrogenation. The strong decrease in hydrogen content towards the surface can be explained by a concentration-dependent diffusion coefficient. The method of Boltzmann ( 1894 ) and Matano ( 1932 ) allows determining the diffusion coefficient for each concentration along a diffusion profile. For sorption and desorption profiles the following equation can be used (Crank 1975 ) $$D\left(C{\prime }\right)=-\frac{1}{2t}\cdot \frac{1}{{\left(\frac{dC}{dx}\right)}_{C{\prime }}}{\int }_{C{\prime }}^{1}xdC$$ 2 where t is the time, C is the normalized concentration at the distance x from the surface, and C’ is the normalized concentration for which the diffusivity is evaluated. Considering that the IR absorbance is proportional to the concentration, in the case of desorption the normalized concentration is defined as $$C=\frac{{A}_{center} -A}{{A}_{center} -{A}_{surface}}$$ 3 Thus, the normalized concentration varies between 1 at the surface ( x = 0) and 0 at the end of the profile ( x = ∞). It is more convenient to plot the concentration as a function of x (Behrens and Zhang 2009 ). Then Eq. ( 2 ) transforms to $$D\left(x{\prime }\right)=-\frac{1}{2\bullet t\bullet {\left(dC/dx\right)}_{x{\prime }}}\cdot \left[{\int }_{x{\prime }}^{\infty }C\bullet dx+x{\prime }\bullet C\left({x}^{{\prime }}\right)\right]$$ 4 Here x’ is the distance from the surface at which the normalized concentration equals C’. A graphical illustration of the evaluation method is given in Fig. 4 . The dehydrogenation profiles in the sanidine can be fitted well with a polynomial C norm = 1 + a∙x + b∙x 1.5 +c∙x 2 + d∙x 0.5 where a, b, c, d are specific fit parameters for each profile. This type of equation has been successfully used to determine the concentration dependence of water diffusivity in silicate melts after dehydration experiments (Behrens 2006 , Behrens and Zhang 2009 ). Polynomial fitting is shown for two experiments in Fig. (4b,c). An upper value of x must be defined, since a good fit cannot be obtained with the polynomial for x →∞. Artefacts easily occur in the BM method at low and at high concentrations when either slopes or integrals have high uncertainty. Therefore, only diffusion data for x ≥ 100 µm and C norm ≥ 0.1 are considered in following. In Fig. 5 the diffusion coefficients obtained by BM analyses are plotted for the successful experiments. The general trend is an increase in diffusivity with increasing hydrogen concentration, expressed as A/A center . The curvature of the lines is not meaningful because it is strongly affected by imperfect polynomial fitting of the data. The increase in diffusion coefficients also supports the proposed defect model. The more protons have been replaced by alkalis, the greater the probability of a reverse reaction, i.e. migrating protons are bound locally again, reducing their overall mobility. For both approaches, the temperature of diffusivity is well described by an Arrhenius law (Fig. 6 a,b). Two selected values of c/c center , the diffusion coefficients are compared in Fig. 6 b with the data of the first approach (constant diffusion coefficient) and the results of the D/H exchange experiments. The difference of diffusivities for c/c center = 0.9 and c/c center = 0.6 is about 0.25 log units, independent on temperature. The values for c/c center = 0.6 agree with the data obtained assuming constant proton diffusivity which demonstrates the consistency of both approaches. At first glance, it is surprising that diffusivities based on dehydrogenation for c/c center = 0.9 are higher by half a log unit compared to self-diffusivities of hydrogen determined by D/H exchange experiments with pre-annealed sanidines. Intuitively, one would expect the opposite, i.e., that due to the temporary binding to dehydrogenated type IIb OH defects, the mobility of the protons is reduced. However, the hydrogen concentrations measured by IR spectroscopy at room temperature only represent the stationary defects, but do not give any information about the concentration of mobile species (Kronenberg et al. 1996 ). As outlined by Behrens ( 2021b ), the mobile hydrogen species in sanidines pre-annealed at ambient pressure are most likely protons while at elevated water pressures H 2 O molecules can enter the feldspar structure and became the main transporter for hydrogen. The concentration of mobile protons ( c H+ ) is probably much lower than that of the hydrogen species bound in the stationary type II OH defects ( c OH,II ). The diffusion coefficient D D/H based on the hydrogen isotope exchange of the stationary OH defects is determined by the concentration ratio of the mobile and stationary hydrogen defects and the diffusivity of the mobile species D H+ : $${D}_{D/H}=\frac{{c}_{OH;II}}{{c}_{{H}^{+}}}\bullet {D}_{{H}^{+}}$$ 5 Assuming that D H+ is not significantly different for A/A center = 0.9 and A/A center = 1 (as adjusted in the pre-annealed sanidine), the difference between D H and D D/H simply reflects that finally 100% of the stationary OH defects are involved in isotope exchange, but only 10% in the dehydrogenation reaction at A/A center = 0.9. With increasing dehydrogenation, c H+ decreases due to the back reaction $${Na}_{i}^{+}+{H}_{OH,II}^{+}={Na}_{OH,II}^{+}+{H}_{i}^{+}$$ 6 where \({Na}_{OH,II}^{+}\) is a sodium ion replacing a localized proton \({H}_{OH,II}^{+}\) needed for local charge compensation of excess aluminum. \({Na}_{i}^{+}\) represents all kinds of sodium interstitials produced by the Frenkel equilibrium reaction $${{Na}_{A}^{+}=Na}_{i}^{+}+{V}_{A}$$ 7 Here, the subscripts A refer to regular alkali sites in the feldspar structure and V indicates a vacancy. The higher D D/H in the natural sanidine compared to the pre-annealed sanidine is due to the presence of water molecules in the former one. These molecules act as transport vehicles for hydrogen isotopes which can easily exchange with hydrogen in stationary defects. The higher diffusion coefficients for oxygen under hydrothermal conditions compared to dry conditions (see Fig. 7 ) also argue for such a mechanism at elevated water pressures. Temperature dependence of hydrogen diffusion Arrhenius parameter for dehydrogenation of sanidine are given in Table 2 . Within error, the activation energies are identical for both evaluation approaches and do not depend on the degree of dehydrogenation, at least at high hydrogen contents. The activation energies for dehydrogenation of sanidine agree well with those for D/H exchange in natural Eifel sanidine (160.2 kJ/mol), pre-annealed Eifel sanidine (159.9 kJ/mole) and adularia from unknown locality (162.3 kJ/mole) reported in Behrens ( 2021b ). A similar value of 172 kJ/mole is given by Kronenberg et al. ( 1996 ) for the removal of hydrogen defects in adularia from Kristallina, Switzerland. Kronenberg et al. ( 1996 ) suggest that proton migration via interstitials is the transport mechanism for hydrogen in their experiments. However, this interpretation is controversial, see the discussion by Doremus ( 1998 ), Kronenberg et al. ( 1998 ) and Behrens ( 2021b ). Table 2 Arrhenius parameter for chemical diffusion of hydrogen in sanidine T range (°C) n log D 0 Ea (D 0 in m 2 /s) (kJ/mol) Fit const. D 605 - 1000 15 -4.37 ± 0.30 168.3 ± 6.0 BM, c/c center = 0.9 605 - 1000 14 -4.39 ± 0.48 162.4 ± 9.7 BM, c/c center = 0.6 720 - 1000 7 -4.71 ± 0.30 161.0 ± 6.4 Notes. n = number of profiles BM: Boltzmann Matano evalution at two different relative concentrations of hydrogen. Much lower diffusivities at moderate temperatures and higher activation energies were found for dehydrogenation of andesine from Halloran Springs, California ( E a = 266–278 kJ/mole, Johnson and Rossman 2013 ) and Eifel sanidine in absence of alkali chloride ( E a ≈ 305 kJ/mole, Behrens 2021a ), see Fig. 7 . These findings point to a different mechanism for removal of hydrogen defects. If the substitution of protons by monovalent cations or the release of hydrogen after oxidation of neighboring ions such as Fe 2+ is not possible, then oxygen must be additionally removed for charge compensation. This is associated with a strong perturbation of the crystal structure and thus requires high activation energies. In the case of alkali-free dehydration of sanidine, such a mechanism is supported by cracking during complete hydrogen removal induced by internal strain (Behrens 2021a ). On the other hand, the activation energy appears to be significantly lower for D/H exchange under hydrothermal conditions (130 kJ/mole at a water pressure of 2 kbar, Behrens ( 2021b )). On the other hand, from the higher diffusivities and the low activation energies under hydrothermal conditions, one cannot necessarily conclude that water molecules have a higher mobility than protons in the feldspar structure. As already mentioned, the exchange rate is determined not only by the mobility of the defects but also by their concentration. In isotope exchange experiments at 6–8 kbar, an increase in hydrogen content of more than 50% was observed in some cases by Behrens ( 2021b ). The incorporated water has a very high mobility, as shown by the chemical diffusion coefficients for H 2 O determined from the sorption profiles (see Fig. 7 , data 9). In Fig. 7 the diffusion data for hydrogen species are compared to alkali diffusion data for Eifel sanidine. It is striking that 22 Na tracer diffusion (Wilangowski et al. ( 2015 ) are very similar to D H values determined by dehydrogenation of Eifel sanidine in presence of alkali chloride, while 43 K tracer diffusion coefficients (Hergemöller et al. ( 2017 ) are several orders of magnitude smaller. My experiments with different alkali chlorides give consistent results which shows that the external source of alkali is not rate-controlling for dehydrogenation. The short Na/K exchange profiles when using sodium chloride as the alkali source support this thesis. As discussed in my previous paper (Behrens 2021a ), the strongly bound type OH II