Smectite Structure Research Articles

Crystal-Chemical Features and Sorption Properties of Natural and Synthetic Smectites

Sorption of uranyl ions from uranyl nitrate UO2(NO3)2 · 6H2O solutions by natural and synthetic layered silicates was studied by X-ray diffraction, IR spectroscopy, and high-resolution inductively coupled plasma mass spectrometry. The modifications of some structural parameters of the studied species after they were reacted with uranyl ions are due to a partial incorporation of uranyls into the interlayer spaces of the smectite structure, or the rotation of the tetrahedra of a tetrahedral network and the distortion of octahedra in an octahedral layer. The natural bottom clay samples from the Pacific Ocean and the Sea of Okhotsk have the best sorption properties. This is due to structural defects and the formation of uranium complexes with the iron ions of the smectite structure. Iron present in clay minerals improves their sorption potentials.

Features of the Hydrothermal Synthesis of Montmorillonite in an Acidic Medium

The process of the hydrothermal crystallization of montmorillonite Na2x(Al2(1 – x),Mg2x)-Si4O10(OH)2 · nH2O (0 < x ≤ 1) is studied in a weakly acidic medium with pH 4–4.5 at 220°С using HF as a mineralizer. The results of the study showed that montmorillonite samples in the acidic medium can be obtained both with the use of the HF mineralizer and without it. Under any conditions of carrying out the synthesis, there is an insignificant amount of corundum in the final product. It is established that a weakly acidic medium favors the formation of a dioctahedral structure of smectites with insignificant isomorphic substitutions (0.5 ≤ x ≤ 1). At a higher aluminum content, the acidic medium does not contribute to the formation of two-dimensional silicon-oxygen layers.

Determination of Minor Contents of Smectite Layers in the Dispersed Dioctahedral K-bearing Micaceous Minerals of the Illite, Aluminoceladonite, and Glauconite Composition

A theoretically substantiated technique has been elaborated for the precise determination of minor contents of smectite layers in the dioctahedral mixed-layer structures with a disordered alternation of K-bearing micaceous and ethylene glycol-saturated smectite layers. It is shown that observed positions of the possible mixed-layer mica–smectite structures in the d(003)obs–d(007)obs or 2θ(003)obs–2θ(007)obs diagrams are distributed along a set of parallel straight lines. Data points of each straight line correspond to the mixed-layer structures with one and the same content of smectite layers. A complete matching of theoretical model was recorded in the positions of d(003)obs–d(007)obs and 2θ(003)obs–2θ(007)obs pairs of the basal reflections determined in diffractograms simulated for 160 two-component mixed-layer mica–smectite structures differing in the thickness of alternating layers and their relative content. We studied samples of the dioctahedral K-bearing micas of the glauconite, illite, and aluminoceladonite composition with different thickness of micaceous layers. Positions of the mixed-layer structures of the studied samples on the 2θ(003)obs–2θ(007)obs and d(003)obs–d(007)obs plots make it possible to visually determine the content of smectite layers in each structure accurate to 0.5%. The content of smectite layers in the studied samples varied from 2 to 15%. We obtained equations relating the thickness of smectite and micaceous layers for a specified content of smectite layers. They make it possible to determine the thickness correlation between specified mica and smectite layers. Analogous equations can be used to calculate the Wsm value in each sample for specified experimental values of d(001)mc and d(001)sm and d(003)obs or d(007)obs. The Wsm values coincide within an error limit of 0.2% with those based on the visual estimates in plots.

Effect of mechanical activation on catalytic properties of Fe2O3-pillared bentonite for Fenton-like reaction

AbstractA layered composite of Fe2O3-pillared bentonite (Fe-P-Bet), used as a catalyst in a Fenton-like process, was prepared and then its mechanical activation was studied in detail. The optimum conditions for preparation of Fe-P-Bet and the effects of mechanical activation on the catalytic activity of Fe-P-Bet were investigated systematically. The impact of the mechanical activation on the catalytic activity was affected by the filling ratio of the grinding medium, by the combination of different sized milling balls, by the rotation speed of the planet carrier, by the milling time and by the powder-to-ball ratio. Compared with the removal ratio of orange II of 14.5%, catalysed by Fe-P-Bet, the removal ratio catalysed by activated Fe2O3-pillared bentonite (A-Fe-P-Bet) was as high as 93.6%, and A-Fe-P-Bet had the merit of good stability and only a small amount of Fe leaching during the Fenton-like process. The catalysts prepared were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and by the BET method; A-Fe-P-Bet has a small particle size and a rough surface. The lattice distortion and decrease in the crystal size of α-Fe2O3, as well as damage to the layer structure of smectite in the bentonite, were the main reasons for the increase in the catalytic activity of A-Fe-P-Bet. The results indicated that mechanical activation was an easy and effective method for improving the catalytic activity of this bentonite-based layered composite and that A-Fe-P-Bet was a safe, ‘green’, catalyst material for use in Fenton-like oxidation during treatment of wastewater.

