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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 (Ea) of just above 30 kJ/mol, the removal of the last few H2O molecules attached strongly to interlayer cations requires Ea > 100 kJ/mol. Dehydroxylation is the loss of structural OH groups that proceeds as evolution of H2O 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 Ea 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.

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.

Clay mineral-bearing mudstones are a prominent component of ancient fluvial-lacustrine deposits, 100s of meters thick, documented by the Mars Science Laboratory (MSL) rover, in Gale crater, Mars. Most of the clay minerals documented by MSL are hypothesized to have formed in situ, at or close to the time of deposition ∼3.5 Ga ago, by aqueous alteration of basaltic detritus. Here we study the mechanisms, controls, and timescales of clay mineral authigenesis in a series of lakes with a wide range of water chemistries from the Western Volcanic District, Victoria, SE Australia, as an analog to the Gale crater mudstones. X-ray diffraction (XRD) analysis reveals that the sediments of most of the Western Volcanic District lakes studied contain mixtures of kaolinite, illite, mixed-layer illite-smectite (I-S), and dioctahedral smectite clay minerals. Comparisons of this mineral assemblage with regional soils and creek bedload material confirm previous assertions of significant inputs of detrital clay minerals into the lakes. A trioctahedral clay mineral phase is also detected, making up to 39 wt.% of bulk sediments. The abundance of trioctahedral clay minerals correlates with contemporary lake hydrology and proxies for past lake water Mg concentration. This indicates in situ formation of trioctahedral clay minerals by the uptake of Mg and Si from lake waters and pore fluids at rates determined by local physico-chemical conditions. Examination of crater lake sediments, where detrital clay mineral input is minimized, demonstrate that neoformed trioctahedral clay minerals are poorly crystalline trioctahedral smectites. Neoformation of trioctahedral smectites also occurs in lakes where detrital clay minerals are more abundant. However, an additional authigenic transformation process is indicated by the proportions of Mg and Si added to detrital clay minerals as well as evidence for the uptake of K from lake waters. The transformation process probably involved the incorporation of Mg into the octahedral sheets of detrital clay minerals, leading to irreversible uptake of K into interlayer sites (illitization). The distribution of trioctahedral smectites and radiocarbon ages from sediment cores show that clay mineral authigenesis occurred before sediment consolidation, on timescales of years to 100s of years. These results support syndepositional interpretations of analogous Mg-rich clay minerals documented by MSL, and their use as proxies for chemical conditions in ancient Gale lakes. In comparison with the Western Volcanic District lakes, clay mineral-bearing lacustrine mudstones from Gale crater exhibit only modest chemical weathering of basaltic detrital materials and rarely contain carbonate minerals in quantities detectable by XRD. These observations highlight significant differences in weathering regimes and regolith mineralogy on ancient Mars that could be linked to lake catchment geomorphology, climate, atmospheric CO2 content, and the absence of biotic processes on Mars.

Clay minerals, along with organic matter, control methane (CH4) gas adsorption properties of sediments and sedimentary rocks. These natural, high-pressure environments of sediments and sedimentary rocks are particularly important in view of hydrocarbon exploration and global warming. The present study investigates swelling of smectites in dry methane at high pressure (1–60 bar), using in situ high-pressure X-ray diffraction and Grand Canonical Ensemble Monte Carlo simulations. The results show that CH4 molecules actively expand the interlayers of divalent cation-exchanged smectites at elevated pressure (e.g., from initial 12.1 Å at 1 bar of N2 to 13.4 Å at 60 bar of dry CH4 of Mg-montmorillonite and from 11.8 Å at 1 bar of N2 to 12.1 Å at 60 bar of dry CH4 of Ca-montmorillonite). The swelling is observed when the interlayer cations are hydrated at the appropriate level of H2O content adsorbed primarily around the interlayer cations. The H2O configuration leaves CH4-accessible interlayer space outside the coordination sphere of the cations. Higher amounts of H2O in the interlayer act as a steric hindrance for CH4 and effectively decrease CH4 intercalation, whereas insufficient H2O content produces interlayer contraction preventing CH4 from entering the interlayer. The swelling was not observed in monovalent cation-exchanged smectites. Adsorption isotherms measured using overdried and fully contracted smectite allowed the determination of the interlayer adsorption component for the higher hydration state of the same sample. The swelling of smectite at an elevated pressure of CH4 is reversible and is driven by energetic factors, whereas entropy change and mechanical work are minor components of the free energy of swelling. The observed phenomenon of smectite swelling explains the occurrence of hysteresis in high-pressure CH4 adsorption isotherms and provides a previously unknown component to methane interaction with sedimentary rocks.

[Abstract The diagenetic history of the Ediacaran sedimentary rocks in the East European Craton (EEC) over the area extending from Arkhangelsk (Russia) in the north to Podolia (Ukraine) in the south was revealed by means of the XRD characterization and K–Ar dating of clay fractions, mudstone porosity measurements and organic geochemistry investigations. Mudstone porosity measurements produced direct evidence of shallow maximum burial of the Ediacaran sediments on the craton (Russia, Lithuania, Belarus, Volyn), not exceeding 1.5 km, and much deeper burial at the cratonic margin, in Podolia and Poland. In general, illitization of smectite and biomarker indices indicates more advanced diagenesis at the cratonic margin. K–Ar dating of authigenic illite–smectite and aluminoceladonite revealed the Palaeozoic age of mineral diagenesis (ca. 450–300 Ma) both on the craton and its margin, with older ages generally observed in the north. When the maximum palaeotemperatures were evaluated from illite–smectite and biomarkers, based on the calibrations from the conventional burial diagenetic sections, a major mismatch was detected for the cratonic area: 100°C–130°C from illite––smectite and tens of oC lower from the lipid biomarkers. This diagenetic pattern was interpreted as the result of short‐lasting (in ky scale) pulses of potassium‐bearing hot fluids migrating from the Caledonian and Variscan orogens deep in the craton interior, effectively promoting illitization in porous rocks without altering the organic matter. Analogous short pulses of fluids were responsible for numerous diagenetic phenomena, including Mississippi Valley‐Type ore deposits, in the American Midwest, in front of the Appalachians. K–Ar dating indicates that the entire Proterozoic sedimentary cover of the Great Unconformity on the EEC remained untouched by measureable post‐sedimentary changes until the early Palaeozoic, thus for over 1000 My, which is an unprecedented finding., Short‐lasting pulses of potassium‐bearing hot fluids migrated from Caledonian and Variscan orogens deep into the craton interior, effectively promoting illitization without altering the organic matter in Ediacaran sediments of EEC. ]

