Organic-rich Miocene mudrocks from the Grabiszyce quarry (southwest Poland) provide a rare natural laboratory for investigating the short-term thermal effects of a basaltic lava flow on immature organic matter (OM) and mineral phases. Samples were collected along a decimeter to meter-scale transect from the basalt-sediment contact and analyzed using organic geochemistry, Rock-Eval, vitrinite reflectance, rock magnetism, and detailed clay mineralogy. Total organic carbon values are the lowest close to the lava contact (similar to 5 wt%) and increase to >10 wt% at distances greater than c.a. 0.3-1 m, consistent with thermal degradation of OM. Vitrinite reflectance increases sharply towards the basalt, from similar to 0.20% Rr to 1.38% Rr within 2-5 cm of the contact, whereas T-max values (425-430 degrees C) remain low or could not be determined near the contact due to very low S2 yields. Hydrogen Indices decrease from 180 mg HC g(-1) TOC in immature samples to as low as 6 mg HC g(-1) TOC at the contact. Molecular compositions show consistent loss of thermally labile compounds and enrichment in stable polycyclic aromatic hydrocarbons towards the lava flow, while unusually unstable biomarkers (e.g., beta beta-hopanes) persist in thermally altered samples, reflecting kinetic inhibition during rapid heating. The preservation limit of tocopherols is constrained to similar to 0.3-0.4% Rr. Rock magnetic parameters show changes consistent with distance towards the lava flow. Enhanced formation of superparamagnetic magnetite within similar to 35 cm of the basalt is documented, which correlates well with PAH concentrations and biomarker maturity ratios. Furthermore, clay mineralogy reveals no illitization of smectite, despite elevated organic maturity, indicating a fluid-deficient, short-lived thermal pulse that is insufficient to trigger potassium-driven mineral reactions. These results demonstrate that OM and magnetic mineralogy respond more rapidly than clay minerals to brief, high-temperature events, highlighting a fundamental decoupling between organic and inorganic maturity indicators under contact metamorphic conditions.

Hydrogen (H) isotopic compositions of clay minerals have long been used to reconstruct the climate, environment, and hydrology of Earth’s past, and more recently, on Mars. Though these approaches often rely on the assumption that primary H isotopic composition is preserved through time, rates and mechanisms of post-formational isotopic exchange with secondary waters are not well understood. The kinetics of isotopic exchange are critical to assessing the fidelity of paleoclimatic reconstructions from clay minerals, with further applications in geoengineering such as the use of clays to effectively immobilize tritium from radioactive waste. Here, we use controlled laboratory experiments coupled with isotopic and mineralogical analyses to assess the kinetics of H exchange in kaolinite–water and montmorillonite–water systems, demonstrating that self-diffusion dominates in the absence of mineralogical alteration – we propose this occurs via hydrogen hopping. We find that H exchange between montmorillonite and water exhibits a lower activation energy (18.7 kJ/mol) than kaolinite (35.0 kJ/mol), likely due to increased structural defects and isomorphous substitutions lowering the energetic barrier for self-diffusion of H. We present an improved model of isotopic exchange kinetics that accurately captures the observed behavior, enabling more realistic extrapolations to geological timescales. The moving r model incorporates a time-dependent rate of exchange that accounts for the progressive decline in exchange rates observed in many heterogeneous systems. By applying this to our results and building a forward model, we demonstrate that some amount of isotopic exchange between secondary fluid and clay mineral H is inevitable over geological timescales – however, this may be analytically indistinguishable and insignificant within paleo-reconstructions depending on system specifics (e.g., the difference in isotopic composition between primary and secondary fluids). We use clay mineral-based thermometry as a case study to quantitatively evaluate the ramifications of post-formational H isotope exchange on paleoclimate reconstructions.