Contribution of cations and layer charges in the smectite structure on zeta potential of Ca-bentonites

The Ca-bentonite occurrences in Edirne-Lalapaşa region of NW Turkey comprise economically important minable deposits which originate from the Eocene and Oligocene formations exposed in the northern part of the Tertiary Thrace Basin. The bentonite levels contain high amounts of dioctahedral Ca-smectite (montmorillonite) formed by devitrification of volcanic glassy fragments and also crystallization from feldspar grains. These differentiations on the formation and structure of smectite were envisaged to cause variations on parameters like zeta potential and cation exchange capacity. In this study, nine different Ca-bentonite samples with relatively high smectite contents were used in order to correlate the zeta potential values of these samples with particle size, pH, cation exchange capacity (CEC) and also smectite structural formulas and layer charges. A new methodology for determining the relationships between zeta potential of bentonite aqueous dispersions in their natural pH and Al3+ and Mg2+ content in the octahedral sheet in smectite structure is proposed. Accordingly, the zeta potential range of −9.5 and −17.4mV was found to reasonably correlate with the octahedral Al3+ and Mg2+. The negative value of zeta potential of the Ca-bentonites studied increases with increasing octahedral Al3+ and decreases with octahedral Mg2+ in the smectite structure.

Report on the natural occurrence of a silica-clay nanocomposite

A natural occurrence of a silica/clay nanocomposite material was investigated by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). High-resolution images show that this nanocomposite material consists of 5–20 nm thick slabs of smectite and tridymite/cristobalite layers with coincident normals. In spite of the brittle glass-like appearance of the nanocomposite material its colloidal properties are similar to those of pure smectite but partial loss of expansion capacity was detected upon glycerol solvation. The structural relationship between smectite and silica is interpreted based on the smectite structure model of Edelman and Favejee (1940) which supposes reversed tetrahedra in the SiO4 layer of the TOT structure. This structure model explains the presence of silica impurities in bentonites used as raw material and several geological standard montmorillonites.

Investigation of the Interlayer Organization of Water And Ions in Smectite from the Combined Use of Diffraction Experiments and Molecular Simulations. A Review of Methodology, Applications, and Perspectives

AbstractInvestigation of the organization of interlayer water and cations in smectite is a permanent topic in clay science for environmental science, civil engineering, materials science, and industrial applications. Experimental X-ray (or neutron) diffraction methods and molecular simulations are key techniques to probe the organization of the smectite structure at a similar molecular length scale. The combination of both of these experimental and numerical methods represents a complementary approach to reveal the structural heterogeneity of real samples, design and model a wide range of smectite structures, and validate the simulation results through comparison with experimental data.This paper first revisits establishment of the original interlayer model as developed in the 1930s for the organization of water and ions in the smectite structure using X-ray diffraction (XRD) techniques. Then, based on a simplified approach, key theoretical tools are provided to calculate XRD pattern 00l reflections for a periodic smectite structure with a wide range of interlayer compositions and organizations using conventional spreadsheet software. In addition to educational purposes, this theoretical description is used to describe the principal parameters governing the positions and intensities of experimental XRD 00l reflections. This calculation toolbox is also used to determine better the layer-to-layer distances considered in molecular simulations and to validate these simulations through a detailed collation procedure using experimental data.Recent examples of the application of such a procedure to collate experimental diffraction data and molecular simulations are presented for the specific case of deciphering the molecular organization of interlayer water and cations in the different smectite hydrates (mono-, bi-, and tri-hydrated layers). The extension of this approach to the interlayer refinement of organo-clays is also detailed, and perspectives regarding the characterization of other lamellar compounds are discussed.