Thermally induced dehydroxylation and oxidative dehydrogenation drive the thermal decomposition of all Fe2+-containing phyllosilicates. Whereas the former produces H2O gas, the latter results in H2 evolution. Six chlorites representing the Mg-Fe2+ series from clinochlore to chamosite and biotite (as an analog of the 2:1 layer in chlorite) were investigated using thermogravimetry coupled to quadrupole mass spectrometry (TG-MS). A fast-ramp heating protocol was applied to identify if and how hydrogen gas was released from the crystal structure and whether it was quantitatively related to structural Fe2+ content. Dehydroxylation and oxidative dehydrogenation were tested under inert and oxidizing conditions.H2 liberation confirmed the H2 gas production by oxidative dehydrogenation, as shown by an evolution of the m/z = 2 signal for chamosites, Fe-rich clinochlores, and biotite heated under nitrogen gas atmosphere. Along with H2 evolution, H2O (m/z = 18) was released, suggesting that dehydroxylation is a trigger for dehydrogenation. The higher the Fe2+ content in the studied chlorites, the more intense the H2 evolution, thus the higher the H2/H2O ratios. The products of ramp-heating to 1000 °C resulted in varying amounts of newly formed Fe3+ (from 7 to 22%), however, biotite that converted into oxybiotite underwent almost complete oxidation, indicating a stronger tendency of 2:1 layer to dehydrogenation. The observed concurrent, but independent mechanisms of H2 and H2O evolution produced a feasible model of the thermal decomposition of chlorites.Despite O2 availability under oxidizing condition, the Fe2+ oxidation was not driven by attaching oxygen anions to the phyllosilicate structure, but also by dehydrogenation. Hydrogen was not detected using MS for any tested sample heated in synthetic air because any H2 if released was instantaneously combined with external O2, which resulted in an excess H2O MS signal not matched by mass loss on the TG profiles of chamosite and biotite. Without coupling of the evolved gas analysis with TG, the excess H2O produced by dehydrogenation in the O2-bearing carrier gas would result in misleading interpretations of dehydroxylation.Methodological and geological implications of the TG-MS experiments are discussed. The oxidation of Fe2+ in all Fe2+-containing phyllosilicates proceeds with simultaneous H2 gas release that is not dependent on oxygen fugacity nor temperature during the mineral formation. Therefore, the correlation between Fe3+/Fe2+ and remaining hydrogen in the structure must be considered during modeling the conditions that involve chlorite as geothermobarometer. H2 release during heating is proposed as an indicator of oxidative dehydrogenation of Fe2+-bearing minerals on Mars.

This study discusses experiment‐induced alterations of shale rocks collected across the ordovician and silurian boundary at a potential site for carbon dioxide sequestration located in the Polish part of the Baltic Basin. Mudstone samples submerged in brine in custom‐built reactors were subjected to CO2 pressures of 30–35 MPa at a temperature of 80°C for 40 days. After the experiments were completed, the mineralogical composition of the studied rocks, their porosity characteristics, and chemistry of brines were analyzed and compared to the original compositions in order to detect experiment‐induced changes. Comparison of mineral composition before and after the experiments demonstrate a set of subtle but systematic mineral changes. The most conspicuous was a presence of Fe in the form of Fe‐oxyhydroxides that was related to the corrosion of the Fe‐bearing steel pipes supplying the experimental setup with CO2. This effect is important from the perspective of industrial carbon dioxide sequestration. However, the second source of Fe ions in experimental brines was related to the decomposition of chlorites, pyrite, and chloritized biotites. The suggested breakdown of pyrite is in line with the modification of the chemical composition of the experimental brines and observed transfer of S ions to the solution. The appearance of Mg and K ions in the experimental brines was related to the decomposition of chlorites and biotites. In turn, observed transfer of Sr cations to the solution could be attributed to the dissolution of biotites and K‐feldspars. The effects of analogue experiments were confronted with the results of geochemical modeling performed using the PHREEQC program. The numerical modeling not only allowed to reproduce all the main mineral and chemical changes postulated on the basis of our analogue experiment, but also enabled to put constraints on the time scale of occurring geochemical processes. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.

The primary aim of this study was the accurate determination of unit-cell parameters and description of disorder in chlorites with semi-random stacking using common X-ray diffraction (XRD) data for bulk powder samples. In the case of ordered chlorite structures, comprehensive crystallographic information can be obtained based on powder XRD data. Problems arise for samples with semi-random stacking, where due to strong broadening of hkl peaks with k ≠ 3n, the determination of unit-cell parameters is demanding. In this study a complete set of information about the stacking sequences in chlorite structures was determined based on XRD pattern simulation, which included determining a fraction of layers shifted by ±1/3b, interstratification with different polytypes and 2:1 layer rotations.A carefully selected series of pure Mg-Fe tri-trioctahedral chlorites with iron content in the range from 0.1 to 3.9 atoms per half formula unit cell was used in the study. In addition, powder XRD patterns were carefully investigated for the broadening of the odd-number basal reflections to determine interstratification of 14 and 7 Å layers. These type of interstratifications were finally not found in any of the samples. This result was also confirmed by the XRD pattern simulations, assuming interstratification with R0 ordering.Based on h0l XRD reflections, all the studied chlorites were found to be the IIbb polytype with a monoclinic-shaped unit cell (β ≈ 97°). For three samples, the hkl reflections with k ≠ 3n were partially resolvable; therefore, a conventional indexing procedure was applied. Two of the chlorites were found to have a monoclinic cell (with α, γ = 90°). Nevertheless, among all the samples, the more general triclinic (pseudomonoclinic) crystal system with symmetry C1 was assumed, to calculate unit-cell parameters using Le Bail fitting.A detailed study of semi-random stacking sequences shows that simple consideration of the proportion of IIb-2 and IIb-4/6 polytypes, assuming equal content of IIb-4 and IIb-6, is not sufficient to fully model the stacking structure in chlorites. Several, more general, possible models were therefore considered. In the first approach, a parameter describing a shift into one of the ±1/3b directions (thus, the proportion of IIb-4 and IIb-6 polytypes) was refined. In the second approach, for samples with slightly distinguishable hkl reflections with k ≠ 3n, some kind of segregation of individual polytypes (IIb-2/4/6) was considered. In the third approach, a model with rotations of 2:1 layers about 0°, 120°, 240° was shown to have the lowest number of parameters to be optimized and therefore give the most reliable fits. In all of the studied samples, interstratification of different polytypes was revealed with the fraction of polytypes being different than IIbb ranging from 5 to 19%, as confirmed by fitting of h0l XRD reflections.

International audience; Phyllosilicates may trap hydrogen (H2) 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 °C. Here, we document the release of H2 and CO during thermal treatment of chloritite (300 °C) and talc (500 °C) from the Trimouns deposits (Eastern Pyrenees, France). Thermal release of gases coupled to stable isotope analysis has been used to recover and characterize H2 and CO, the two detected gases. Hydrogen content may be as high as 7 ppm with δDH2 values ranging from −258‰ to – 224‰ for sub-pure chloritite and − 140‰ for pure talc. CO content ranges between 3 ppm and 35.3 ppm with very homogeneous δ13CCO values between −27.6‰ and − 25.7‰. This study supports the idea that H2 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 H2 produced by dehydrogenation and both talc and chloritite. Dehydrogenation of phyllosilicates is a potential source term of H2 in numerous magmatic-hydrothermal settings and must thus be accounted for in the budget of the H2 geochemical cycle

Understanding the formation of pore space, especially in low porosity shales (as source rocks and as unconventional resources), is critical to the oil and gas industry, since pores control the space available for hydrocarbon and participate in hydrocarbon transport. We examined 87 Ordovician and Silurian mudstone samples collected from four wells located in the Pomeranian part of the Baltic Basin (northern Poland), one of the primary Polish targets for hydrocarbon exploration. These samples represent the Pelplin, the Pasłęk, the Jantar, the Prabuty, and the Sasino Formations, which still requires more detailed porosity studies. Our study aimed to identify factors controlling porosity development, by applying bulk techniques (organic petrology and TOC analyses, quantitative mineralogy, and porosimetry) as well as nano-to microscale techniques (thin section petrography, electron microscopy). The studied samples are mainly argillaceous mudstones. The results of porosimetry measurements, combined with image analysis, indicate that the pores of all studied rocks are dominated by micropores (pores