Bentonite is mined globally for use in commercial and industrial applications. In these applications, smectite content and composition are the paramount factors of the bentonite material and control its properties. As bentonite composition and properties can vary significantly over a large mining district or within a single mine, quality control is required including: mineral composition, especially smectite content; cation exchange capacity (CEC); exchangeable cation composition; and smectite crystallochemical features. Differences in bentonite composition locally or over a spatial area stem from the different geological settings present throughout bentonitization. The study aims were to: (1) determine the layer charge (LC) variation of dioctahedral smectite over the Bavarian mining district and within individual mines in the area; and (2) assess the error in smectite content calculations based on CEC data resulting from the actual range of experimentally determined LC values. This information has been missing in the scientific literature, as previous LC methods were laborious or subject to assumptions, making a comprehensive study over a large spatial area impractical. This study employed the use of the recently developed efficient and precise spectroscopic 'O-D method', which enabled the LC measurement of 40 samples from eight mines in the Bavarian bentonite mining district, covering an area of 250 km(2), within the North Alpine Foreland Basin. Results showed LC values calibrated against the alkylammonium method (LC (AAM)) generally ranged between 0.29 and 0.30 eq per formula unit (FU), with only 10% of samples showing LC values >0.31 eq/FU. This narrow LC range has positive implications for the accuracy of determining smectite content calculated from CEC data, during routine quality control of Bavarian and other bentonites. The average error of the CEC-based smectite contents resulting from LC variations was, on average, +/- 3 wt.%.

Retention of the pristine hydrogen isotope composition (expressed as delta D) formed during mineral formation in equilibrium with water is the basis for any paleoenvironmental reconstructions and for tracing mineral reactions using hydrogen (or H and O) isotope composition in kaolinite. Not only do post-formation reactions cause partial equilibration with ambient water, altering the pristine signature, but also result in the decoupling of H and O diffusion. In this study, kaolinite samples of different structural order (expressed, for example, as the Hinckley Index) were tested for their susceptibility to H isotope exchange under D2O vapor in an open system. The H isotope exchange was detected by recording the kaolinite's structural OH and OD stretching mode using infrared spectroscopy. A number of different in-situ and ex-situ testing protocols under diverse temperature ranges (90 degrees C, 125-275 degrees C, 300-700 degrees C), show consistent kinetics and mechanism. In each kaolinite sample, the H isotope exchange rate increases with temperature; however, the major control on the reaction rate is the kaolinite crystallite's structural order, not the particle (agglomerated crystallites) size. The hydrogen isotope composition in kaolinite can be altered independently of oxygen atoms, likely via proton hopping mechanism through structural defects. The H isotope exchange reaction under vapour shows a two-mode kinetics: the time-to-the-quarter (TTTQ; t(1/4)) dependence dominates in the first part of the reaction or in low-temperature reactions, transitioning to the log(10)t (Elovich) relationship and with lower activation energy, in advanced stages and in higher temperatures. Extrapolating the TTTQ model to temperatures < 90 degrees C allows estimating the rate of H isotope exchange between water vapor and kaolinite. The paper presents the prediction of preservation of pristine delta D signature and thus reliability of using isotope data in kaolinites of different origins and structural order when exposed to pore water / water vapor over geological timescales.