Kinetics of uranyl ions sorption on heterogeneous smectite structure at pH 4 and 6 using a continuous stirred flow-through reactor

The <2μm clay fractions of three smectite samples (BA1, PS2 and PS3) were submitted to UO22+ sorption experiments at pH4 (1×10−4) and pH6 (2.0×10−1) in a continuous stirred flow-through reactor. The aim was to evaluate the influence of heterogeneous structure of smectite during the adsorption and desorption processes, where the breakthrough curves (BTCs) were used to model the mass balance. Kinetic models were applied to describe the adsorption – desorption kinetics for a liquid-solid system. Sample PS2 (K-Illite/beidellite/Na,Ca-montmorillonite) adsorbed at pH4 more 34% and 138% of UO22+ than samples BA1 (beidellite/Ca,Na-montmorillonite) and PS3 (K-illite/Na,Ca-montmorillonite) where the dominant mechanism was the ion-exchange (outer-sphere complexation). The UO22+ amount adsorbed at pH6 decreased significantly, indicating changes in the sorption mechanism. The desorption experiments at pH4 suggested a complete reversibility of the UO22+ previously adsorbed, whereas lower reversibility of the UO22+ previously adsorbed and a stronger complexation occurred at pH6. The kinetics was highly influenced by the structural properties of heterogeneous smectite and the surface complexation mechanism involved. Samples submitted to sorption experiments at pH4 and 6 were analysed by X-ray photoelectron spectrometry (XPS). Two binding energies were identified at 380.8±0.3eV and 382.4±0.3eV, before and after desorption processes at pH4. The proportion of the higher binding energy component decreased after desorption process, suggesting that the UO22+ weakly adsorbed was previously desorbed. The results obtained at pH6 show two different binding energies at 380.3±0.3eV and 381.8±0.3eV, whose proportions remained unchanged before and after desorption process, indicating the higher retention of UO22+ on smectite.

Biogeochemical dissolution of nontronite by Shewanella oneidensis MR-1: Evidence of biotic illite formation

Microbial Fe-reduction in smectite structure plays a significant role in illitization accompanying with the structural/chemical modification of smectite, closely linked to the physico-chemical properties of clays, Fe-liberation, water chemistry, elemental cycles, and fault behavior. Biotic dissolution of smectite is a major process that promotes illitization, however direct evidence of illite formation is not clearly understood. In the present study, a combination of spectroscopic, microscopic, and chemical analyses revealed evidence of illitization in bio-reduced smectite, with elemental composition measured on a nanoscale as incubation time increased. Fe-reducing bacteria (FeRB), Shewanella oneidensis MR-1 was inoculated in M1 medium with nontronite (NAu-1) less than 0.2μm as an electron acceptor and Na-lactate as a sole electron donor at 30°C in the anaerobic chamber for up to 120days. The alkalinity was maintained at pH8.0 in the whole experiment to enhance illite formation. The extent of Fe(III) reduction measured by 1,10-phenanthroline assay reached up to 10.6% in the experiment while less than ~1% of reduction was measured in no-bacteria control. In biotic and abiotic control, increases of elemental concentrations (Si, Al, and Fe) in the supernatant indicated the dissolution of nontronite. The progress of bio-reduced nontronite reaction can be explained as follows: altered nontronite (AN) with a scouring surface texture→K-nontronite (KN) with frayed edges→euhedral lath shaped illite. A progressive morphology change in bio-reduced nontronite corresponded to an increase in Al/Si and K/(K+2Ca) that ranged between 0.13 and 0.28 and 0.16 to 1.0, suggesting the biotic reductive dissolution of nontronite and neoformation of illite. The precipitation of biotic amorphous silica supported the reductive dissolution of nontronite. In contrast, there was no clear evidence of mineral precipitation in no-bacteria control. Following treatment with Li and ethylene glycol for the long-term incubation (70 and 120days), the X-ray diffraction (XRD) profiles confirmed illitization by displaying a 10-Å peak shoulder at around 8.9° 2θ in the bio-reduced nontronite. Indeed, a direct microscopic observation of distinct illite packets of 16nm in thickness with d001=1.0nm in the wavy nontronite matrix with various spacings (d001=1.2–1.3nm) strongly suggested biotic illite formation.