defects in the sanidine are likely protons incorporated on interstitial sites as a charge balance for excess aluminum. Replacement by alkalis requires energy, and the smaller sodium ions are preferred over the larger potassium ions. Accordingly, the diffusion out of hydrogen requires a corresponding counterflux of sodium. However, since in the annealed sanidine the Na concentration is about 30 times higher than the H concentration, the local charge balance could be rapidly established even with an order of magnitude slower Na diffusivity. Conclusions Hydrogen defects can be incorporated as thermodynamically stable species in feldspars that crystallize at high temperatures in hydrous magmas. The nature of the defects and, hence, their stability in subsequent processes depends on the composition of the magmas. Thus, depending on the conditions, protons may be incorporated together with aluminum as a substitute for silicon, or water molecules may be associated with vacancies in the structure. The new experiments support the hypothesis that strongly bound, isolated OH defects in feldspars can be removed by diffusion of protons if alkalis are present for charge balance. If this is not the case, massive disruption of the crystal framework is required for dehydration, since oxygen ions must be removed in addition to protons to balance the charge. Under these conditions, the transport of hydrogen will preferentially occur by diffusion of neutral water molecules. If the defect centers are not irreversibly destroyed after hydrogen release, then they act as traps for protons and water molecules, respectively. As a result, the dehydration rate decreases with increasing degree of hydrogen removal. The situation is different for feldspars in which the hydrogen was incorporated under metamorphic conditions, for example during recrystallization. Here, the destruction of hydrogen defects is typically irreversible, and the mobility of released water molecules is not reduced by temporary binding in the defect centers. This appears to be the case with the Kristallina andesine studied by Kronenberg et al. ( 1996 ). Declarations Acknowledgement. My special thanks go to Otto Dietrich for the excellent preparation of feldspar sections. I thank Fabian Hergemöller for stimulating thoughts on proton and alkali diffusion. The research was supported by the researcher unit FOR2881 of the German Science foundation (DFG). 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Skogby H (2006) Water in Natural Mantle Minerals I: Pyroxenes. Rev Mineral Geochem 62:155-167 Wilangowski F, Abart R, Divinski SV, Stolwijk NA (2015) Radiotracer Experiments and Monte Carlo Simulations of Sodium Diffusion in Alkali Feldspar: Evidence Against the Vacancy Mechanism. Defect Diffusion Forum 363: 79-84 Yund RA (1986) Interdiffusion of NaSi-CaAl in Peristerite. Phys Chem Mineral 13:11-16 Yund RA, Anderson TF (1974) The effect of fluid pressure on oxygen isotope exchange between feldspar and water. Geochim Cosmochim Acta 42:235-239 Additional Declarations No competing interests reported. Supplementary Files Supplementhydrogenprofilesalkalisupporteddehydrogenation10022023.xlsx Cite Share Download PDF Status: Published Journal Publication published 23 Jun, 2023 Read the published version in Physics and Chemistry of Minerals → Version 1 posted Editorial decision: Major revision 17 May, 2023 Reviews received at journal 14 May, 2023 Reviews received at journal 19 Apr, 2023 Reviewers agreed at journal 18 Apr, 2023 Reviewers agreed at journal 23 Mar, 2023 Reviewers invited by journal 15 Feb, 2023 Editor assigned by journal 11 Feb, 2023 Submission checks completed at journal 11 Feb, 2023 First submitted to journal 10 Feb, 2023 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. 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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-2572968","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":175278743,"identity":"31d7d71b-6df6-4f1f-9b20-4b5b6e647357","order_by":0,"name":"Harald Behrens","email":"data:image/png;base64,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","orcid":"","institution":"Leibniz University Hannover","correspondingAuthor":true,"prefix":"","firstName":"Harald","middleName":"","lastName":"Behrens","suffix":""}],"badges":[],"createdAt":"2023-02-10 12:44:28","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2572968/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2572968/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00269-023-01242-9","type":"published","date":"2023-06-23T21:17:04+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":32939745,"identity":"ddaf7d8d-3e9c-4cd0-a427-241e7ee6adb4","added_by":"auto","created_at":"2023-02-14 21:03:12","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":23903,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea,b.\u003c/strong\u003e Examples of hydrogen profiles in sanidine after desorption experiments with alkali chloride. Lines represent fits by Eqn. (1), i.e., assuming constant diffusion coefficients.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-2572968/v1/faf55288a746fd15ff3b4730.png"},{"id":32939746,"identity":"c1b9c00c-8728-4fe7-b90d-559aae2838fa","added_by":"auto","created_at":"2023-02-14 21:03:12","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":14673,"visible":true,"origin":"","legend":"\u003cp\u003eVariation of alkali contents along the \u003cem\u003ea\u003c/em\u003e axis after a desorption experiment in which a sanidine cuboid was embedded in NaCl powder. Lines illustrate the trend of the data. The sigmoidal shape of the curves is due to the concentration dependence of the Na-K interdiffusion coefficient (Christoffersen et al. 1983, Schäffer et al. 2014).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-2572968/v1/d2cb5588495f942e28ce277a.png"},{"id":32940385,"identity":"6a32c0c1-c1fe-4413-930f-b1b53a4d645f","added_by":"auto","created_at":"2023-02-14 21:11:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":12214,"visible":true,"origin":"","legend":"\u003cp\u003eVariation of the hydrogen concentration at the surface of the crystals after desorption experiments with alkali chloride. The relative abundance of hydrogen at the surface compared to the center, defined as the absorbance ratio \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003esurface\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/A\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e, was determined from fitting as shown in Fig. 1a,b. The dashed line is a guide for the eye to illustrate the evolution with temperature.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-2572968/v1/a501e603dc07d8e7c51bea90.png"},{"id":32939751,"identity":"8c886915-ce11-49ee-b020-c6990d2ea13b","added_by":"auto","created_at":"2023-02-14 21:03:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":73071,"visible":true,"origin":"","legend":"\u003cp\u003eBoltzmann Matano analysis of dehydrogenation profiles.\u003cstrong\u003e (a)\u003c/strong\u003e Illustration of the method for desorption experiments. \u003cstrong\u003e(b) \u003c/strong\u003eNormalized concentration vs. distance for the example shown in Fig. 1a. \u003cstrong\u003e(c) \u003c/strong\u003eNormalized concentration vs. distance for the example shown in Fig. 1b. Lines in (b) and (c)\u003cstrong\u003e \u003c/strong\u003erepresent fitting of the data with a polynomial \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003enorm\u003c/em\u003e\u003c/sub\u003e = 1 + a∙x + b∙x\u003csup\u003e1.5\u003c/sup\u003e +c∙x\u003csup\u003e2\u003c/sup\u003e + d∙x\u003csup\u003e0.5\u003c/sup\u003e where a, b, c, d are specific fit parameters. Basic assumption is that the surface concentration of hydrogen equals zero. The size of the symbols corresponds to its errors. Thin dashed lines mark the concentrations for which diffusion data are plotted in Fig. 6.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-2572968/v1/c83a5a0e24307c427dd33c41.png"},{"id":32939749,"identity":"16c0d1dc-703b-43b9-ac78-6c9b94d6f3e6","added_by":"auto","created_at":"2023-02-14 21:03:12","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":15753,"visible":true,"origin":"","legend":"\u003cp\u003eDiffusion coefficients in function of the concentration of hydrogen expressed as the absorbance ratio \u003cem\u003eA/A\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e, determined by the Boltzmann-Matano method.\u003cem\u003e \u003c/em\u003eOnly data for x ≥ 100 µm and \u003cem\u003eA/A\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter \u003c/em\u003e\u003c/sub\u003e\u0026lt;0.9 are plotted\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-2572968/v1/ec62174d0043e8da65d02883.png"},{"id":32939747,"identity":"af01be9f-881f-451a-b1d1-4cf414d21106","added_by":"auto","created_at":"2023-02-14 21:03:12","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":25645,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature dependence\u003cstrong\u003e \u003c/strong\u003eof chemical diffusion of hydrogen in sanidine at ambient pressure. \u003cstrong\u003e(a)\u003c/strong\u003e\u0026nbsp; \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eH\u003c/em\u003e\u003c/sub\u003e assuming constant diffusivity. \u003cstrong\u003e(b)\u003c/strong\u003e Chemical diffusivities of hydrogen for two different hydrogen contents (black solid lines) derived by the Boltzmann-Matano analysis. Dashed lines represent self-diffusivities of hydrogen determined by D/H exchange experiments in natural and heat-treated sanidine (Behrens 2021b). The pnk line corresponds to the Arrhenius fit in (a).