Bentonite barriers in repositories for high-level radioactive waste disposal are expected to interact with Portland cement, which is commonly used during the underground construction of repositories. These reactions occur at a high pH (>12), leading to the formation of XRD-amorphous calcium silicate hydrate (C-S-H) phase and its crystalline analogue, tobermorite, as well as Ca-carbonate, Ca- and Mg-silicate phases, including zeolites. The cation exchange capacity (CEC) is a key parameter frequently used to evaluate bentonite's ability to swell and adsorb cationic species released from nuclear waste over its lifespan. In addition to being influenced by the residual smectite content, newly formed phases generated during bentonite-cement reactions may also contribute to the bulk CEC. Moreover, the extremely high pH of the reacted material when in contact with water raises questions about the reliability of CEC measurements under such conditions. In this study, the CEC of both reacted and unreacted bentonite-cement mixtures was tested using hexamminecobalt(III) ([Co(NH3)(6)](3+)) index cation and a spectrophotometric method, under natural pH of the suspension. The extinction of the [Co(NH3)(6)](3+) solution was almost constant even at high pH, up to 12.5. Fresh, unwashed bentonite-cement mixtures showed higher-than-predicted CEC values due to the strong contribution of smectite edge charges which develop at high pH. Extended washing of the material via dialysis lowered the pH and reduced the excess Ca2+ cations content. After dialysis (up to 4 days), the CEC of bentonite-cement mixtures increased and exceeded the CEC predicted from the smectite content alone. In long-term bentonite-cement reactions (3-36 months, at 80 degrees C), an initial equilibration occurred rapidly, within days, followed by the slow crystallization of tobermorite at the expense of smectite. However, the large effective dimension of the [Co(NH3)(6)](3+) cation prevents it from entering the tobermorite structure (as it does to zeolite structures), which results in insignificant contribution of tobermorite to the bulk CEC. All CEC values measured with the [Co(NH3)(6)](3+) index cationion reacted bentonite-cement samples were found to be a combination of the exchange by residual smectite and the contribution from new-formed C-S-H phase. The C-S-H phase develops its own surface CEC, which is highly variable and increases as its structural Ca/Si ratio decreases. Soda-soluble silica present in the raw bentonite lowers the Ca/Si ratio in the newly formed C-S-H phase, thereby increasing the CEC of the bulk sample. Also, extensive leaching of Ca2+ cations in an open system during cement maturation decreases the Ca/Si ratio in the newly formed C-S-H phase, enhancing its CEC contribution. In conclusion, CEC measurements on reacted bentonite-cement mixtures must be interpreted with caution as they likely reflect not only the residual smectite content but also the contributions from newly formed C-S-H phases and other reaction products.

This study investigated the rehydration kinetics of homoionic model smectites and various bentonites following dehydration, addressing gaps in the understanding of how both dehydration and rehydration temperature and the type of interlayer cation affect this process, critical in basic studies involving swelling clays and in their industrial applications. The results showed that higher dehydration temperatures reduced the extent of rehydration and were correlated with a decrease in layer charge, particularly in smectites exchanged with divalent cations. This was linked to the fixation of small interlayer cations, resulting in the collapse of high-charge layers and, therefore, slower reaction progress. Smectites exhibited fast initial rehydration followed by slow equilibration stage. Under typical laboratory conditions (20 degrees C, RH = 30 %), 25 % of water was readsorbed within several minutes whereas 90 %- within four hours, emphasizing the necessity to prevent rehydration during experiments to ensure accuracy of various measurements. Regardless of the experimental conditions, reaction profiles were fitted to contraction and 2D/3D diffusion kinetic models. The activation energy for rehydration (similar to 50-70 kJ/mol) was comparable to that of dehydration, reflecting similar energy barriers in overcoming adsorption/desorption of tightly bound water. Notably, bentonites exhibited rehydration behaviour similar to that of smectites, with the kinetics also influenced by the layer charge of the smectites. Similarities in behaviour of model smectites and bentonites will aid in predicting the long-term rehydration behaviour of bentonite barriers in high-level radioactive waste underground repositories.