Thermally unstable hopanes with a biological 17β,21β (H)- configuration (ββ hopanes), hopenes, hopanoic acids and hopanols were identified as important constituents of Ediacaran to Cambrian sedimentary rocks of the East European Craton. Relatively high abundances of ββ hopanes in relation to αβ hopanes were identified in the Petersburg area and eastern Belarus, while in Volyn samples these compounds were found in traces. In addition, polar hopanoids including hopanols and hopanoic acids were found in most of the Petersburg and some Belarus and Volyn sedimentary rocks. The estimated equivalent of vitrinite reflectance for samples of lower maturity, measured based on C31ββ/(αβS + αβR + ββ) ratio is in the range 0.28–0.49% Rr, while for those of higher thermal maturity this parameter corresponds to 0.41–0.57% Rr. The values of the C31 22S/(S + R) ratio are in agreement with above data and are in the range of 0.1–0.3 for the Belarus and Petersburg samples, of 0.3–0.4 for Volyn, and of 0.4–0.5 for Lithuania, where ββ hopanes and hopanols were not detected or are present as traces. Moreover, there is good correlation (R2 = 0.8) between C31ββ/(αβS + αβR + ββ) and C31ENE/(H + ENE) ratio values (defined as ratio of C31 hopenes to αβ hopanes), which proves that less-stable ββ hopanes and hopenes are enriched in the same immature sedimentary rocks. The remarkable occurrence of a significant portion of sedimentary hopanes retaining the biological stereochemical configuration confirms that exceptionally immature Precambrian rocks can be found in some instances when these strata have undergone only a mild burial temperature history.

Detrital grain assemblages in mudrocks establish a key starting condition for the diagenetic pathways that drive the evolution of rock properties in unconventional petroleum systems. This case study is based on petrographic examination (light microscopy, elemental mapping, and cathodoluminescence imaging) of 41 subsurface samples of Silurian mudrocks from the western margin of the East European Craton (EEC) in Poland. About 2/3 of the samples have grain assemblages composed by >90% of grains of quartz, feldspar, mica, and clay of extrabasinal derivation. Mechanical compaction is the principal cause of porosity decline in these terrigenous-argillaceous mudrocks (tarls). Where thermal maturation has reached a level sufficient to induce smectite illitization, minute amounts of authigenic silica (≪1 vol%) are observed in the small (~100 nm) primary intergranular pores that survived compaction. Organic-rich mudrocks in this succession also display prominent compaction and contain little organic-matter-hosted porosity. Although these organic-rich tarls may have served as effective source rocks as unimpeded compaction drove expulsion of generated hydrocarbons, they do not in general have sufficient porosity and stiffness to serve as effective reservoirs. Around 27% of the sample set has grain assemblages that include biogenic grains in amounts over 10%. Most of these are calcareous tarls (calcareous-terrigenous argillaceous mudrocks), but do not display discernible carbonate cements. Only three of the studied samples contain grain assemblages that originally included a significant component of biogenic silica. These three samples contain microquartz cement (up to 30% of rock volume) and migrated pore-filling organic matter that has developed secondary porosity. Where thermally mature and sufficiently thick, these local intervals of silica-cemented mudrocks may offer interesting targets for development of unconventional gas.

The aim of the present study was to test the applicability of the O-D method, a newly developed spectroscopic layer charge measurement technique, to measure the charge present on wettable surfaces of mixed-layer illite-smectite (I-S). The O-D method employs the exact position of the sharp, high frequency O-D stretching band of adsorbed D2O at 2685–2700 cm−1, which was found to correlate linearly with the layer charge for dioctahedral smectites. The O-D method has therefore potential to measure the charge present on the wettable surfaces of I-S crystallites, i.e. surfaces of smectite monolayers and external surfaces of illite fundamental particles. The O-D method was found to be applicable to mixed-layer I-S. The I-S' surface charge was measured for 41 well-documented I-S samples coming from bentonites and hydrothermal deposits, and representing smectite contents between 8 and 100%. The measured charge averaged 0.47 ± 0.03 e− per formula unit and was very narrowly distributed irrespectively of smectite content or sample origin. Such a result is close to earlier estimates based on chemical analyses. It was concluded that the O-D method measures only the surface charge of fundamental particles, without the influence of the illitic charge from the K-occupied interlayer. No difference in the surface charge among samples with and without smectite monolayers was observed, which is in agreement with the polar model of illite fundamental particles and confirms the smectitic nature of their basal surfaces. On the other hand, a systematic increase of surface charge upon decreasing fraction size was noticed for ultrafine fractions. This trend suggests a low polarity of illite fundamental particles at the initial stages of nucleation.

Sub-aerial weathering of black shales drives the gradual leaching of sulphur- and organic-bound heavy metal elements, which are usually abundant in these rocks due to depositional conditions. The formation of oxygen functional groups in kerogen, however, can lead to an opposing mechanism - metal adsorption and binding, similar to a process common in soils. An increase in cation exchange capacity (CEC) measured previously using metal complexes on black shales oxidised under laboratory conditions implies that the same phenomenon may occur in a naturally oxidised black shale. This idea was tested on a unique, well-developed and -preserved Permian palaeoweathering profile containing two neighbouring but diverse black shales from the Devonian/Carboniferous boundary in the Holy Cross Mountains (Poland). In the studied black shale beds, the oxygen groups formed in kerogen in the partially-weathered zone were found to be responsible for significant changes in adsorption properties measured using hexamminecobalt(III) and Cu(II)‑triethylenetetramine cations, which are common probes for CEC. Compared to a pristine part of black shales, the partially weathered zone was depleted of total organic carbon (TOC), sulphur, and sulphur- and organic-bound metals, and highly enriched in Cu, which is generally present in low levels in the nascent shales. In the partially weathered zone, where TOC content is reduced, apparent CEC values surpass the CECs predicted from the contents and structures of clay minerals, and correlate linearly with the content of oxygen groups developed during weathering. The adsorption properties of carboxyl groups in the oxidised kerogen are suggested as being responsible for the syn- or post-weathering enrichment in Cu caused by the remobilisation of older Cu-sulphide ores present in the area. As opposed to natural weathering, aggressive oxidation, e.g. under laboratory conditions produces a high proportion of cross-linked oxygen groups that do not participate in metal cation adsorption. The CEC values of artificially oxidised samples reached a limit corresponding to those of naturally oxidised shales.

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 (δ2H) of smectites. To evaluate this problem, δ2H of different cation-saturated (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 δ2H reproducibility for the determination of smectite hydroxyl hydrogen. The lowest adsorbed water contribution and the most reproducible and possibly accurate δ2H for smectite hydroxyl hydrogen was obtained for K-saturated smectites dried for both 4 and 24 h at 220 °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 δ2H and facilitated much greater sample throughput than classical methods for smectitic clays. This study proposes a protocol for hydroxyl δ2H determination in smectitic clay minerals, and reveals the effect of H-isotope fractionation of adsorbed water during sample preparation.

The selective self-assembly of small molecules in the interlayer of kaolinite is a fundamentally important and technologically relevant process, typically studied ex situ by X-ray diffraction (XRD). Near-infrared (NIR) spectroscopy is now introduced to provide a complementary local structural description of intercalation with improved control of experimental conditions. New NIR- and XRD-based proxies were developed and applied to the real-time monitoring of N-methylformamide (NMF) intercalation in two reference kaolinites differing in stacking order. The commonly employed XRD-based formalism was found to overestimate reaction progress. The bonding of NMF in the interlayer was independent of reaction progress and kaolinite type. Both NIR and XRD recorded identical sigmoidal kinetics. Isothermal NIR monitoring (25–80 °C) yielded time-temperature superimposable sigmoidals with an apparent activation energy of ~60 kJ/mol, common to both samples. All NIR and XRD data series could be described as linear combinations of empty and fully intercalated kaolinite. The filling of the interlayer was too fast to be observed. The sigmoidal curves were instead modeled as the log-normally distributed response of an ensemble of intercalating entities, presumably crystallites. The multiplicative standard deviation of the distribution, which determines its steepness, is a sample-specific, temperature-independent property of kaolinite.