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-2572968/v1/031a20a52370d4d5db7ea68e.png"},{"id":32940384,"identity":"e262865b-c474-481f-906d-1124ada037fa","added_by":"auto","created_at":"2023-02-14 21:11:12","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":17139,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of diffusion of hydrogen and oxygen species in feldspars. D/H interdiffusion is marked in black, H\u003csub\u003e2\u003c/sub\u003eO diffusion in blue and oxygen diffusion in red und proton diffusion in pink. Note that oxygen diffusion data are considered for Eifel sanidine only. For additional oxygen diffusion data and a detailed discussion see the review of Farver (2010). Tracer diffusion data for Na and K in sanidine (in green) are shown to constrain the mechanism of proton removal. Data sources:\u003c/p\u003e\n\u003cp\u003e[1] D-H interdiffusion, sanidine SV, pre-annealed, 1 atm, Behrens (2021b)\u003c/p\u003e\n\u003cp\u003e[2] D-H interdiffusion,, sanidine SV, virgin, 1 atm, Behrens (2021b)\u003c/p\u003e\n\u003cp\u003e[3] D-H interdiffusion, sanidine SV, p\u003csub\u003eH2O\u003c/sub\u003e = 2 kbar, Behrens (2021b)\u003c/p\u003e\n\u003cp\u003e[4] D-H, adularia A1, pre-annealed, 1 atm, Behrens (2021b)\u003c/p\u003e\n\u003cp\u003e[5] H\u003csub\u003e2\u003c/sub\u003eO desorption, plag An31, 1 atm, Johnson and Rossman (2012)\u003c/p\u003e\n\u003cp\u003e[6] H\u003csub\u003e2\u003c/sub\u003eO desorption, adularia, 1 atm, Kronenberg et al. (1996)\u003c/p\u003e\n\u003cp\u003e[7] H\u003csub\u003e2\u003c/sub\u003eO desorption, sanidine SV, virgin, 1 atm, Behrens (2021a)\u003c/p\u003e\n\u003cp\u003e[8] H\u003csub\u003e2\u003c/sub\u003eO desorption, plag An66, 1 atm, Behrens (2021a)\u003c/p\u003e\n\u003cp\u003e[9] H\u003csub\u003e2\u003c/sub\u003eO sorption, sanidine SV, p\u003csub\u003eH2O\u003c/sub\u003e = 6-8 kbar, this study\u003c/p\u003e\n\u003cp\u003e[10] \u003csup\u003e17\u003c/sup\u003eO,\u003csup\u003e18\u003c/sup\u003eO diffusion, sanidine, p\u003csub\u003eH2O\u003c/sub\u003e = 1 kbar, Freer et al. (1997)\u003c/p\u003e\n\u003cp\u003e[11] \u003csup\u003e18\u003c/sup\u003eO diffusion, sanidine, 1 atm, dry, Derdau et al. (1998)\u003c/p\u003e\n\u003cp\u003e[12] \u003csup\u003e22\u003c/sup\u003eNa tracer diffusion, sanidine, 1 atm, dry, Wilangowski et al. (2015)\u003c/p\u003e\n\u003cp\u003e[13] \u003csup\u003e43\u003c/sup\u003eK tracer diffusion, sanidine, 1 atm, dry, Hergemöller et al. (2017)\u003c/p\u003e\n\u003cp\u003e[14] Dehydrogenation supported by alkali, sanidine SV, pre-annealed, 1 atm, this study\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-2572968/v1/1ad52df443192da092bb87f2.png"},{"id":44733846,"identity":"43af1ec7-81f0-4459-91f3-85b9b573146a","added_by":"auto","created_at":"2023-10-16 22:11:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":473057,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2572968/v1/f78abc1e-69d5-4fae-9328-996505d94dbf.pdf"},{"id":32939752,"identity":"50a7ba46-ad9d-44d7-b53d-49a5829f2049","added_by":"auto","created_at":"2023-02-14 21:03:13","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":20025652,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementhydrogenprofilesalkalisupporteddehydrogenation10022023.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-2572968/v1/ec9bd2e1e383f52dbab2c18b.xlsx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Hydrogen defects in feldspars: Alkali-supported dehydrogenation of sanidine","fulltext":[{"header":"Introduction","content":"\u003cp\u003eHydrogen is a common impurity in nominally anhydrous minerals such as olivine, pyroxene, feldspar or quartz (e.g., Beran and Libowitzki 2006, Johnson \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2006\u003c/span\u003e, Skogby \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The incorporated hydrogen can have major impact on the properties of the minerals. In the case of feldspars, diffusion of network-forming elements such as silicon, aluminum and oxygen is strongly enhanced by dissolved hydrogen species (e.g., Yund and Anderson \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1974\u003c/span\u003e, Giletti et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1978\u003c/span\u003e, Grove et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1984\u003c/span\u003e, Yund \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1986\u003c/span\u003e, Elphick et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1988\u003c/span\u003e, Farver and Yund \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1990\u003c/span\u003e, Baschek und Johannes \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1995\u003c/span\u003e, Cherniak \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e, Farver \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). The enhancement of intracrystalline diffusion is important not only on the local scale, but affects also long-range processes in rocks, i.e. the tectonic response to accumulating stress induced by plate movement or the mobilization of elements in rocks as a prerequisite for the formation of ore deposits. Thus, there is large interest in better understanding the properties of dissolved hydrogen in feldspars.\u003c/p\u003e \u003cp\u003eIn feldspars formed or recrystallized under metamorphic conditions, most of the hydrogen is typically present in inclusions. These can be fluid inclusions as well as hydrous glasses or minerals. Regarding the kinetic properties of feldspars, such localized hydrogen pools have little impact. More important are hydrogen species incorporated as point defects in the feldspar structure. Using polarized IR spectroscopy, Johnson and Rossman (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2003\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) identified different types of structural hydrogen defects in feldspar. According to their studies, volcanic feldspars contain only structural OH groups while plutonic and pegmatitic feldspars may host also other hydrogen species, i.e. H\u003csub\u003e2\u003c/sub\u003eO molecules and ammonium cations. A common OH defect in alkali feldspars and plagioclase, denoted as \u003cem\u003etype IIa OH\u003c/em\u003e bands by Johnson and Rossman (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2004\u003c/span\u003e), is characterized by a broad IR band system centered around 3300 cm\u003csup\u003e− 1\u003c/sup\u003e with the maximum intensity closely aligned with the crystallographic \u003cem\u003ea\u003c/em\u003e axis.\u003c/p\u003e \u003cp\u003eA different IR absorption spectrum is observed for Eifel sanidine with a band at 3050 cm\u003csup\u003e− 1\u003c/sup\u003e being most intense in the \u003cem\u003eb\u003c/em\u003e direction, while a band near 3400 cm\u003csup\u003e− 1\u003c/sup\u003e being most intense in the \u003cem\u003ea\u003c/em\u003e direction. Johnson and Rossman (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) classified the corresponding defect in Eifel sanidines as \u003cem\u003etype IIb OH\u003c/em\u003e. However, results of the experimental studies of Behrens (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e) suggest, that hydrogen incorporation in Eifel sanidines is more complex and at least two different OH defects with different mobilities coexist in the structure.\u003c/p\u003e \u003cp\u003eThis paper is the third part of a series of studies on hydrogen defects in feldspar. In the first part, the thermal stability of hydrogen defects in feldspars and the partitioning of water between a gas phase and feldspars (plagioclase and K-rich alkali feldspar) was investigated at ambient and elevated pressure (Behrens \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e). Diffusion of hydrogen species and hydrogen isotope exchange between coexisting fluid/gas phases was the main topic of the second part (Behrens \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e). Indication was found that different transport mechanism for hydrogen are dominating at wet and at dry conditions. At elevated water pressures, water molecules diffusing via interstitial sites act as transport vehicles for hydrogen. Under dry conditions, diffusion of protons resulting from dissociation of extrinsic defects such as AlOH groups is the dominant process.\u003c/p\u003e \u003cp\u003eThe latter mechanism is in the focus of the third paper of this series. Dehydrogenation experiments were performed with sanidine in contact with alkali chloride. Behrens (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e) has already demonstrated that the release of hydrogen from natural sanidine is strongly accelerated when alkali ions are available for charge compensation of out-diffusing protons. Here, this process is investigated in detail over a large temperature range. The data are compared to self- diffusion of hydrogen determined by isotope exchange experiments (Behrens \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e) and alkali diffusion data (Neusser et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Schäffer et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Wilangowski et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Hergemöller et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) to constrain the mechanisms of dehydrogenation.\u003c/p\u003e"},{"header":"Samples and experiments","content":"\u003cp\u003eCuboids of sanidine were used to probe hydrogen diffusion in three orthogonal directions. The sanidine SV has gemstone quality and is described in detail in the first paper (Behrens \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e). Based on cleavage planes in (010) and (001) crystals were oriented. The pleochroisms of the OH vibration bands at 3400 cm\u003csup\u003e− 1\u003c/sup\u003e and 3050 cm\u003csup\u003e− 1\u003c/sup\u003e in polarized IR absorption spectra gives further constraints of the crystallographic axis in the cuboid. The crystals were heated for at least 7 days in air at 900°C to remove large part of the mobile \u003cem\u003etype OH IIa\u003c/em\u003e defects.