Effectiveness of microwave assisted acid treatment on dioctahedral and trioctahedral smectites. The influence of octahedral composition

The effect of the microwave assisted acid treatment (MAT) on the structure and texture of dioctahedral (montmorillonite and nontronite) and trioctahedral (saponite and stevensite) smectites were studied by SEM, nitrogen adsorption, XRF, XRD and FTIR. The effectiveness of this treatment is notably influenced by the chemical composition of the octahedral sheet. Thus, trioctahedral smectites seem to be very reactive to the microwave assisted acid treatment. In saponite and stevensite this treatment caused in the first 20min the progressive Mg2+ depletion of the octahedral sheet, the gradual destruction of the smectite structure and the formation of an amorphous silica phase which contributes to a noticeable increase of the BET specific surface area. After 20min of microwave assisted acid treatment the increase in SSA is of 315m2/g in saponite and 244m2/g in stevensite. The assistance of microwaves during the acid treatment allows us to get, in 20min, specific surface areas equivalent to those obtained after 48h with conventional heating methods. On the contrary, dioctahedral smectites seems to be less reactive to the microwave assisted acid treatment.

Genesis of smectite in siliciclastics and pyroclastics of the Eocene İslambeyli Formation in the Lalapaşa region, NW Thrace, Turkey

AbstractThe Eocene İslambeyli Formation in the Lalapaşa region (NW Thrace, Turkey) consists predominantly of siliciclastic clayey clastics at the bottom and of claystone, tuff and tuffaceous claystone/ sandstone/limestone in the middle and uppermost parts of this formation. Some tuff-tuffaceous beds of the middle–upper parts of the formation are economically viable bentonite depoits. The İslambeyli Formation exhibits vertical variations in the mineralogy, elemental compositions, and smectite-forming processes. Smectite was formed by weathering and diagenetic processes in the fluvial-shore environments in the lower part and by diagenetic alteration in a shallow-marine environment in the middle–upper parts of the formation. Ca-smectite flakes were formed by two processes: direct precipitation; and by means of a dissolution-precipitation mechanism from feldspar and mica. Dissolution-precipitation was most prevalent in the siliciclastic lower part of the formation. The amounts of Al, Fe, Mg and Ca required to form smectite and accessory illite were supplied mainly from the alteration of feldspars, mica and glass shards. The origin of smectite can also be explained by the inferred solution compositions given the parent phases in this formation, and the devitrification of glass shards in pyroclastic-rich middle–upper parts of the formation. In the upper beds, the observed decrease of K and Fe in the smectite structure coincided with both the increase in the amount of smectite and the increase in solution pH suggesting that precipitation of smectite developed over a prolonged period and under arid conditions.

Electrochemical activity of iron in acid treated bentonite and influence of added nickel

Bentonite originated from Mečji Do, Serbia, was submitted to acid treatment at 70°C for 30min, while only the concentration of applied HCl varied. The obtained acid treated samples were used to modify glassy carbon (GC) electrode. The effect of applied acid treatment on the electrochemical behavior of GC electrodes modified with these materials was investigated. Furthermore, the effect of the introduction of nickel into acid treated samples was studied. The incorporation of nickel into acid treated bentonite was achieved by either ion exchange or impregnation/decomposition method. The obtained samples were characterized using the following methods: inductively coupled plasma (ICP), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and electron spin resonance (ESR) spectroscopy. The electrochemical behavior of these samples was tested by cyclic voltammetry in 0.1moldm−3 H2SO4 solution. The ICP, FTIR and ESR results exhibited a slight decrease of iron content in the acid treated samples. XRD and FTIR results confirmed that the conditions applied for the acid treatment were mild enough for the smectite structure to be preserved. The electrocatalytic test showed that the current response of Fe2+/Fe3+ oxidation/reduction process increased on the GC electrodes separately modified with each of the acid treated samples in comparison with current obtained on the GC electrode modified with untreated sample. These results indicated that applied acid treatment probably increased the accessibility of the electroactive iron within smectite. Cyclic voltammograms obtained for the GC electrodes modified with acid treated bentonite materials showed greater anodic charge (qa) than cathodic charge (qc). This difference might be due to iron detachment from smectite structure during the oxidation process. Further modification of the selected acid treated sample with nickel species resulted in decreased current response of the Fe2+/Fe3+ oxidation/reduction process and improved the reversibility of this process.