Supercritical CH4 and subcritical CO2 and N2 gas adsorption measurements, combined with scanning electron microscopy (SEM) have been used to determine CH4 sorption capacity and pore characteristics for immature, mature and overmature shales from the Baltic Basin (Poland). Organic matter (OM) maturity exerts a dominant control on porosity evolution in micro- and mesoscale. In the Baltic Basin shales, the initial formation of micro- ( The CH4 adsorption in the Baltic Basin shales is predominantly controlled by OM micropore structure. The mesopore surface area and volume do not play an important role in CH4 sorption. The proposed adsorbed CH4 density equivalent (maximal absolute CH4 adsorption divided by micropore volume), revealed that the CH4 loading potential decreases in micropores with increasing maturity. The highest CH4 loading potential is linked to OM before metagenesis (Ro

Smectites are able to retain molecular tightly bound water (TBW) at temperatures above 100 °C, even after prolonged drying. The presence of TBW affects the stable isotope ratios, the dehydroxylation behavior of smectites and smectite-rich samples and also has implications in measuring various properties of clay-rich rocks. Five reference smectites, in Mg-, Ca-, Na-, and Cs-exchanged forms were subjected to different drying protocols followed by the determination of TBW contents using precise thermogravimetric (TG) analysis. Activation energies (Ea) of the removal of different water fractions at temperatures up to 1000 °C were determined in non-isothermal TG experiments using model-independent methods. Additionally, 4A and 13X zeolites were examined in both cases as apparent OH-free references. After drying at 110 °C, all smectites still contained up to 3 water molecules per interlayer cation. The TBW contents in smectites were found to be primarily dependent on the isothermal drying temperature. For a given temperature, TBW contents decreased with respect to the type of interlayer cation in the following order: Mg > Ca > Na > Cs. The influence of the time of drying and the smectite layer charge were found to be negligible. The Ea of dehydration below 100 °C, as determined by the Friedman method, was quite constant within the 45–60 kJ/mol range. The Ea of TBW removal increased along with the degree of reaction from 90 to 180 kJ/mol, while the Ea of dehydroxylation was found in the 159–249 kJ/mol range, highly depending on the sample’s octahedral sheet structure and the interlayer cation. The Mg2+ cation can hold H2O molecules even beyond 550 °C, making it available during dehydroxylation or—for geologic-scale reactions—pass H2O to metamorphic conditions. High similarities between the TBW contents and the Ea of dehydration for smectites and cationic (low Si/Al-) zeolites lead to the conclusion that TBW in smectites is remarkably similar to zeolitic water in terms of cation bonding and diffusion characteristics. The optimal drying protocol for smectites is to substitute interlayer cations with cations of a low-hydration enthalpy, such as Cs, and to dry a sample at 300 °C, provided that the sample is Fe-poor. Fe-rich smectites should be dried at 200 °C to avoid dehydroxylation that occurs below 300 °C.

The mineralogical and chemical compositions of Lower Carboniferous (Tournaisian) marine black shale from the Kowala quarry, the Holy Cross Mountains, Poland, were investigated. This study focuses on disturbances in palaeoenvironmental proxies caused by palaeoweathering, which progressively changed the major and trace element abundances. Palaeomagnetic investigations reveal that the Devonian – Carboniferous succession was weathered during the Permian-Triassic by the infiltration of oxidizing fluids related to karstification following post-Variscan exhumation. The weathering process led to vermiculitization of chlorite, partial dissolution of calcite and replacement of pyrite by hematite and goethite. Moreover, the concentrations of some trace metals, including Co, Cu, Pb, Mo, Ni, As and U, significantly decreased. Consequently, some elemental abundance ratios that are used as environmental proxies, including U/Th, Ni/Co and V/Cr, were altered. Elements that are bound to iron sulphides (e.g., Mo) appear to be especially prone to mobilization by even a lightly weathered black shale. The documented weathering, including changes in elemental concentrations, can potentially create misinterpretations of the original palaeoenvironmental conditions. In addition, the palaeoweathering of the studied samples appears to have substantially changed the carbon, oxygen, nitrogen and molybdenum stable isotope values. The nitrogen and molybdenum stable isotope ratios, in particular, appear to be most sensitive to the effects of weathering and therefore are good indicators of (palaeo)weathering processes. The major cause of these changes is the decay of organic matter and pyrite. For the organic carbon stable isotopes ratios, the main factor that controlls this process appears to be the preferential degradation of labile organic matter. A combination of the total organic carbon (TOC), total sulphur (TS) content, Mo concentration and stable isotope compositions seems to be the most useful for identify (palaeo)weathering. Our results suggest that reductions in TS and Mo in tandem with diminished Mo stable isotope values in the absence of obvious changes to the TOC content provide the most compelling evidence of (palaeo)weathering.

In the natural, high-pressure environments of sediments and sedimentary rocks, clay minerals, along with organic matter, contribute to a large portion of the available surface area and microporosity in which methane (CH4) can be adsorbed. However, most of the questions concerning the location of adsorption sites, e.g. interlayer accessibility for CH4, remain unknown. Here, we have separately investigated the textural and structural control on CH4 high-pressure adsorption on pure clay minerals by combining high- and low-pressure gas adsorption techniques, transmission electron microscopy, and thermogravimetric and X-ray diffraction analyses on various cationic forms of montmorillonite, beidellite, and two illites, with different contents of adsorbed water. The results show that CH4 adsorption capacity is controlled by N2-accesible micropore volume. CH4 adsorption sites are located mainly outside of the interlayer galleries unless they are open wide enough (e.g. pillared by organic cations or incompletely dried divalent cations). However, the structure-independent factors like the crystallite planar dimensions and the way that they assemble are the main control on CH4 adsorption capacity. This might be the reason for the inconsistencies in CH4 adsorption reported for clay minerals in the literature. CO2 adsorption measurements, assumed to provide a good proxy for the estimation of CH4 adsorption capacity in natural mudrocks, may overestimate the CH4 adsorption potential in geomaterials containing a significant amount of exchangeable divalent cations due to higher penetrability of CO2 than CH4 in the interlayers of expandable clay minerals.