\u003c/p\u003e\u003cp\u003eExperiments were performed at ambient pressure in a large temperature range from 605 to 1000°C (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). At temperatures below 800°C, the samples were embedded in solid alkali chloride. Some of these experiments were performed in a closed glass apparatus, similar to that used for D-H exchange experiments (Behrens \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). A dry atmosphere was set by evacuating and filling the apparatus several times with air pre-dried over phosphorus pentoxide. In other experiments, the salt-sanidine assembly was loaded in a Pt-boat and continuously flushed with a pre-dried air stream.\u003c/p\u003e\u003cdiv class=\"gridtable\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" 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\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eConditions and results of dehydrogenation experiments.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e\u003ccolgroup cols=\"11\"\u003e\u003c/colgroup\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNo\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eT\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003et\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSection\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e \u003cp\u003ea\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003eb\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c10\" namest=\"c9\"\u003e \u003cp\u003ec*\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c11\"\u003e \u003cp\u003ealkali source, comments\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e(°C)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e(h)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ec\u003csub\u003eS\u003c/sub\u003e/c\u003csub\u003eC\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003elog D\u003csub\u003eH\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003ec\u003csub\u003eS\u003c/sub\u003e/c\u003csub\u003eC\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003elog D\u003csub\u003eH\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003ec\u003csub\u003eS\u003c/sub\u003e/c\u003csub\u003eC\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003elog D\u003csub\u003eH\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c11\"\u003e\u0026nbsp;\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSV-M1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e605\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1488\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(010)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.713\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-14.40\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.512\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e-14.43\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eKCl powder, closed apparatus\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSV-L1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e650\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e504\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eperp a\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.457\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e-13.69\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eKCl powder, closed apparatus\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSV-H1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e720\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e288\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(001)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.305\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-13.30\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.232\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e-13.26\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eNaCl powder, flushed by dry air\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003ecracks 30 µm in a, 70 µm in b\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSV-P1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e720\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e150\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(010)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.538\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-13.26\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.604\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e-13.23\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eKCl powder, closed apparatus\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSV-K1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e750\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e97\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(010)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.293\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e-13.00\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eKCl powder, flushed by dry air\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSV-I1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e783\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(010)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.265\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-12.84\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eNaCl powder\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003ecracks 25 µm in a, 80 µm in c*\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSV-Q1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e900\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(010)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.212\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e-11.85\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eK\u003csub\u003e0.95\u003c/sub\u003eNa\u003csub\u003e0.05\u003c/sub\u003e vapor\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSV-S1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e900\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(010)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.408\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-11.72\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eK\u003csub\u003e0.95\u003c/sub\u003eNa\u003csub\u003e0.05\u003c/sub\u003e melt\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.467\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-11.65\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.452\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e-11.65\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSV-FH\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1000\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.17\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(010)\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.153\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-11.49\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.058\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e-11.40\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eNaCl vapor\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003ecracks 220 µm in a\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"11\"\u003eNotes.\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd colspan=\"11\"\u003ea, b, c* refer to the crystallographic orientation of the profiles\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd colspan=\"11\"\u003eDiffusion coefficients D\u003csub\u003eH\u003c/sub\u003e determined assuming constant diffusivity (Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Units are m\u003csup\u003e2\u003c/sup\u003e/s.\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd colspan=\"11\"\u003ec\u003csub\u003eS\u003c/sub\u003e/c\u003csub\u003eC\u003c/sub\u003e is the hydrogen concentration at the surface compared to the center, based on fitting by Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e)\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003cp\u003eAt temperatures above the melting points of the alkali chlorides, closed gold capsules were used. Either the cuboid was placed directly in the melt, or it was set on a small table above the melt so that the sample is only in contact with the vapor phase. This set-up was shown in Fig.\u0026nbsp;9 of Behrens (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAlkali chlorides were either pure NaCl or pure KCl or K\u003csub\u003e0.95\u003c/sub\u003eNa\u003csub\u003e0.05\u003c/sub\u003eCl. NaCl was chosen because of the high mobility of Na\u003csup\u003e+\u003c/sup\u003e in sanidine (Wilangowski et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) and because the smaller size of Na\u003csup\u003e+\u003c/sup\u003e compared to K\u003csup\u003e+\u003c/sup\u003e favors substitution for H\u003csup\u003e+\u003c/sup\u003e on interstitial sites (Behrens \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e). However, a serious disadvantage of using NaCl is that the cation exchange in the sanidine causes severe stress and cracking (Scheidl et al. 2014, Petrishcheva et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Therefore, K-rich chlorides were used in most of the experiments. At the time the experiments were done, the partition coefficients between alkali chloride and alkali feldspar were not yet known. It was assumed that K\u003csub\u003e0.95\u003c/sub\u003eNa\u003csub\u003e0.05\u003c/sub\u003eCl is close to the equilibrium composition of a melt coexisting with the sanidine. Partitioning coefficients determined by Neusser et al. (2009) imply that the equilibrium chloride composition for a sanidine with 85 mol% Or component is more sodic. However, as discussed below, this does not affect the results of this paper.\u003c/p\u003e\u003ch2\u003eAnalytics\u003c/h2\u003e\u003cp\u003eAfter experiment, the cuboids were cut in two different orientations as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e of Behrens (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e). After grinding and polishing to a thickness of about 500 µm, hydrogen profiles were recorded from the rim to the center using an IR microscope A590 coupled with an FTIR spectrometer Bruker IFS88. A globar light source, a KBr beamsplitter and an MCT (mercury cadmium telluride) detector were applied in these measurements. 