The water retention mechanism of a Cs+and Na+exchanged montmorillonite: effect of relative humidity and ionic radius on the interlayer

Dioctahedral smectites are clay minerals that widely used as a crucial component for the edifice of natural barriers in industrial wastes and wastewater treatment. The hydration behaviours of these mineral types are very sensitive by environmental surroundings changes. The present work focuses the evolution of the interlayer water amount as a function of the relative humidity rates (RH), varied along a humidifying-drying cycle, in the case of two dioctahedral smectites with different charge location (i.e. Wyoming montmorillonite SWy-2 and Beidellite SbId-1). The CEC (i.e. cation exchange capacity) of the studied sample is saturated by two monovalent cations (i.e., Cs+and Na+) characterized by a different ionic radius. The X-ray diffraction (XRD) profile modelling tools based on the indirect method that consist on the comparison of experimental00lreflections with other calculated from structural models is used to perform these aims. The qualitative XRD profile investigation suggests heterogeneous hydration behaviour at different stage of the cycle for all studied samples. The water content fluctuation, confirmed by the appearance of a hydrationhysteresis, within the interlamellar space of diverse complexes is an irreversible process during the shrinking–rewetting sequence. Comparing to Na+cations, the presence of the Cs+ions in exchangeable sites of both smectite structures is accompanied by more orderliness of systems, which retunes the lowest water molecules amount over the applied cycle.

Relation between molecular structure of smectite and liquefaction of mudstone

As the degrees of water soaking increase, breakage of the smectite microstructure will occur, and overall energy of the mode will dramatically decreases, resulting in mudstone with macroscopic phenomena such as argillization and liquefaction.

Stability of lanthanum-saturated montmorillonite under high pressure and high temperature conditions

Smectite has been used to capture radioactive cations through adsorption in deep radioactive waste repositories in various parts of the world. Smectite is also important in the transport of water and some trace element cations such as rare earth elements (REE), which are captured in its structure, back to the mantle in subduction environments. Such captures are based on the ionic strength of the surrounding solution and the adsorption coefficient of smectite. However, captured cations can be released from the smectite structure once the ionic strength of the solution changes. In this work, the stability of a particular smectite (montmorillonite) structure saturated with lanthanum was verified at high pressures (up to 12GPa) and room temperature and at high pressure and high temperature (HPHT) concomitantly. La3+-montmorillonite remains stable up to 12GPa at room temperature with a small variance in its vibrational mode. At HPHT, however, the structure becomes muscovite-like and rich in La3+. When in contact with a Ca2+-enriched solution, La3+ is partially replaced by Ca2+ in the new phase, returning to its original Ca2+-montmorillonite phase, whereas another part remains La-muscovite-like. These results were confirmed by X-ray diffraction and scanning electron microscopy with energy dispersive X-ray spectroscopy.

The role of Fe(III) bioreduction by methanogens in the preservation of organic matter in smectite

Organic matter in sediments is largely associated with clay minerals and can be preserved over geological time. However, microbial activity can possibly influence this association and release organic matter from clay minerals via reductive or oxidative dissolution of clay minerals. In this study, the relationship between bioreduction of structural Fe(III) in smectite and organic matter release from smectite structure was investigated. A model organic compound, 12-aminolauric acid (ALA) was intercalated into the interlayer region of an iron-rich smectite (nontronite, NAu-2). Two methanogens: mesophilic Methanosarcina mazei and thermophilic Methanothermobacter thermautotrophicus were selected to reduce structural Fe(III) in ALA-intercalated nontronite. As a comparison, sodium dithionite was used to chemically reduce structural Fe(III) in the same mineral. The results showed that the intercalation of ALA into the nontronite interlayer decreased both the rate and the extent of Fe(III) bioreduction. Furthermore, methanogenesis was more inhibited by the presence of intercalated ALA in the nontronite structure relative to pure nontronite. After the bioreduction, the intercalated ALA was partially released, and the extent of release was positively correlated with the extent of Fe(III) reduction. A low reduction extent of bioreduction (<30%) resulted in little ALA release, whereas a nearly complete chemical reduction by sodium dithionite released all intercalated ALA. SEM observations and aqueous chemistry data suggested that reductive dissolution was a main mechanism for the observed ALA release. Because naturally prevalent biological reduction is a slow process with a low reduction extent relative to chemical reduction, the results of this study demonstrated that organic matter preserved within smectite structure should not be released by the mechanism of iron reduction.