Dehydroxylation-rehydroxylation experiments were performed on various dioctahedral aluminous 2:1 layer clay minerals using the precise thermogravimetry (TG) system equipped with a continuous N2 gas flow enriched with H2O vapor of controlled concentration. The setup and rehydroxylation conditions applied are specifically sensitive to study in-situ rehydroxylation of smectites with various octahedral compositions and vacancy, saturated with counterions in various sizes and valences. Under the conditions applied, the pyrophyllite structure does not rehydroxylate, illite shows a minor ability to rehydroxylate, and smectites present an entire range of potential rehydroxylation, from remaining in the completely dehydroxylated state to an almost complete rebuild of the octahedral sheet. The mass gained during single- or multi-isotherm rehydroxylation matches the mass lost during the subsequent (secondary) dehydroxylation. In smectites, the Al-pyrophyllite-like arrangement is preferentially rehydroxylated, but at higher degrees of rehydroxylation, OH groups in other octahedral arrangements are also rebuilt. Among the temperatures tested, smectites and illite rehydroxylate most intensively during isothermal heating at 400 °C (H2O content = 0.28 mol H2O/m3), which is the onset temperature of dehydroxylation of their rehydroxylated forms. The smectites capability to rehydroxylate comes from an interplay of the counterions' size, the tetrahedral substitution in the 2:1 layers, and the negative charge generated in the former-octahedral sheet that controls the ability of cations to move from the surface of the tetrahedral sheet or from the octahedral sheet back to the interlayer. The conventional (time)1/4 power law and the logarithmic kinetic models of rehydroxylation produced good fits of the experimental data. Calculation of activation energies (Ea) for the rehydroxylation reaction produced values from 30 kJ/mol to 173 kJ/mol, highly dependent on the interlayer cation, 2:1 layer structure, and the experimental protocol. The kinetic models developed for rehydroxylation at temperature near ambient remain valid for the experiments up to 400 °C. At 500 °C the mechanism of rehydroxylation is influenced by other factors.

Measuring and differentiating effective and ineffective porosity in oil- and gas-shale remains a challenging step in the calculation of hydrocarbon reserves. Shale composition, typically dominated by clay minerals and organic matter (OM), forms a complex pore system, making the proportion between effective and ineffective porosity variable and difficult to determine in the course of porosity measurement. In this study, total porosity was measured from a set of rock chips using both light kerosene (kerosene immersion porosimetry-KIP) and deionized (DI) water (water immersion porosimetry-WIP) as saturation-immersion fluids. Both KIP and WIP measurements were performed on samples equilibrated at 40% and 80% relative humidity (RH) and combined with water adsorption isotherms to estimate the range of clay-bound water (CBW). A combination of these procedures, and the use of kerosene and water as the saturation-immersion fluids, is hereafter called dual-liquid porosimetry (DLP). To test the method, two different burial diagenetic sequences of mudstone with different mineralogy and OM content were used. The results demonstrate that all the porosity values show a consistent declining trend with depth regardless of sample origin and type of saturation-immersion fluid used in the measurement process. Samples saturated with DI water were susceptible to swelling and the swelling decreased with sample depth, maturation and cementation, and increased with illite-smectite mineral content and degree of expandability. A consistently lower calculated grain density for KIP samples compared with WIP samples reflected incomplete pore saturation by kerosene. Kerosene saturation can be restricted by the presence of residual water that blocks kerosene pathways to small pores. This limitation, however, reflects geologic formation conditions making KIP measurement a good approximation for maximum liquid hydrocarbon-available porosity. Equilibrating the rock chips at 40% and 80% RH provides a rapid evaluation of the range of CBW that may represent ineffective porosity under various formation conditions. CBWmin (40% RH) constitutes roughly 1W–2W smectite equivalent layers and gives an estimation of bound water close to high hydrocarbon saturation. CBWmax (80% RH) represents the maximum possible bound water content in formations with higher water saturation. For samples, with low porosity and a significant content of expandable clay minerals and cation exchange capacity, CBW can constitute up to 100% of rock porosity. In order to avoid the limitations and assumptions of WIP (swelling) and KIP (incomplete saturation of nanopores), a combination of grain density and bulk density measured by WIP and KIP, respectively, was proposed. The paper further tests and discusses the pitfalls of saturation-immersion techniques.

The phase composition of partially dehydroxylated specimens of two Clay Mineral Society standards, named KGa-1 and KGa-2 samples, were studied by powder X-ray diffraction patterns (XRD), infrared (IR) spectroscopy, and thermogravimetric (TG) data. Each specimen was preheated for 6 h in isothermal conditions, and the heating temperatures were selected to cover the entire range of dehydroxylation for both kaolinites (380–550 °C for KGa-2 and 400–600 °C for KGa-1). Products preheated at 550 and 600 °C are named metakaolinite, as their XRD patterns showed no traces of kaolinite reflections. The contribution of metakaolinite to the experimental XRD pattern and IR spectrum of each preheated specimen (including 3550–3750 and 500–1300 cm−1 ranges) was determined by the simulation of the experimental pattern with a sum of the XRD patterns or IR spectra corresponding to the untreated sample and metakaolinite. This procedure showed that the contents of metakaolinite determined independently by diffraction and spectroscopic methods for each of the preheated specimen have almost identical values. Although the modeled and experimental XRD and IR spectra generally match, they contain particular ranges where the intensity differences are particularly significant. The misfits suggest the presence of a third phase forming in partially dehydroxylated kaolinite. To reveal the diffraction and spectroscopic features of the kaolinite and possible intermediate phase (or phases), the contribution of metakaolinite was subtracted from the experimental XRD pattern and diffuse reflectance infrared Fourier transform (DRIFT) spectrum of each preheated specimen. Analysis of the metakaolinite-free XRD patterns and DRIFT spectra supports the idea that each preheated specimen, along with metakaolinite and initial kaolinite, contains a kaolinite-like component termed the intermediate phase. To determine the contents and specific diffraction features of the intermediate phases, the contribution of initial, untreated kaolinite was subtracted from the metakaolinite-free XRD patterns for each preheated specimen. In both samples, the content of the intermediate phase, C inter, decreases with the growth of metakaolinite from 23–25% at the beginning of dehydroxylation to 2% when the XRD-determined content of metakaolinite is 98%. The analysis of the diffraction features of the intermediate phases formed during partial dehydroxylation of the KGa-1 and KGa-2 samples indicates that they represent a new type of defective kaolinite-like structure, in which, along with the one-dimensional periodicity along the c * axis and the layer displacement vectors typical for natural kaolinite samples, the octahedral sheets of the individual layers of the structure are partially dehydroxylated.

Cation exchange capacity (CEC) is among most important properties of sedimentary rocks, broadly applied in various means of geosciences research and industry. The organic matter (OM) in ancient black shales is thought to be a negligible source of cation exchange capacity (CEC), due to the loss of polar functional groups from extensive diagenetic transformations that occur during burial. OM in modern soils and sediments contains weakly bound hydrogen on carboxyl and phenolic hydroxyl groups, providing negatively charged functional groups that facilitate CEC on the order of hundreds of cmol(+)/kg. Kerogen in ancient sediments may either retain a portion of polar oxygen groups or these functional groups can be (re)gained upon drying, revealing an overlooked source of charge in black shales. Analyzing an extensive series of shales from the Baltic Basin (Poland) and Marcellus Shale (USA) with varying OM content and diagenesis we found that CEC, measured using Hexamminecobalt(III), of heated samples (≥ 200 °C) is up to ten times greater than CEC measured on air-dry samples. Moreover, CEC measured on the heated samples is greater than theoretical CEC estimated from clay minerals composition. The excess CEC correlates with the content of oxygen-rich groups determined with OM pyrolysis and infrared spectroscopy. Carboxyl groups formed in OM due to thermal oxidation at temperatures ranging from 200 °C to 310 °C, in the presence of oxygen and under vacuum, are responsible for excess CEC. Our results reveal that kerogen in black shales is not chemically inert in the case of cation exchange and the OM can provide a considerable portion of the apparent CEC measured in bulk rock samples.