50–100 scans were accumulated for each spectrum with a spectral resolution of 2 cm\u003csup\u003e− 1\u003c/sup\u003e. The spectrometer provides a partial polarized beam which is very well suited for precise measurements of diffusion profiles. A circular aperture corresponding to a focus diameter of 50 µm was applied, corresponding to an effective diameter of the exited area of 60 µm (Behrens \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e)). Some profiles were recently re-measured with an IR microscope Bruker IRscope II using a slit aperture to adjust an area of 20 x 100 µm\u003csup\u003e2\u003c/sup\u003e in the focus plane. Aligning the long side parallel to the edge of the section, a spatial resolution of ≈ 40 µm could be achieved. The profiles measured by both setups show no significant difference.\u003c/p\u003e\u003cp\u003eExamples of IR absorption spectra of the sanidine in the range of OH stretching vibrations can be found in Behrens (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003eb\u003c/span\u003e). Die absorbance \u003cem\u003eA\u003c/em\u003e of the main OH peak was determined after subtraction of a linear baseline. Hydrogen concentration was calculated by the Lambert-Beer law as described in these papers.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eCuboids were well preserved after experiments with K-rich chlorides while pronounced crack formation in the near-surface region occurred in the runs with NaCl (SV-H1, SV-I1 and SV-FH). Cracks are aligned parallel to each other, but the orientation does not correspond to any of the feldspar cleavages. The lengths of the cracks depend on the orientation of the surface from which crack growth started, and for a given surface the cracks have all similar length (see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These observations agree well with those of Abart and co-workers (Neusser et al. 2021, Scheidl et al. 2014, Petrishcheva et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) who performed similar alkali exchange experiments with Eifel sanidines. In these papers a detailed description of the mechanisms of alkali exchange and crack formation is given. The driving force for crack formation is the coherency stress created by the replacement of the large K\u003csup\u003e+\u003c/sup\u003e ion by the smaller Na\u003csup\u003e+\u003c/sup\u003e ions at the diffusion front.\u003c/p\u003e \u003cp\u003eHydrogen profiles obtained from two experiments are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea,b. The complete set of experiments can be found in the electronic supplement. A common feature is that extrapolations of the measured profiles do not pass through the origin.\u003c/p\u003e \u003cp\u003eFor one experiment using NaCl as the alkali source (SV-H1), a traverse from rim to the center was analyzed by electron microprobe CAMECA Camebax. Measurement conditions were the same as reported in Behrens (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e). The basic feldspar composition was preserved along the profile, only the Na/K ratio had changed. The alkali exchange profile was much shorter than the hydrogen profile (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), and the crack tips evolved even beyond the region with measurable variation in alkali abundance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eDue to the limited spatial resolution of the IR beam it is not possible to determine the concentration of hydrogen in the sanidine directly at the contact to the surrounding media. The first measurement point was typically 50 \u0026micro;m away from the surface and represents the average concentration over a range of 50\u0026thinsp;\u0026plusmn;\u0026thinsp;30 \u0026micro;m. The measured profiles can be explained by a remaining hydrogen content in the surface area of the sanidine as well as by a very strong decrease of the concentration near the surface. Both approaches are discussed in following.\u003c/p\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eProfile evaluation assuming constant diffusivity\u003c/h2\u003e \u003cp\u003eIn the first approach, the absorbance-distance profiles were evaluated by the model of one-dimensional diffusion between an infinite reservoir (the gas phase) and a semi-infinite medium (the crystal) with the boundary condition of a constant surface concentration. Since the absorbance of the OH stretching vibration band is proportional to concentration of hydrogen, the measured absorbance can be directly used for the determination of the diffusion coefficient. Assuming a constant diffusion coefficient \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eH\u003c/em\u003e\u003c/sub\u003e, the solution of Fick's second law for these boundary conditions is (Crank \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1975\u003c/span\u003e)\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\frac{A-{A}_{center}}{{A}_{surface}-{A}_{center}}=1-e rf\\left(\\frac{x}{\\sqrt{4\\bullet {D}_{H}\\bullet t}}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003et\u003c/em\u003e is the run duration, \u003cem\u003eA\u003c/em\u003e is the absorbance at the distance \u003cem\u003ex\u003c/em\u003e from the surface, \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003esurface\u003c/em\u003e\u003c/sub\u003e are absorbances in the center and at the surface, respectively. The term \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eH\u003c/em\u003e\u003c/sub\u003e characterizes the chemical diffusion of hydrogen during dehydrogenation. The measured profiles were fitted to Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e with \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003esurface\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eH\u003c/em\u003e\u003c/sub\u003e being adjustable parameter. The fitted curves are shown as solid lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea,b and fit results are also included in the electronic supplement. For all profiles, the correlation coefficient r\u003csup\u003e2\u003c/sup\u003e of the fit was \u0026gt;\u0026thinsp;0.95, in many cases even \u0026gt;\u0026thinsp;0.99. This could be regarded as a good confirmation of the evaluation approach.\u003c/p\u003e \u003cp\u003eA unique feature of all experiments is that the fit curves do not pass through the origin, i.e. some hydrogen remained in the crystal at the contact to alkali chloride. The relative abundance of hydrogen at the surface, expressed by the absorbance ration \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003esurface\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/A\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e, varies between 0.06 and 0.71. There is a rough trend of decreasing surface concentration of hydrogen with increasing temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). However, the scatter of the data is high, and the correlation coefficient is small (r\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.45). One reason for the variation in \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003esurface\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/A\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e is that the water pressure (\u003cem\u003ep\u003c/em\u003e\u003csub\u003e\u003cem\u003eH2O\u003c/em\u003e\u003c/sub\u003e) in the experiments was low but it was not fixed at constant value. Another reason is some variation in the hydrogen concentration in the center of the sample between 81 and 115 ppm (see supplement). The trend of \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003esurface\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/A\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e with temperature is consistent with the structural interpretation of the type IIb OH defects in sanidine proposed in Behrens (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003eb\u003c/span\u003e). Accordingly, this defect consists of a proton on an interstitial site compensating the charge deficiency of a nearby Al\u003csup\u003e3+\u003c/sup\u003e. Replacing the proton by a larger alkali requires energy for local expansion of the feldspar structure and, hence, substitution of hydrogen by alkali is a thermally activated process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eBoltzmann-Matano (BM) analysis\u003c/h2\u003e \u003cp\u003eIn the second approach it is assumed that the hydrogen content drops to zero at the surface during dehydrogenation. The strong decrease in hydrogen content towards the surface can be explained by a concentration-dependent diffusion coefficient. The method of Boltzmann (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1894\u003c/span\u003e) and Matano (\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1932\u003c/span\u003e) allows determining the diffusion coefficient for each concentration along a diffusion profile. For sorption and desorption profiles the following equation can be used (Crank \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1975\u003c/span\u003e)\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$D\\left(C{\\prime }\\right)=-\\frac{1}{2t}\\cdot \\frac{1}{{\\left(\\frac{dC}{dx}\\right)}_{C{\\prime }}}{\\int }_{C{\\prime }}^{1}xdC$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003et\u003c/em\u003e is the time, \u003cem\u003eC\u003c/em\u003e is the normalized concentration at the distance \u003cem\u003ex\u003c/em\u003e from the surface, and \u003cem\u003eC\u0026rsquo;\u003c/em\u003e is the normalized concentration for which the diffusivity is evaluated. Considering that the IR absorbance is proportional to the concentration, in the case of desorption the normalized concentration is defined as\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$C=\\frac{{A}_{center} -A}{{A}_{center} -{A}_{surface}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThus, the normalized concentration varies between 1 at the surface (\u003cem\u003ex\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0) and 0 at the end of the profile (\u003cem\u003ex\u003c/em\u003e = \u0026infin;). It is more convenient to plot the concentration as a function of x (Behrens and Zhang \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Then Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) transforms to\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$D\\left(x{\\prime }\\right)=-\\frac{1}{2\\bullet t\\bullet {\\left(dC/dx\\right)}_{x{\\prime }}}\\cdot \\left[{\\int }_{x{\\prime }}^{\\infty }C\\bullet dx+x{\\prime }\\bullet C\\left({x}^{{\\prime }}\\right)\\right]$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eHere \u003cem\u003ex\u0026rsquo;\u003c/em\u003e is the distance from the surface at which the normalized concentration equals \u003cem\u003eC\u0026rsquo;.