In order to track structural stable hydrogen (H)- and oxygen (O)-isotope exchange between smectite and water over a thermal gradient, samples were studied from the Alternative Buffer Materials (ABM) 2 and 5 experiments conducted at the Aspo Hard Rock Laboratory in Sweden for five different bentonite materials. The ABM experiments were designed to investigate the stability of different bentonite materials of interest as buffers around canisters containing spent nuclear fuel in long-term underground repositories. The ABM2 and 5 experiments consisted of compacted bentonite discs stacked vertically around a central heating system in a host-rock borehole for several years, with this configuration intended to simulate heating that occurs during radionuclide decay. The ABM experiments provided a unique opportunity to track isotopic exchange between smectite and water in a long-term, imperfect, natural but still semi-controlled environment. Clear differences were observed for Hisotope compositions between the ABM2 and ABM5 packages. O-isotope compositions showed similar trends between the two ABM packages. In both ABM packages H- and O-isotope exchange was more progressed at higher temperatures but the mechanisms for H- and O-isotope exchange proceeded differently. H-isotope compositions nearly reached modeled equilibrium values at temperatures between 130 and 150 degrees C in the absence of any smectite alteration, which suggests proton exchange as the exchange mechanism. The percentage of exchange calculated for oxygen showed that it was far from reaching equilibrium and was more resistant to exchange than hydrogen. A dissolution/re-precipitation mechanism involving fine smectite most simply explains calculated percentages of oxygen exchanged that range between initial and modeled equilibrium O-isotope compositions.

The recent and efficient spectroscopic, "O-D (using D2O water) method", for layer charge (LC) determination of materials containing smectitic, wettable surfaces was used to identify subtle LC changes of thermally reacted samples (between similar to 80 and 250 degrees C) from the Alternative Buffer Material (ABM) tests 2 and 5 conducted at the Aspo Hardrock Laboratory (HRL), Sweden. The tests were in situ intermediate-scale field experiments designed to simulate an engineered barrier around canisters containing radioactive waste within a deep geological repository. The tests consisted of several different bentonites compacted into 31 (ABM2) and 30 (ABM5) discs, stacked vertically around a central heater, and emplaced in a crystalline rock borehole in the HRL. Previous investigations of the ABM2 and 5 bentonites did not reveal specific mineralogical reactions of smectites, which may be observed at temperatures above 100 degrees C. The data presented in the present study, however, showed LC changes, in the ABM5 test, within the individual bentonite discs along a thermal gradient from the outside edge of the discs toward the heater contact, at which the temperature reached a maximum of 250 degrees C. Considerably less LC changes were observed for the bentonites in ABM2, in which heating reached a maximum of only similar to 140 degrees C. Moreover, the LC data presented in the present study were found to be consistent with previously obtained cation exchange capacity (CEC) results and explained the observed changes in CEC along the thermal gradients across the discs. Both CEC and LC displayed a strong decreasing trend toward the heater contact in ABM5, and very weak increases toward the heater contact in ABM2. Of the several bentonites used in the tests, those with an initially higher LC, generally showed the greatest decrease of apparent LC after being reacted at high temperatures in the ABM5 package. The bentonites with an initially low to medium smectitic LC, appeared to undergo the least changes at the high temperatures achieved in the ABM5 package. The observed decrease of apparent LC is interpreted as resulting from the adsorption of interlayer cations at the siloxane surface, possibly in the ditrigonal cavity of the tetrahedral sheet, causing partial neutralization of the negative charge of the 2:1 layer. The minute increase in LC in the ABM2 package also showed a correlation with CEC but was within the measurement uncertainty.

The aim of the present study was to examine changes in the layer charge (LC) of smectite heated for an extended period of time under two contrasting conditions: wet (hydrothermal experiments) and dry (oven heating). Understanding smectite's behavior at elevated temperatures is crucial, e.g., in the case of high-level radioactive waste storage facilities, where bentonites (smectite-rich rocks) are planned to be used as one of the barriers. A set of two montmorillonites and one beidellite in four cationic forms, Mg2+, Ca2+, Fe2+, and K+, was heated at 100 or 200 degrees C for up to 157 days. In addition, LC regeneration tests involving cation exchange and hydrothermal treatment of oven-dried samples were performed. LC was measured using the spectroscopic OD method, whereas X-ray diffraction was used to verify the changes detected by the OD method. In the dry-heating experiments, most samples showed stable LC values up to 146 days of heating, except the LC decrease for Fe2+ and Mg2+exchanged montmorillonite, which was interpreted as resulting from the formation of inner-sphere complexes and/or penetration of these cations into the octahedral sheet. The LC values of samples after hydrothermal experiments were more variable, possibly due to the cation exchange, and dissolution and reprecipitation processes. In the case of Fe2+-exchanged samples reacted hydrothermally at 200 degrees C, kaolinite was formed at the expanse of smectite, and collapsed interlayers formed in K+-exchanged samples reacted under the same conditions. Cation exchange and hydrothermal treatment were not always successful in restoring the initial LC of samples, however, hydrothermal rehydration reaction was efficient in the case of samples dried at 100 degrees C. The paper presents the mechanism of interlayer cations fixation and liberation, which change the properties of smectite-rich bentonite barriers for nuclear waste disposal under partially dry vs. water-saturated conditions. Reversible after drying at 100 degrees C and irreversible after 200 degrees C cation fixation match common temperature limits of bentonite exposure to radioactively-produced heat.