The study is aimed at employing molecular dynamics simulations to study the interactions between smectite and major oil constituents, along with the impact of different brine chemistry (type of cation in a solution) and ionic strength on the compound mobility in fine pores. Clay minerals with smectitic type of mineral outer surface (smectite, illite-smectite, illite) are major constituents in many unconventional oil and gas reservoirs. Therefore, smectite was used as a model compound to test interactions of oil compounds with mineral surfaces. Natural crude oil is a complex mixture, which - for the purpose of simulating an influence of surrounding fluid on its interactions with smectite surfaces - needs to be simplified. The following molecules were selected as organic model compounds: decane, quinolone, nonanoic acid, benzamide, and heptylamine. Various protonation states were considered. The first two sets of simulations showed remarkably different interactions of the selected organic molecules with the smectite surface. In the first one, three water/oil coverage models of the surface were considered. In the second one, two different pore sizes were taken into account: 3 nm and 5 nm. Significant differences were readably visible between polar and non-polar organic molecules. Also, the degree of protonation was found to be a crucial factor affecting the molecules’ affinity to the mineral surface. The molecules that are well soluble in cyclohexane (here representing crude hydrocarbons phase) formed droplets in the water phase, whereas organic compounds miscible in water were concentrated at the water-monolayer film on the smectite surface. Neutral organic molecules in the interlayer influences hydration spheres of the cations and their distance from the surface, while organic cations tend to substitute for inorganic ones. The third set of calculations aimed at testing mobility of organic model compounds within a wide-open smectite interlayer space (acting as a narrow pore model), under one-dimensional pressure. The results show significant differences in the pressure that allows expulsion of organic molecules in different systems and different ions from a narrow slit. The mobility of oil compounds depends mainly on differences in the type of molecules and the size of effective hydration sphere of the cations adsorbed on a mineral surface, which is clearly visible in the simulations.

Layer charge (LC) is a fundamental property of smectite but its measurement remains challenging and tedious to apply on a high-throughput basis. The present study demonstrates that the position of a sharp, high-energy O—D stretching band of adsorbed D2O (νO—D, at ~2686–2700 cm−1), determined by infrared spectroscopy, correlates with LC and provides a simple method for its measurement. Twenty nine natural dioctahedral smectites and 14 reduced-charge montmorillonites with LC determined previously by different methodologies were saturated with D2O and examined by attenuated total reflectance infrared spectroscopy (ATR-IR). The samples included smectites in Mg, Ca, Na, Li, K, and Cs forms and covered the full range of the smectite LC (0.2 to 0.6 e per formula unit). Statistically significant correlations were found between νO—D and LC values determined with each of the two main methods of LC determination: the structural formula method (R2 = 0.96, σ = 0.02, ~0.2 < LC < 0.6) and the alkylammonium method (R2 = 0.92, σ = 0.01, 0.27 < LC < 0.37). These correlations were based on Li- and Na-saturated smectites, respectively, but other cationic forms can be employed provided that the exchangeable cations are of sufficiently high hydration enthalpy (e.g. Mg2+ or Ca2+, but not K+ or Cs+). The new method is fast, low-cost, implemented easily in laboratories equipped with ATR-FTIR, and applicable to samples as small as ~5 mg.

Sedimentological, mineralogical, stable carbon and oxygen isotope determinations and biomarker analyses were performed on siderite concretions occurring in terrestrial silts to understand their formation and to characterize the sedimentary and diagenetic conditions favouring their growth. High δ13C values (6·4‰ on average) indicate that siderite precipitated in an anoxic environment where bacterial methanogenesis operated. The development of anoxic conditions during shallow burial was induced by a change in sedimentary environment from flood plain to swamp, related to a rise of the ground-water table. Large amounts of decaying plant debris led to efficient oxygen consumption within the pore-water in the peat. Oxygen depletion, in combination with a decrease in sedimentation rate, promoted anoxic diagenetic conditions under the swamp and favoured abundant siderite precipitation. This shows how a change in sedimentary conditions can have a profound impact on the early-diagenetic environment and carbonate authigenesis. The concretions contain numerous rhizoliths; they are cemented with calcium-rhodochrosite, a feature which has not been reported before. The rhodochrosite cement has negative δ13C values (−16·5‰ on average) and precipitated in suboxic conditions due to microbial degradation of roots coupled to manganese reduction. The exceptional preservation of the epidermis/exodermis and xylem vessels of former root tissues indicates that the rhodochrosite formed shortly after the death of a root in water-logged sediments. Rhodochrosite precipitated during the initial stages of concretionary growth in suboxic microenvironments within roots, while siderite cementation occurred simultaneously around them in anoxic conditions. These suboxic microenvironments developed because oxygen was transported from the overlying oxygenated soil into sediments saturated with anoxic water via roots acting as permeable conduits. This model explains how separate generations of carbonate cements having different mineralogy and isotopic compositions, which would conventionally be regarded as cements precipitated sequentially in different diagenetic zones during gradual burial, can form simultaneously in shallow burial settings where strong redox gradients exist around vertically oriented permeable root structures.

Talc is an important industrial mineral with a broad range of applications. Particle size and crystal structure have a significant influence on the potential uses. The present study examined the influence of grinding and ultrasound treatment on talc from a new deposit, Gemerska Poloma, in Slovakia. The general knowledge that grinding produces progressive structural disorder leading to amorphization, whereas sonication has a negligible effect on the talc crystal structure, was confirmed by X-ray diffraction (XRD), infrared (IR) spectroscopy, and transmission electron microscopy (TEM). Partial reduction of particle size along with delamination was observed by XRD after sonication, low-angle laser light scattering (LALLS), scanning electron microscopy (SEM), and TEM. The specific surface area (SSA) increased slightly after prolonged sonication, but grinding initially caused a rapid increase in SSA followed by a drastic decrease after prolonged grinding time of up to 120 min which was attributed to the aggregation of amorphized talc. Sonication and grinding had different influences on the thermal behavior of the talc studied. Sonication decreased slightly the dehydroxylation temperature, whereas grinding added a significant mass loss at low temperature, arising from the dehydration of hydrated Mg cations released from the talc structure during amorphization. The initial high whiteness value of talc decreased slightly after grinding or sonication. Thermogravimetry was suggested as a useful tool to track and predict changes in the talc structure upon sonication and grinding.

The mineralogical complexity of mudstone reservoirs has led to the increased usage of multimineral optimizing petrophysical models for estimating porosity, water, and hydrocarbon volumes. A key uncertainty in these models is the log response parameter assigned for each log equation related to each volumetric variable. Default parameter values are commonly used and often need to be modified by considering subjective local knowledge or intuition to achieve a result that is considered acceptable. This paper describes the methods developed at Chevron for calibration of mineral log response parameters using core data. Mineral log response parameters are controlled by the major and trace element chemistry of the individual minerals in the formation rock matrix. BestRock™ uses a nonlinear approach to optimize whole-rock chemistry with mineralogy to calculate individual mineral structural formulas and trace element associations from which certain log response parameters can then be calculated. Accurate quantitative phase analysis (QPA) to determine mineral content is a critical step in the process, which is achieved here by rigorous sample preparation methods and QPA by x-ray diffraction (QXRD). The QXRD in combination with whole-rock elemental analyses are processed using Chevron’s BestRock optimization software to provide refined quantities of the mineral species present in the formation, their structural formulas, and their predicted wireline log responses. Calibrated petrophysical models are built from the information obtained from the QXRD and BestRock results. The method described herein provides an independent and robust method for determining petrophysical parameters that is independent of the interpreter, quick to implement, and supported by quantitative measurements.