\u003c/em\u003e A graphical illustration of the evaluation method is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The dehydrogenation profiles in the sanidine can be fitted well with a polynomial \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003enorm\u003c/em\u003e\u003c/sub\u003e = 1 + a∙x\u0026thinsp;+\u0026thinsp;b∙x\u003csup\u003e1.5\u003c/sup\u003e +c∙x\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;+\u0026thinsp;d∙x\u003csup\u003e0.5\u003c/sup\u003e where a, b, c, d are specific fit parameters for each profile. This type of equation has been successfully used to determine the concentration dependence of water diffusivity in silicate melts after dehydration experiments (Behrens \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2006\u003c/span\u003e, Behrens and Zhang \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePolynomial fitting is shown for two experiments in Fig.\u0026nbsp;(4b,c). An upper value of x must be defined, since a good fit cannot be obtained with the polynomial for x \u0026rarr;\u0026infin;. Artefacts easily occur in the BM method at low and at high concentrations when either slopes or integrals have high uncertainty. Therefore, only diffusion data for x\u0026thinsp;\u0026ge;\u0026thinsp;100 \u0026micro;m and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003enorm\u003c/em\u003e\u003c/sub\u003e \u0026ge; 0.1 are considered in following.\u003c/p\u003e \u003cp\u003eIn Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e the diffusion coefficients obtained by BM analyses are plotted for the successful experiments. The general trend is an increase in diffusivity with increasing hydrogen concentration, expressed as \u003cem\u003eA/A\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e. The curvature of the lines is not meaningful because it is strongly affected by imperfect polynomial fitting of the data. The increase in diffusion coefficients also supports the proposed defect model. The more protons have been replaced by alkalis, the greater the probability of a reverse reaction, i.e. migrating protons are bound locally again, reducing their overall mobility.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor both approaches, the temperature of diffusivity is well described by an Arrhenius law (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea,b). Two selected values of \u003cem\u003ec/c\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e, the diffusion coefficients are compared in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb with the data of the first approach (constant diffusion coefficient) and the results of the D/H exchange experiments. The difference of diffusivities for \u003cem\u003ec/c\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e = 0.9 and \u003cem\u003ec/c\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e = 0.6 is about 0.25 log units, independent on temperature. The values for \u003cem\u003ec/c\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e = 0.6 agree with the data obtained assuming constant proton diffusivity which demonstrates the consistency of both approaches. At first glance, it is surprising that diffusivities based on dehydrogenation for \u003cem\u003ec/c\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e = 0.9 are higher by half a log unit compared to self-diffusivities of hydrogen determined by D/H exchange experiments with pre-annealed sanidines. Intuitively, one would expect the opposite, i.e., that due to the temporary binding to dehydrogenated type IIb OH defects, the mobility of the protons is reduced.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eHowever, the hydrogen concentrations measured by IR spectroscopy at room temperature only represent the stationary defects, but do not give any information about the concentration of mobile species (Kronenberg et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). As outlined by Behrens (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e), the mobile hydrogen species in sanidines pre-annealed at ambient pressure are most likely protons while at elevated water pressures H\u003csub\u003e2\u003c/sub\u003eO molecules can enter the feldspar structure and became the main transporter for hydrogen. The concentration of mobile protons (\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003eH+\u003c/em\u003e\u003c/sub\u003e) is probably much lower than that of the hydrogen species bound in the stationary \u003cem\u003etype II OH defects\u003c/em\u003e (\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003eOH,II\u003c/em\u003e\u003c/sub\u003e). The diffusion coefficient \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eD/H\u003c/em\u003e\u003c/sub\u003e based on the hydrogen isotope exchange of the stationary OH defects is determined by the concentration ratio of the mobile and stationary hydrogen defects and the diffusivity of the mobile species \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eH+\u003c/em\u003e\u003c/sub\u003e:\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$${D}_{D/H}=\\frac{{c}_{OH;II}}{{c}_{{H}^{+}}}\\bullet {D}_{{H}^{+}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eAssuming that \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eH+\u003c/em\u003e\u003c/sub\u003e is not significantly different for \u003cem\u003eA/A\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e = 0.9 and \u003cem\u003eA/A\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e = 1 (as adjusted in the pre-annealed sanidine), the difference between \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eH\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eD/H\u003c/em\u003e\u003c/sub\u003e simply reflects that finally 100% of the stationary OH defects are involved in isotope exchange, but only 10% in the dehydrogenation reaction at \u003cem\u003eA/A\u003c/em\u003e\u003csub\u003e\u003cem\u003ecenter\u003c/em\u003e\u003c/sub\u003e = 0.9. With increasing dehydrogenation, \u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003eH+\u003c/em\u003e\u003c/sub\u003e decreases due to the back reaction\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$${Na}_{i}^{+}+{H}_{OH,II}^{+}={Na}_{OH,II}^{+}+{H}_{i}^{+}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({Na}_{OH,II}^{+}\\)\u003c/span\u003e\u003c/span\u003e is a sodium ion replacing a localized proton \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({H}_{OH,II}^{+}\\)\u003c/span\u003e\u003c/span\u003e needed for local charge compensation of excess aluminum. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({Na}_{i}^{+}\\)\u003c/span\u003e\u003c/span\u003erepresents all kinds of sodium interstitials produced by the Frenkel equilibrium reaction\u003cdiv id=\"Equ7\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ7\" name=\"EquationSource\"\u003e\n$${{Na}_{A}^{+}=Na}_{i}^{+}+{V}_{A}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eHere, the subscripts \u003cem\u003eA\u003c/em\u003e refer to regular alkali sites in the feldspar structure and \u003cem\u003eV\u003c/em\u003e indicates a vacancy.\u003c/p\u003e \u003cp\u003eThe higher \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eD/H\u003c/em\u003e\u003c/sub\u003e in the natural sanidine compared to the pre-annealed sanidine is due to the presence of water molecules in the former one. These molecules act as transport vehicles for hydrogen isotopes which can easily exchange with hydrogen in stationary defects. The higher diffusion coefficients for oxygen under hydrothermal conditions compared to dry conditions (see Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) also argue for such a mechanism at elevated water pressures.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eTemperature dependence of hydrogen diffusion\u003c/h2\u003e \u003cp\u003eArrhenius parameter for dehydrogenation of sanidine are given in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Within error, the activation energies are identical for both evaluation approaches and do not depend on the degree of dehydrogenation, at least at high hydrogen contents. The activation energies for dehydrogenation of sanidine agree well with those for D/H exchange in natural Eifel sanidine (160.2 kJ/mol), pre-annealed Eifel sanidine (159.9 kJ/mole) and adularia from unknown locality (162.3 kJ/mole) reported in Behrens (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e). A similar value of 172 kJ/mole is given by Kronenberg et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) for the removal of hydrogen defects in adularia from Kristallina, Switzerland. Kronenberg et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) suggest that proton migration via interstitials is the transport mechanism for hydrogen in their experiments. However, this interpretation is controversial, see the discussion by Doremus (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1998\u003c/span\u003e), Kronenberg et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) and Behrens (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021b\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\u003eArrhenius parameter for chemical diffusion of hydrogen in sanidine\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"13\"\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 \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c12\" colnum=\"12\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c13\" colnum=\"13\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eT range (\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003en\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003elog D\u003csub\u003e0\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"5\" nameend=\"c13\" namest=\"c9\"\u003e \u003cp\u003eEa\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003e(D\u003csub\u003e0\u003c/sub\u003e in m\u003csup\u003e2\u003c/sup\u003e/s)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"5\" nameend=\"c13\" namest=\"c9\"\u003e \u003cp\u003e(kJ/mol)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFit const. D\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e605\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-4.