Three types of smectite with various crystallochemical compositions and interlayer cations as well as kaolinite and illite were fired at 800 degrees C and 650 degrees C and tested for their rehydroxylation's (RHX) potential and kinetics in the presence of water vapor at 200 degrees C-350 degrees C. A dehydroxylated structure of 2:1 Al-rich mineral with the interlayer pillared by a large cation (like K+ or Cs+) is wide enough to allow H2O diffusion which results in advanced RHX, that is, restoring up to several tens of percent of the original OH content. Such 2:1 layer structures (beidellite, illite) do not follow the time-to-the-quarter (TTTQ) kinetics during RHX and show a non-Arrhenius behavior for isothermal RHX. Because TTTQ kinetics is assumed in an RHX dating of ceramic artifacts in archaeometry, fired-clay ceramics prepared from material dominated by Al-rich 2:1 minerals was found unfeasible for RHX dating.Kaolinite and Mg2+- or Ca2+-exchanged smectites show the Arrhenius behavior and follow the TTTQ kinetics of RHX, resulting in apparent Ea increase with the progress of reaction (up to alpha similar to 0.2) of similar to 20-50 kJ/mol in beidellite and 15-35 kJ/mol in montmorillonite. We suggest using 'hydroxylation' rather than 'rehydroxylation' for structurally-disordered fired clays gaining OH groups in an unknown position.

Many kaolinites are known to exhibit limited intercalation capacity which affects their usage. Some reports have linked this lack of reactivity to particular structural features or to slow kinetics; others recommended increasing intercalation temperature as a remedy. The purpose of the current study was to investigate systematically the N-methylformamide (NMF) intercalation capacity of three kaolinites differing in layer stacking order (KGa-1b, KGa-2, and Imerys Hywite Alum) in the 5–150°C temperature range. Near-infrared spectroscopy (NIR) was employed to record the full kinetics of intercalation in closed systems with excess NMF. Increasing intercalation temperature accelerated the reaction, but the NMF uptake decreased and eventually vanished. Complementary thermogravimetric analysis (TGA) confirmed this unexpected trend. All kaolinites exhibited the same behavior, but the amount of inert material was in the order of their stacking-fault concentration at all temperatures: KGa-2 > Hywite > KGa-1b. Subjecting the samples to stepwise temperature changes showed that, once intercalated, the NMF could not deintercalate and was removed from equilibrium with the surrounding fluid. Thus, intercalation capacity was not a unique feature of the material because it depended on thermal history. As stacking order and thermal history had no detectable effect on the NMF-hosting environment, the unusual temperature dependence was attributed tentatively to the adverse effect of temperature on the adsorption of NMF on the edges of the crystallites, which is a prerequisite for intercalation.