The Boom Clay Formation of early Oligocene age, which occurs underground in northern Belgium, has been studied intensively for decades as a potential host rock for the disposal of nuclear waste. The goal of the present study is to determine a reference composition for the Boom Clay using both literature methods and methods developed during this work. The study was carried out on 20 samples, representative of the lithological variability of the formation. The bulk-rock composition was obtained by X-ray diffraction using a combined full-pattern summation and singlepeak quantification method. Siliciclastics vary from 27 to 72 wt.%, clay minerals with 25–71 wt.% micas, 0–4 wt.% carbonates, 2–4 wt.% accessory minerals (mainly pyrite and anatase) and 0.5–3.5 wt.% organic matter. This bulk-rock composition was validated independently by majorelement chemical analysis. The detailed composition of the clay-sized fraction was determined by modelling of the oriented X-ray diffraction patterns, using a larger sigma star (σ*) value for discrete smectite than for the other clay minerals. The

Surface characteristics of sedimentary opal-A and -CT were investigated for a large collection of samples from the Monterey Formation, California, based on the bulk rock mineral and chemical analysis, cation exchange capacity (CEC), and water content. Two approaches were used: (1) modeling bulk CEC and adsorbed water content for the entire data set using the contents of opal and clay minerals measured by XRD, and (2) correcting the chemical composition and CEC of the most pure opal samples for the mineral impurities quantified by XRD. Modeling indicates that the bulk rock data can be best explained by mixing an illite-smectite (CEC = 59 meq/100 g, 7–8% H2O), consistent with the XRD characteristics of the clay fraction, with opal-A (8 meq/100 g, 3.4% H2O), and opal-CT (13 meq/100 g, 3.7% H2O). Correcting the chemical composition of the most pure opal samples leaves a large excess of cations (Al, Fe, Na, K, Ca, and Mg). Iron is suspect to form traces of separate (oxy-)hydroxide phases, not detected by XRD, while Al for Si substitution in the opal structure produced local negative charge, which was compensated by Na, K, Ca, and Mg exchange cations. A perfect balance of positive and negative charges is observed if the clay admixture in pure opals has the composition of montmorillonite. The concentration of heterogeneous impurities in silica network in opal leads to smectite formation on or within the diatom frustules. These dispersed smectite particles, perhaps monolayers, can be missed during the bulk rock mineral quantification. The recalculated CEC of the opal, assuming the occurrence of dispersed smectite particles, varies from 3 to 11 meq/100 g, which is slightly less than that evaluated by modeling all the rock samples in the set, and corresponds to ~10–50% of the total opal charge quantified by the degree of Al for Si substitution. The remaining charge of the opal structure represents non-exchangeable cations. As opposed to smectite, opal CEC may depend on the size of cation used for the CEC measurement. For opals in the Monterey Formation the content of water removable at 200 °C can be modeled as a sum of a constant value and a variable value dependent on CEC; the latter component is similar to the H2O-CEC relationship that is typical for smectite. The combined system of a constant H2O + variable H2O in opal can potentially be applied for mineral modeling programs in wireline log formation evaluation in diatomaceous hydrocarbon reservoirs.

The multi-cycle heating and cooling thermogravimetric (TG) method was used to study the kinetic behavior of three kaolinite samples: defect-free Keokuk kaolinite, KGa-2 with a very low degree of structural order, and KGa-1 having intermediate structural order. In each cycle, the maximum cycle temperature (MCT) was set to 25 °C higher than the preceding cycle. The TG patterns consist of a set of subsequent DTG maxima representing the portions of OH groups that did not dehydroxylate in previous cycles. Each stage of partial dehydroxylation consists of two kinetic mechanisms and for each of them the experimental dα/d t values that characterize the reaction rate of the dehydroxylated fraction, α, within a period of the reaction time, t , were computed. One mechanism corresponds to a zero-order reaction that occurs in each cycle and indicates that the reaction is homogeneous and each non-dehydoxylated layer is transformed into metakaolinite layer without formation of intermediate derivatives. For this step of the cycles activation energy, E a , was calculated from the linear relationship between ln(dα/d t ) and reciprocal temperature, T ; for KGa-2 kaolinite, the E a varies from 32.0 to 38.1 kcal/mol; in KGa-1, E a varies from 37.1 to 40.4 kcal/mol, whereas in Keokuk, E a varies from 42.7 to 47.5 kcal/mol. The particular variation of the E a is discussed in terms of structural and morphological features of the samples. The kinetic mechanism of the second step of reaction corresponds to the temperature range higher than the first step of the same heating cycle. The second step starts from the point where α = α P that was found to vary between 0.25 and 0.45. The acceleration of the reaction rate of dehydroxylation within this interval decreases with increasing α and T , and the mechanism observed for each of the studied samples is independent of its stacking order, average particle size, and particle size distribution. The f (α) is a function of the reaction mechanism in the second step and has the form ![Formula][1] where n is an empirical parameter and its value was found from

Selective sorption and/or fixation of cations with low hydration energies (e.g. K+, NH 4 + , Rb+, Cs+) by vermiculites is a well known phenomenon in soil science and it has been described by many investigators since the 1950s. Because most of the available studies deal with trioctahedral vermiculites, cation fixation in dioctahedral vermiculites is not as well understood as fixation by trioctahedral structures. The objective of the present study was to investigate the influence of NH 4 + saturation on the structure of a natural dioctahedral vermiculite. Because no dioctahedral vermiculite standard reference material was available, two natural dioctahedral vermiculite-rich soil clay samples were used in the study. The clays were saturated with NH 4 + using different protocols to simulate natural processes that likely take place in soils. The degree of NH 4 + fixation by the dioctahedral vermiculite was evaluated using X-ray diffraction, elemental N analysis, and infrared spectroscopy. All the treatments that involved NH 4 + saturation caused NH 4 + fixation and irreversible collapse (i.e. contraction to ~10 A) of at least a portion of the previously hydrated (vermiculitic) interlayers. Air drying of the NH 4 + -saturated samples greatly enhanced the degree of the collapse. The results indicated that the collapse of dioctahedral vermiculite leads to the formation of a NH4-illite-like phase that is likely to occur in some soils and sediments that are rich in organic matter. The formation of a NH4-illite-like phase by NHNH 4 + fixation in vermiculitic interlayers needs to be taken into consideration in studies that deal with the clay mineralogy of sedimentary basins.

An overlap of bands produced by the O−H stretching vibrations of H2O (O–Hw) and structural OH (O−Hs) in smectite hampers the study by infrared spectroscopy (IR) of both their layer and interlayer structure. The present study re-evaluated the D2O saturation of smectite as a tool to enable separation of the overlapping bands at ambient conditions. Real-time monitoring by Attenuated Total Reflectance infrared spectroscopy (ATR-IR) was employed during in situ sample drying and H2O or D2O saturation at ambient temperature. Six dioctahedral and one trioctahedral pure smectites in Ca2+-, Na+-, and Cs+-cationic forms were studied to explore variability in total layer charge, charge location, and interlayer cation. The IR data showed the interlayer O−Dw signature at 2700–2200 cm−1 as a proxy for the O−Hw signature in the 3700–3000 cm−1 region. In addition to the expected liquid-like bands of D2O in the interlayer, these O−Dw spectra exhibited an additional sharp stretching feature in the 2695–2680 cm−1 range. No significant cation dependence of the sharp band position was observed between pairs of Ca- and Na-smectites for relative humidity (RH) between 60 and 80%, despite the large difference in the ionic potential between these interlayer cations. The intensity of the sharp band was found to be almost insensitive to changes in water content within the range 60–80% RH. The sharp band frequency decreased linearly with increasing total charge of the 2:1 layer (and can be used as a proxy for it), but no effect of charge location could be discerned. In agreement with early studies, this band was attributed to D2O located on the surface of the interlayer, pointing one O−D group toward the siloxane surface. Based on its high frequency, this band was indicative of free O−D oscillators, with very little or no involvement in hydrogen bonding (“dangling OD”). By analogy to the spectra of D2O-smectites, the spectrum of H2O-smectites also involves a sharp O−Hw analog at ~3630 cm−1 overlapping with typical OHs bands (e.g. Al2OH). As a result of this overlap, the sharp 3630 cm−1 O−Hw contribution was often missed or attributed solely to O−Hs.