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u0026plusmn;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c9\" namest=\"c8\"\u003e \u003cp\u003e0.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c11\" namest=\"c10\"\u003e \u003cp\u003e168.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e\u0026plusmn;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e6.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBM, c/c\u003csub\u003ecenter\u003c/sub\u003e = 0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e605\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-4.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u0026plusmn;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c9\" namest=\"c8\"\u003e \u003cp\u003e0.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c11\" namest=\"c10\"\u003e \u003cp\u003e162.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e\u0026plusmn;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e9.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBM, c/c\u003csub\u003ecenter\u003c/sub\u003e = 0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e720\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-4.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u0026plusmn;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c10\" namest=\"c8\"\u003e \u003cp\u003e0.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e161.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003e\u0026plusmn;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e \u003cp\u003e6.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"13\"\u003eNotes.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"13\"\u003en\u0026thinsp;=\u0026thinsp;number of profiles\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"13\"\u003eBM: Boltzmann Matano evalution at two different relative concentrations of hydrogen.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eMuch lower diffusivities at moderate temperatures and higher activation energies were found for dehydrogenation of andesine from Halloran Springs, California (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e = 266\u0026ndash;278 kJ/mole, Johnson and Rossman \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) and Eifel sanidine in absence of alkali chloride (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e \u0026asymp; 305 kJ/mole, Behrens \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e), see Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. These findings point to a different mechanism for removal of hydrogen defects. If the substitution of protons by monovalent cations or the release of hydrogen after oxidation of neighboring ions such as Fe\u003csup\u003e2+\u003c/sup\u003e is not possible, then oxygen must be additionally removed for charge compensation. This is associated with a strong perturbation of the crystal structure and thus requires high activation energies. In the case of alkali-free dehydration of sanidine, such a mechanism is supported by cracking during complete hydrogen removal induced by internal strain (Behrens \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eOn the other hand, the activation energy appears to be significantly lower for D/H exchange under hydrothermal conditions (130 kJ/mole at a water pressure of 2 kbar, Behrens (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e)). On the other hand, from the higher diffusivities and the low activation energies under hydrothermal conditions, one cannot necessarily conclude that water molecules have a higher mobility than protons in the feldspar structure. As already mentioned, the exchange rate is determined not only by the mobility of the defects but also by their concentration. In isotope exchange experiments at 6\u0026ndash;8 kbar, an increase in hydrogen content of more than 50% was observed in some cases by Behrens (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e). The incorporated water has a very high mobility, as shown by the chemical diffusion coefficients for H\u003csub\u003e2\u003c/sub\u003eO determined from the sorption profiles (see Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, data 9).\u003c/p\u003e \u003cp\u003eIn Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e the diffusion data for hydrogen species are compared to alkali diffusion data for Eifel sanidine. It is striking that \u003csup\u003e22\u003c/sup\u003eNa tracer diffusion (Wilangowski et al. (\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) are very similar to \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eH\u003c/em\u003e\u003c/sub\u003e values determined by dehydrogenation of Eifel sanidine in presence of alkali chloride, while \u003csup\u003e43\u003c/sup\u003eK tracer diffusion coefficients (Hergem\u0026ouml;ller et al. (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) are several orders of magnitude smaller. My experiments with different alkali chlorides give consistent results which shows that the external source of alkali is not rate-controlling for dehydrogenation. The short Na/K exchange profiles when using sodium chloride as the alkali source support this thesis. As discussed in my previous paper (Behrens \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e), the strongly bound \u003cem\u003etype OH II defects\u003c/em\u003e in the sanidine are likely protons incorporated on interstitial sites as a charge balance for excess aluminum. Replacement by alkalis requires energy, and the smaller sodium ions are preferred over the larger potassium ions. Accordingly, the diffusion out of hydrogen requires a corresponding counterflux of sodium. However, since in the annealed sanidine the Na concentration is about 30 times higher than the H concentration, the local charge balance could be rapidly established even with an order of magnitude slower Na diffusivity.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eHydrogen defects can be incorporated as thermodynamically stable species in feldspars that crystallize at high temperatures in hydrous magmas. The nature of the defects and, hence, their stability in subsequent processes depends on the composition of the magmas. Thus, depending on the conditions, protons may be incorporated together with aluminum as a substitute for silicon, or water molecules may be associated with vacancies in the structure.\u003c/p\u003e \u003cp\u003eThe new experiments support the hypothesis that strongly bound, isolated OH defects in feldspars can be removed by diffusion of protons if alkalis are present for charge balance. If this is not the case, massive disruption of the crystal framework is required for dehydration, since oxygen ions must be removed in addition to protons to balance the charge. Under these conditions, the transport of hydrogen will preferentially occur by diffusion of neutral water molecules. If the defect centers are not irreversibly destroyed after hydrogen release, then they act as traps for protons and water molecules, respectively. As a result, the dehydration rate decreases with increasing degree of hydrogen removal.\u003c/p\u003e \u003cp\u003eThe situation is different for feldspars in which the hydrogen was incorporated under metamorphic conditions, for example during recrystallization. Here, the destruction of hydrogen defects is typically irreversible, and the mobility of released water molecules is not reduced by temporary binding in the defect centers. This appears to be the case with the Kristallina andesine studied by Kronenberg et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1996\u003c/span\u003e).\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgement.\u003c/h2\u003e \u003cp\u003eMy special thanks go to Otto Dietrich for the excellent preparation of feldspar sections. I thank Fabian Hergem\u0026ouml;ller for stimulating thoughts on proton and alkali diffusion. The research was supported by the researcher unit FOR2881 of the German Science foundation (DFG).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBaschek G, Johannes W (1995) The estimation of NaSi-CaAl interdiffusion rates in peristerites by homogenization experiments. Eur J Mineral 7:295-307\u003c/li\u003e\n\u003cli\u003eBehrens H (2006) Water diffusion in silicate glasses and melts. Adv Sci Tech 46: 79-88\u003c/li\u003e\n\u003cli\u003eBehrens H (2021a) Hydrogen defects in feldspars: defect properties and implications for water solubility in feldspar Phys Chem Minerals, 48: 8\u003c/li\u003e\n\u003cli\u003eBehrens H (2021b) Hydrogen defects in feldspars: D/H interdiffusion and H\u003csub\u003e2\u003c/sub\u003eO diffusion in alkali feldspars. Phys Chem Minerals, 48: 27\u003c/li\u003e\n\u003cli\u003eBehrens H, Zhang Y (2009) H\u003csub\u003e2\u003c/sub\u003eO diffusion in peralkaline to peraluminous rhyolitic melts. Contrib Mineral Petrol 157, 765-780\u003c/li\u003e\n\u003cli\u003eBehrens H, Zhang Y, Xu Z (2004) H\u003csub\u003e2\u003c/sub\u003eO diffusion in dacitic and andesitic melts. Geochim Cosmochim Acta 68, 5139-5150\u003c/li\u003e\n\u003cli\u003eBeran A, LiIbowitzky E (2006) Water in Natural Mantle Minerals II: Olivine, Garnet and Accessory Minerals. Rev Mineral Geochem 62:169-191\u003c/li\u003e\n\u003cli\u003eBoltzmann L (1894) Intergration der Diffusionsgleichung bei variablen Diffusions-koefficienten. Ann Phys 53: 959-964\u003c/li\u003e\n\u003cli\u003eCherniak DJ (2010) Cation diffusion in feldspars. Rev Mineral Geochem 72:691-733\u003c/li\u003e\n\u003cli\u003eChristoffersen R, Yund RA, Tullis J (1983) Interdiffusion of K and Na in alkali feldspars: homogenization experiments: Am Mineral 68(11\u0026ndash;12): 1126-1133.