The Central Asian Orogenic Belt constitutes the Kazakhstan and Mongolian oroclinal collages located between the Tarim–North China collage (TNC) to the south and the Siberian craton to the north (Fig. 1a) [1]. While the oroclinal bending of the western Kaza-khstan collage is widely accepted by the paleomagnetic and tectonic community, the oroclinal bending model of the eastern Mongolian collage is disputed [2,3]. Despite differences, all studies report: (i) difference in Late Paleozoic paleolatitudes between Siberia and the peri-Siberian terranes accreted during Cambrian–Ordovician [4,5], the Amuria and the TNC; (ii) Permian consumption of the Paleo-Asian Ocean (PAO) and (iii) Permian–Triassic counterclockwise rotations of the latter two blocks during scissor-like closure of the Mongol OkhotskOcean(MOO)tothenorth[6]. Recentlyacquired[2,3]andunpublished paleomagneticdatashowaconsistent evolutionforthePAOOrdovicianstrata oftheMongolianAltaiWedge(MAW), theDevonianandCarboniferousoceanic rocksoftheTrans-AltaiZone(TAZ),the northerlyProterozoicLakeZone(LZ) andtheMongolianPrecambrianblocks (MB)constitutingtheAmuria(Fig.1a

Smectites are a group of minerals traditionally analyzed by thermal methods due to their exceptionally large adsorbed-water contents and the presence of OH groups, which makes them unique among all common soil- and rock-forming minerals. The dehydration reaction of smectite is a low-temperature endothermic effect that ends typically below 200°C. Although the removal of bulk interlayer water requires activation energy ( E a ) of just above 30 kJ/mol, the removal of the last few H 2 O molecules attached strongly to interlayer cations requires E a > 100 kJ/mol. Dehydroxylation is the loss of structural OH groups that proceeds as evolution of H 2 O molecules out of the smectite structure and occurs in the 300–900°C range. In trioctahedral species, dehydroxylation is combined with recrystallization and proceeds usually at > 700°C. In dioctahedral species, the temperature of dehydroxylation is controlled by the type of octahedral vacancy, having trans-vacant and cis-vacant distinguished by the boundary at ~ 600°C, and by the octahedral cation–OH bond strength, following the order Mg > Al > Fe. The E a of dehydroxylation correlates linearly with the temperature of maximum dehydroxylation; from > 170 kJ/mol for Cs + -exchanged beidellite and nontronite, through ~ 300 kJ/mol in Mg-rich montmorillonite, to > 500 kJ/mol in trioctahedral saponite. Dehydration and dehydroxylation of smectites can be accompanied by a number of other phenomena, such as dehydrogenation or defluorination. At high temperatures, smectite amorphization and recrystallization occurs. Unless amorphized and/or recrystallized, smectites can undergo rehydration and rehydroxylation, which are opposite reactions to dehydration and dehydroxylation, respectively. This review discusses the details of the above-mentioned thermal reactions of smectites, focusing on thermogravimetric methods, evolved gas analysis, and structural alterations. Factors affecting the accuracy and precision of thermal analysis of smectite are discussed along with examples of best laboratory practices. The paper also provides the most recent description and critical evaluation of smectite reaction kinetics.

Layer charge is an important property of 2:1 phyllosilicates originating from isomorphic substitutions in the structure, from vacancies in the octahedral sheet and from unsatisfied bonds at the edges of the crystals. It is of particular interest in the case of smectites because it affects important properties of this mineral. Several methods have been proposed for determining the layer charge of smectites, namely the structural formula method (SFM), the alkylammonium method (AAM), the NH4-method, the K-saturation method, and the O-D method. Most of these methods have been used extensively in the past and they all have advantages and shortcomings. The SFM and the AAM are based on different principles and are considered as primary methods. They have been used for a long time but they are time consuming and they provide contrasting layer charge values, with the AAM yielding consistently lower layer charge values than the SFM, especially for low-charge smectites. The remaining methods have been developed more recently and have been calibrated over their primary counterparts. They are applied easily and are capable of producing a large amount of data within a short time. In this sense they can be used both for geological interpretations and for assessment of bentonite deposits at an industrial scale.