The characterization of pore networks is extremely important in understanding transport and storage phenomena in unconventional gas and oil reservoir rocks. An ultrasmall-angle X-ray scattering (USAXS) measurement has been performed on Silurian black shales from the Baltic Basin, Poland, from a wide range of depths along a burial diagenetic sequence. This study provides insight into the nature of the pore structure, including the pore size distribution, total porosity, and fractal dimensions of the rocks. Samples were measured in both their air-dried state, equilibrated at ∼50% relative humidity, and prior to dehydration by drying at 200 °C to make a comprehensive comparison of the pore structure changes induced by dehydration. Two trends were observed: porosity values decreased with depth as expected from the models of porosity evolution with burial and increased upon sample dehydration. The USAXS-measured porosity values show very good correspondence with the measurements by immersion porosity methods. ...

In sedimentary basins, immediate equilibration with surface and pore waters of Ar, released from K-bearing minerals during their diagenesis or weathering, has been a paradigm for geochemistry and geochronology. Consequently, K–Ar and Ar–Ar isotope geochronology techniques applied to sedimentary rocks are based on an assumption that no measurable external radiogenic 40Ar (“excess argon”) has been locked in the rock components during their formation and alteration. Our results indicate that the reaction of micaceous sedimentary and diagenetic clay minerals (illite, glauconite) with acid produces microporous silica that retains a great fraction of the initial argon, releasing potassium to the solution. In all tested cases the evolution of K–Ar isotope ages followed the very same pattern: the apparent K–Ar isotope age increased enormously after acid treatment and dropped significantly after silica removal (with hot Na2CO3), but never decreased lower than the initial K–Ar isotope age of the untreated sample. The amorphous silica content and the apparent K–Ar age increased with the acid reaction time. Using the molecular dynamics simulations, the clay-acid reaction by-product was shown to bend and wrap, producing three-dimensional, protonated and hydrated silica. As a consequence of dramatically different hydration energies of Ar and K, potassium is instantaneously released and hydrated outside the residual structure while Ar atoms remain inside the silica network, adsorbed on the surface. This is, to our knowledge, the first experimental evidence that the excess argon can be retained in solid mineral reaction products formed under pressure and temperature close to those of the Earth surface (1 atm

Over the past decade interest in shale properties has increased due to the commercial success of gas shale plays. Despite their commercial importance, porosity measurement from gas shale samples is still challenging due to their extremely low permeability and complex pore structure. This leads to a significant uncertainty in the economic assessment of these plays. The current energy industry standard technique for measuring porosity in gas shales is based on methodology developed by the Gas Research Institute (GRI) that involves crushing a rock and aggressive pretreatment. The objective of this study is to develop an alternative method of measuring total porosity in gas shales. A porosity measurement using a liquid saturation and immersion technique with deionized water was adopted and modified for such applications. The water immersion porosimetry (WIP) technique was used to measure total porosity of shale samples from an Eastern Europe Silurian gas shale play and the Haynesville Shale from East Texas, USA. The samples were characterized for whole rock quantitative mineral and elemental composition, along with cation exchange capacity (CEC) and organic matter. The results from the WIP measurements are compared with other standard techniques including the GRI method and mercury intrusion porosimetry (MIP). An assessment of the advantages, potential errors, pitfalls and reproducibility of this method are also presented. The experimental results indicate that WIP provides (i) highly reproducible porosity, grain density, and bulk density measurements for gas shales, (ii) the average absolute experimental uncertainty is ± 0.22 porosity unit (p.u.), compared to the reported uncertainty level of 0.5 p.u. for GRI measurements, (iii) standard MIP techniques systematically underestimate the porosity and grain density compared to WIP, because mercury cannot access the entire pore structure in shales, and (iv) grain density values obtained by the GRI method in samples with high organic matter content are higher compared to WIP measurements, probably because of dissolution of solid organic matter during solvent extraction pretreatment.

The geochemical and fossil record preserved in the Ediacaran age (635–551 Ma) Doushantuo Formation of South China has been extensively examined to explore the impact of changing climate and the oxidation state of the oceans on the development and distribution of early multicellular life. In the Yangtze Gorges area, this formation shows many of the geochemical trends and features thought to typify global ocean chemistry in the Ediacaran Period, but there are indications that post-sedimentary processes modified these signals. This study of clay minerals and organic matter builds a more detailed picture of the type and degree of post-sedimentary alteration at different stratigraphic levels of the formation and focuses on how this alteration influenced stable carbon and oxygen isotope records. In the cratonward Jiulongwan and Huajipo sections of the Doushantuo Formation, its lower part (Members 1 and 2) consists largely of dolomitic shale, rich in authigenic saponite that crystallized in an alkaline sedimentary basin. Saponite has been altered to chlorite via corrensite across tens of meters of strata in lower Member 2, with increased alteration downward toward the cap dolostone. The greater chloritization is accompanied by lower δ 18 O and higher δD values of trioctahedral clays. This pattern of alteration of trioctahedral clays is likely due to hydrothermal fluid activity in the underlying, relatively permeable Nantuo Formation and cap dolostone. A concomitant increase of solid bitumen reflectance toward the base of the formation supports this idea. In the uppermost part of the formation in the Yangtze Gorges area (Member 4), a typical open water marine dolomitic shale rich in illite and organic matter, increases in the methylphenanthrenes ratio index and solid bitumen reflectance correlate with decrease of the bulk rock K/Al ratio upward, providing evidence for hot fluid migration above the nearly impermeable shale. Clay from the upper part of the formation is enriched in 18 O, but not in D, relative to clay from the lower parts, indicating progressive 18 O-enrichment of hydrothermal fluids that percolated upward and laterally through permeable 18 O-rich carbonates. A maximum hydrothermal-alteration temperature of ∼200 °C is estimated from a calibration curve for illitization during burial diagenesis, but given that the hydrothermal activity probably occurred in short pulses, the temperature could have been much higher. K–Ar ages are consistent across different size fractions of fine illite from Member 4 shale (∼430 Ma) and from a K-bentonite bed near the base of Member 2 in the Jiuqunao section (∼325 Ma), ∼25 km from Jiulongwan and Huajipo. These age values show that the diagenetic illite of the Doushantuo Formation is a product of either deep burial diagenesis overprinted by spatially limited hydrothermal activity or of two localized hydrothermal events. Patterns of carbonate 13 C and 18 O depletion in the basal Doushantuo Formation are similar to chloritization trends and 18 O variation in diagenetic clay minerals. Given independent evidence for 13 C depletion of hydrothermal fluids, these trends indicate carbonate–fluid isotope exchange commensurate with the degree of post-sedimentary alteration, supporting a model of lithologically controlled differential diagenesis induced by hydrothermal fluids as the main control on C and O isotope variability in this stratigraphic interval. This model could potentially explain other notable δ 13 C excursions higher up in Member 3.