\u003c/li\u003e\n\u003cli\u003eCrank J (1975). The mathematics of diffusion, 2nd edition. Clarendon press, Oxford 414 pp.\u003c/li\u003e\n\u003cli\u003eDerdau D, Freer R, Wright K (1998) Oxygen diffusion in anhydrous sanidine feldspar. Contrib Mineral Petrol 133:199-204\u003c/li\u003e\n\u003cli\u003eDoremus RH (1998) Comment on \u0026ldquo;Stationary and mobile hydrogen defects in potassium feldspar\u0026rdquo;. Geochim Cosmochim Acta 62:377-378\u003c/li\u003e\n\u003cli\u003eElphick SC, Graham, CM, Dennis PF (1988) An ion microprobe study of anhydrous oxygen diffusion in anorthite: a comparison with hydrothermal data and some geological implications. Contrib Mineral Petrol 100:490-495\u003c/li\u003e\n\u003cli\u003eFarver JR, Yund RA (1990) The effect of hydrogen, oxygen and water fugacity on oxygen diffusion in alkali feldspar. Geochim Cosmochim Acta 54:2953-2964 \u003c/li\u003e\n\u003cli\u003eFarver JR (2010) Oxygen and hydrogen diffusion in minerals. Rev Mineral Geochem 72:447-507\u003c/li\u003e\n\u003cli\u003eFreer R, Wright K, Kroll H, G\u0026ouml;ttlicher J (1997) Oxygen diffusion in sanidine feldspar and a critical appraisal of oxygen isotope-mass-effect measurements in non-cubic materials. Phil Mag A75:485-503\u003c/li\u003e\n\u003cli\u003eGiletti BJ, Semet MP, Yund RA (1978) Studies in diffusion-III. oxygen in feldspars: an ion microprobe determination. Geochim Cosmochim Acta 42:45-57\u003c/li\u003e\n\u003cli\u003eGrove TL, Baker MB, Kinzler RJ (1984) Coupled CaAl\u0026ndash;NaSi diffusion in plagioclase feldspar: experiments and applications to cooling rate speedometry. Geochim Cosmochim Acta 48: 2113-2121 \u003c/li\u003e\n\u003cli\u003eHergem\u0026ouml;ller F, Wegner M, Deicher M, Wolf H, Brenner F, Hutter H, Abart R, Stolwijk NA (2017) Potassium self‑diffusion in a K‑rich single‑crystal alkali feldspar. Phys Chem Minerals 44:345\u0026ndash;351\u003c/li\u003e\n\u003cli\u003eJohnson EA (2006) Water in nominally anhydrous crustal minerals: Speciation, concentration, and geologic significance. Rev Mineral Geochem 62:117-154\u003c/li\u003e\n\u003cli\u003eJohnson EA, Rossman GR (2003) The concentration and speciation of hydrogen in feldspars using FTIR and \u003csup\u003e1\u003c/sup\u003eH MAS NMR spectroscopy. Am Mineral 88:901-911\u003c/li\u003e\n\u003cli\u003eJohnson EA, Rossman GR (2004) A survey of hydrous species and concentrations in igneous feldspars. Am Mineral 89:586-600\u003c/li\u003e\n\u003cli\u003eJohnson EA, Rossman GR (2013) The diffusion behavior of hydrogen in plagioclase feldspar at 800\u0026ndash;1000 \u0026deg;C: implications for reequilibration of hydroxyl in volcanic phenocrysts. Am Mineral 98:1779-1787\u003c/li\u003e\n\u003cli\u003eKronenberg AK, Yund RA, Rossman GR (1996) Stationary and mobile hydrogen defects in potassium feldspar. Geochim Cosmochim Acta 60:4075-4094\u003c/li\u003e\n\u003cli\u003eKronenberg AK, Yund RA, Rossman GR (1998) Reply to the comment by Robert H. Doremus on \u0026ldquo;Stationary and mobile hydrogen defects in potassium feldspar\u0026rdquo;. Geochim Cosmochim Acta 62: 379-382.\u003c/li\u003e\n\u003cli\u003eMatano C (1932-3) The relation between the diffusion coefficients and concentrations of solid metals (the nickel-copper system). Jpn J Phys 8: 109-113\u003c/li\u003e\n\u003cli\u003eNeusser G, Abart R, Fischer F-D, Harlov D, Norberg N (2012) Experimental Na/K exchange between alkali feldspar and an NaCl-KCl salt melt: chemically induced fracturing and element partitioning. Contrib Mineral Petrol 164:341-358\u003c/li\u003e\n\u003cli\u003ePetrishcheva E, Rieder M, Predan J, Fischer FD, Giester G, Abart R (2019) Diffusion‑controlled crack propagation in alkali feldspar. Phys Chem Minerals 46:15-262\u003c/li\u003e\n\u003cli\u003eSch\u0026auml;ffer A-K, Petrishcheva E, Habler G, Abart R, Rhede D, Giester G (2014) Sodium-potassium interdiffusion in potassium-rich alkali feldspar II: composition and temperature dependence obtained from cation exchange experiments. Am J Sci 314(9):1300-1318.\u003c/li\u003e\n\u003cli\u003eScheidl K, Sch\u0026auml;ffer A-K, Petrishcheva E, Habler G, Fischer F-D, Schreuer J, Abart R (2013) Chemically induced fracturing in alkali feldspar. Phys Chem Minerals 41: 1-16.\u003c/li\u003e\n\u003cli\u003eSkogby H (2006) Water in Natural Mantle Minerals I: Pyroxenes. Rev Mineral Geochem 62:155-167\u003c/li\u003e\n\u003cli\u003eWilangowski F, Abart R, Divinski SV, Stolwijk NA (2015) Radiotracer Experiments and Monte Carlo Simulations of Sodium Diffusion in Alkali Feldspar: Evidence Against the Vacancy Mechanism. Defect Diffusion Forum 363: 79-84\u003c/li\u003e\n\u003cli\u003eYund RA (1986) Interdiffusion of NaSi-CaAl in Peristerite. Phys Chem Mineral 13:11-16\u003c/li\u003e\n\u003cli\u003eYund RA, Anderson TF (1974) The effect of fluid pressure on oxygen isotope exchange between feldspar and water. Geochim Cosmochim Acta 42:235-239 \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":"physics-and-chemistry-of-minerals","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"pcmi","sideBox":"Learn more about [Physics and Chemistry of Minerals](http://link.springer.com/journal/269)","snPcode":"269","submissionUrl":"https://submission.nature.com/new-submission/269/3","title":"Physics and Chemistry of Minerals","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"igneous feldspar, hydrogen defects, infrared spectroscopy, dehydrogenation, diffusion","lastPublishedDoi":"10.21203/rs.3.rs-2572968/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2572968/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn the first two papers of this series (Behrens 2022a,b), incorporation of hydrogen in the feldspar structure, partitioning of hydrogen between feldspars and gases/fluids and self diffusion of hydrogen in feldspars has been discussed, with particular focus on sanidine. Here, the results of reactions between sanidine containing strongly bonded hydrogen defects and (Na,K)Cl are presented. Experiments were performed at ambient pressure at temperatures of 605\u0026ndash;1000\u0026deg;C and hydrogen profiles were measured by IR microspectroscopy. Profiles can be interpreted by an incomplete dehydrogenation at the crystal surface or a strong concentration dependence of hydrogen diffusivity. Both is consistent with hydrogen located on interstitial sites and difficult to substitute by the larger alkali ions. Chemical diffusivities of hydrogen derived from fitting of the profiles or Boltzmann-Matano analysis are similar to self diffusivities determined by D/H exchange experiments. Activation energies are also comparable. Comparison to sodium and potassium diffusion data for sanidine (Wilangowski et al \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e, Hergem\u0026ouml;ller et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) support a mechanism of proton diffusion charge-compensated by Na\u003csup\u003e+\u003c/sup\u003e diffusion for hydrogen removal in the sanidines under dry conditions.\u003c/p\u003e","manuscriptTitle":"Hydrogen defects in feldspars: Alkali-supported dehydrogenation of sanidine","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-02-14 21:03:07","doi":"10.21203/rs.3.rs-2572968/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2023-05-17T17:19:11+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2023-05-14T22:49:58+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2023-04-20T02:59:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"b56872bd-1ec4-427b-aa69-f9192a63a387","date":"2023-04-18T22:42:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"50b52a1f-69f8-4753-9959-0391004aafe5","date":"2023-03-23T17:41:40+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2023-02-15T18:52:28+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2023-02-11T12:50:01+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2023-02-11T12:49:59+00:00","index":"","fulltext":""},{"type":"submitted","content":"Physics and Chemistry of Minerals","date":"2023-02-10T12:35:38+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"physics-and-chemistry-of-minerals","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"pcmi","sideBox":"Learn more about [Physics and Chemistry of Minerals](http://link.springer.com/journal/269)","snPcode":"269","submissionUrl":"https://submission.nature.com/new-submission/269/3","title":"Physics and Chemistry of Minerals","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"0056ef3a-a090-41e4-9ba3-7ceba66df0c8","owner":[],"postedDate":"February 14th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2023-10-16T21:54:01+00:00","versionOfRecord":{"articleIdentity":"rs-2572968","link":"https://doi.org/10.1007/s00269-023-01242-9","journal":{"identity":"physics-and-chemistry-of-minerals","isVorOnly":false,"title":"Physics and Chemistry of Minerals"},"publishedOn":"2023-06-23 21:17:04","publishedOnDateReadable":"June 23rd, 2023"},"versionCreatedAt":"2023-02-14 21:03:07","video":"","vorDoi":"10.1007/s00269-023-01242-9","vorDoiUrl":"https://doi.org/10.1007/s00269-023-01242-9","workflowStages":[]},"version":"v1","identity":"rs-2572968","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2572968","identity":"rs-2572968","version":["v1"]},"buildId":"_2-kVJe1T_tPrBINL-cwx","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}