The Interaction Between Bentonite and Water Vapor. I: Examination of Physical and Chemical Properties

AbstractThe influence of water vapor on bentonites or smectites is of interest in many different fields of applied mineralogy such as nuclear-waste sealing or casting in the foundry industry. The water vapor affects the smectite surface and perhaps its structure probably leading to mostly unfavorable changes in its properties. In this first part of the present study, the influence of hot water vapor (200°C) on the physicochemical and mineralogical properties of smectite-group minerals was studied. After the steam treatment, turbidity measurements, methylene-blue sorption, water adsorption, and cation exchange capacity (CEC) were measured on both untreated and treated samples. Mineralogical changes were monitored by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) was used to measure O, Al, and Si. Only a few parameters showed differences between the untreated and vapor-treated samples. Sedimentation volumes (SV) decreased following the treatment. As shown by XRD and XPS, the crystalline structure of smectite remained unaffected by the steam treatment. Equivalent sphere diameters (ESD) were not affected systematically by the steam treatment. Differences in CEC values between untreated and treated samples were observed, but only for smectites with monovalent interlayer cations. From the variety of different measurements the conclusion of the present study was that steam treatment changes the charge properties at or near the smectite particle surface.

Structure of mixed-layer corrensite-chlorite revealed by high-resolution transmission electron microcopy (HRTEM)

Mixed-layer corrensite-chlorite in a glauconitic sandy-clayey rock has been investigated and the three-dimensional stacking structure of corrensite was determined for the first time using high-resolution transmission electron microscopy (HRTEM). We are also able to identify the corrensite-chlorite transition mechanism. The crystals consist of corrensite and chlorite packets excluding successive smectite layers, consistent with the result of XRD analysis previously reported for the same specimen. One-dimensional HRTEM imaging of corrensite with dark contrast corresponding to the cation sheets indicated two types of the smectite-like interlayers in corrensite, probably containing one atomic plane and without any distinct material, which results in the corrensite basal heights of ca. 26.5 and 24.4 Å, respectively, in TEM. Two-dimensional HRTEM imaging revealed that the polytypic stacking sequence in the chlorite-like layer [the two 2:1 layers and the brucite-like sheet (B-sheet) between them] in the corrensite unit is always IIbb type. The intralayer displacements of the two 2:1 layers in the unit are well ordered to show a “two-layer” character, which can be regarded as combination of two different one-layer chlorite polytypes belonging to IIbb. These regulated features of corrensite structure indicate that corrensite precipitated directly from solution probably in an environment with a high water/rock ratio, without inheriting smectite structures, during the smectite-to-chlorite transition. The number of the successive B-sheets in the corrensite-chlorite interstratification is always odd. Along with frequent observation of the transition from the smectite-like interlayer to the B-sheet and similarity of polytypic stacking sequence between corrensite and chlorite, this result strongly supports the transformation from corrensite to chlorite, by replacing the smectite-like interlayer with the B-sheet

Constraining clay hydration state and its role in active fault systems

AbstractTo understand the role of hydrated clay minerals in active fault systems, a humidity chamber connected to an X‐ray diffractometer was used to determine the adsorption of water onto and/or into the crystal structure of smectite. This new type of analysis was carried out under specific temperature and humidity conditions, using powdered clay size fractions (&lt; 2 µm) of rock samples from the San Andreas Fault (USA) and the Nankai Trough (Japan). Pressure cannot be controlled, but does not significantly affect clay swelling at shallow conditions. Air‐dried samples show a discrete smectite phase that swells after traditional ethylene glycolation to an interlayer distance of 1.5 and 1.7 nm. Using the humidity chamber, however, the samples show a shorter interlayer distance, between 1.09 and 1.54 nm. Based on our analysis, we show that (i) ethylene glycol overestimates the size of the interlayer space, and therefore water content, so is a crude maximum only; (ii) interlayer swelling occurs in smectite clay minerals at all temperatures between 25 and 95°C; and (iii) particle orientation increases with increasing humidity, indicating a higher mobility of smectite from interlayer hydration. Detailed characterization of the hydration state of smectite under original conditions is critical for understanding of clay‐fluid interaction, the mechanical behavior during fault displacements, and fluid budgets at depth. We propose that humidity chamber experiments should be the new standard procedure to constrain swelling characteristics of natural and synthetic clay minerals.

Composition, structure, and properties of iron-rich nontronites of different origins

The composition, structure, and properties of smectites of different origins have been studied by X-ray diffraction, IR spectroscopy, scanning electron microscopy, and microprobe analysis. The results showed that nontronites of different origins differ in composition, properties, morphology, and IR spectroscopic characteristics. Depending on the degree of structural order and the negative charge of iron-silicate layers in nontronites, the shift of the 001 reflection to smaller angles as a result of impregnation with ethylene glycol (this shift is characteristic of the smectite group) occurs differently. The calculated values of the parameter b (from 9.11 to 9.14A) are valid for the extreme terms of dioctahedral smectite representatives: nontronites.