Atmospheric water adsorbed onto clay mineral surfaces, of smectite group minerals in particular, can contaminate hydrogen released from structural hydroxyl groups during hydrogen-isotope analysis. Interlayer cation composition and cation hydration enthalpy, in particular, can affect the magnitude of this excess hydrogen yield and hence the hydrogen isotope composition (delta 2H) of smectites. To evaluate this problem, delta 2H of different cationsaturated (Ca2+, Na+, K+), dried forms of six smectite standards from the Clay Minerals Society Source Clays were measured using a modified sample drying and on-line High-Temperature-Conversion-Elemental-Analysis (TCEA) Continuous-Flow-Isotope-Ratio-Mass-Spectrometry (CF-IRMS) protocol. More negative interlayer cation hydration enthalpies (Ca2+ > Na+ > K+) led to higher residual adsorbed water contents, which produced poorer delta 2H reproducibility for the determination of smectite hydroxyl hydrogen. The lowest adsorbed water contribution and the most reproducible and possibly accurate delta 2H for smectite hydroxyl hydrogen was obtained for Ksaturated smectites dried for both 4 and 24 h at 220 degrees C prior to isotopic analysis and transferred to a "Zero-Blank" autosampler in <= 2.5 min. This approach provided the lowest measurement error for hydroxyl delta 2H and facilitated much greater sample throughput than classical methods for smectitic clays. This study proposes a protocol for hydroxyl delta 2H determination in smectitic clay minerals, and reveals the effect of H-isotope fractionation of adsorbed water during sample preparation.

Phyllosilicates may trap hydrogen (H-2) in the crust, but they may also produce it through various processes, including oxidative dehydrogenation. The dehydrogenation temperature depends on the type and composition of the phyllosilicates considered, but it may be as low as 300 degrees C. Here, we document the release of H-2 and CO during thermal treatment of chloritite (300 degrees C) and talc (500 degrees C) from the Trimouns deposits (Eastern Pyrenees, France). Thermal release of gases coupled to stable isotope analysis has been used to recover and characterize H-2 and CO, the two detected gases. Hydrogen content may be as high as 7 ppm with delta D-H2 values ranging from -258 parts per thousand to - 224 parts per thousand for sub-pure chloritite and - 140%0 for pure talc. CO content ranges between 3 ppm and 35.3 ppm with very homogeneous delta C-13(CO) values between -27.6 parts per thousand and - 25.7 parts per thousand. This study supports the idea that H-2 was produced during experiments by dehydrogenation. The origin of CO remains enigmatic, but its carbon isotope composition suggests a link to the few amounts of graphite documented in chloritite and talc from the deposit. This work also reports extensive hydrogen isotope fractionation between H-2 produced by dehydrogenation and both talc and chloritite. Dehydrogenation of phyllosilicates is a potential source term of H-2 in numerous magmatic-hydrothermal settings and must thus be accounted for in the budget of the H-2 geochemical cycle.

Both natural and anthropogenically-generated hydrogen gas occurs in sedimentary rocks and geotechnical barriers. Due to high-pressure conditions, a significant portion of H-2 can be physisorbed in available micropores of clay minerals, which have been proven to control the gas adsorption properties of rocks. This study investigates H-2 adsorption on clay minerals naturally occurring in rocks, by combining the high-pressure H-2 experiments with sample characterization analyzed using low-pressure adsorption of N-2 and CO2, and structural analysis using X-ray diffraction. We found that H-2 adsorption depends strongly on the mineral texture, which is not related directly to its structure. Most of H-2 is adsorbed in the micropores accessible to CO2. H-2 intercalates in the smectitic interlayers with a basal spacing larger than 10.8 angstrom. The density of adsorbed H-2 is about double that of free H-2 gas under given pressure and temperature, effectively increasing the gas storage capacity of rocks. (C) 2022 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

Physical interactions of molecular hydrogen (H 2 ) with rocks and minerals have become an aim of multi-disciplinary studies after H 2 gas was identified in various geological environments