Smectite Interlayers Research Articles

Microtextures, microchemistry, and mineralogy of basaltic glass alteration, Jeju Island, Korea, with implications for elemental behavior

The meteoric alteration of basaltic glass in hyaloclastite (0.55 to 0.2 Ma) from Jeju Island, Korea, was examined using high-resolution electron microscopy, in situ sample preparation using focused ion beam, and microchemical analysis. The glass particles showed a sequential development of alteration textures toward the glass interior as follows: oscillatory zoning of smectite and amorphous bands/hemispheres, congruent dissolution of glass with smectite infilling in interparticle pore, and the replacement of nanogranular amorphous aggregate (NAA) for porous leached glass (LG) followed by a partial crystallization of smectite. Chemical compositions of LG widely varied from partial to complete leaching of almost all kinds of cations, to a preferred leaching of Na only. The order of leaching in LG followed Na + >Mg 2+ >Ca 2+ = Fe 2+ >Al 3+ >Ti 4+ . Leached cations were removed from the hyaloclastite-water system or incorporated into secondary phases. Sodium was all but removed from the system. Magnesium was largely lost with partial incorporation into smectite. Calcium was significantly retained in NAA, partly incorporated into the smectite interlayer, and partly lost. Iron and Al were partitioned between NAA and smectite. Titanium was mostly retained in NAA and Ti-rich silicates. The molar ratio of major cations to Si was reversed from Ca 2+ >Mg 2+ >Na + in basaltic glass to Na + >Mg 2+ >Ca 2+ in groundwater. The geochemical evolution of pore water was influenced by the microtextural and microchemical evolution of glass particles, and vice versa. Systematic microtextural, microchemical, and mineralogical analyses of natural glass alteration are critical to understanding the long-term behavior of glass alteration.

Acidities of confined water in interlayer space of clay minerals

The acid chemistry of confined waters in smectite interlayers have been investigated with first principles molecular dynamics (FPMD) simulations. Aiming at a systematic picture, we establish the model systems to take account of the three possible controlling factors: layer charge densities (0 e, 0.5 e and 1.0 e per cell), layer charge locations (tetrahedral and octahedral) and interlayer counterions (Na + and Mg 2+). For all models, the interlayer structures are characterized in detail. Na + and Mg 2+ show significantly different hydration characteristics: Mg 2+ forms a rigid octahedral hydration shell and resides around the midplane, whereas Na + binds to a basal oxygen atom and forms a very flexible hydration shell, which consists of five waters on average and shows very fast water exchanges. The method of constraint is employed to enforce the water dissociation reactions and the thermodynamic integration approach is used to derive the free-energy values and the acidity constants. Based on the simulations, the following points have been gained. (1) The layer charge is found to be the direct origin of water acidity enhancement in smectites because the neutral pore almost does not have influences on water dissociations but all charged pores do. (2) With a moderate charge density of 0.5 e per cell, the interlayer water shows a p Ka value around 11.5. While increasing layer charge density to 1.0 e, no obvious difference is found for the free water molecules. Since 1.0 e is at the upper limit of smectites’ layer charge, it is proposed that the calculated acidity of free water in octahedrally substituted Mg 2+-smectite, 11.3, can be taken as the lower limit of acidities of free waters. (3) In octahedrally and tetrahedrally substituted models, the bound waters of Mg 2+ show very low p Ka values: 10.1 vs 10.4. This evidences that smectites can also promote the dissociations of the coordinated waters of metal cations. The comparison between the two Mg 2+-smectites reveals that different layer charge locations do not lead to obvious differences for bound and free water acidities.

Comparison of Reactivity of Nanoscaled Zero‐Valent Iron Formed on Clay Surfaces

Clay minerals were used as template‐supporting matrices to synthesize nanoscaled zero‐valent iron (ZVI). The size and distribution of the synthesized ZVI particles were related to clay structure, origin of negative charges, and surface charge density. The average particle size ranged from 0.5 nm (perpendicular to the clay layers) for the ZVI intercalated in smectite interlayers, to 1 to 2 nm for the ZVI residing on kaolinite and pyrophyllite surfaces, and ∼5 nm for the ZVI formed on illite and smectite external surfaces. The reactivity of the ZVI was evaluated based on the rate and extent of reduction of nitrobenzene to aniline. The results revealed that the reactivity of ZVI decreased with increasing particle size, and the ZVI intercalated in smectite clay manifested the highest reactivity. The reaction efficiency of the ZVI associated with smectite clay was estimated to be as high as 90% based on the equivalency of electron transfer from ZVI to nitrobenzene. The great reactivity and high efficiency of smectite‐intercalated ZVI, along with many other advantageous properties of smectite (e.g., a layer structure that protects ZVI from aggregation and oxidation), warrants its potential utilization in the development of in situ remediation and treatment technologies for inorganic and organic contaminants.

Sorption and desorption of carbamazepine from water by smectite clays

Carbamazepine is a prescription anticonvulsant and mood stabilizing pharmaceutical administered to humans. Carbamazepine is persistent in the environment and frequently detected in water systems. In this study, sorption and desorption of carbamazepine from water was measured for smectite clays with the surface negative charges compensated with K +, Ca 2+, NH 4 + , tetramethylammonium (TMA), trimethylphenylammonium (TMPA) and hexadecyltrimethylammonium (HDTMA) cations. The magnitude of sorption followed the order: TMPA-smectite ⩾ HDTMA-smectite > NH 4-smectite > K-smectite > Ca-smectite ⩾ TMA-smectite. The greatest sorption of carbamazepine by TMPA-smectite is attributed to the interaction of conjugate aromatic moiety in carbamazepine with the phenyl ring in TMPA through π–π interaction. Partitioning process is the primary mechanism for carbamazepine uptake by HDTMA-smectite. For NH 4-smectite the urea moiety in carbamazepine interacts with exchanged cation NH 4 + by H-bonding hence demonstrating relatively higher adsorption. Sorption by K-, Ca- and TMA-smectites from water occurs on aluminosilicate mineral surfaces. These results implicate that carbamazepine sorption by soils occurs primarily in soil organic matter, and soil mineral fractions play a secondary role. Desorption of carbamazepine from the sorbents manifested an apparent hysteresis. Increasing irreversibility of desorption vs. sorption was observed for K-, Ca-, TMA-, TMPA- and HDTMA-clays as aqueous carbamazepine concentrations increased. Desorption hysteresis of carbamazepine from K-, Ca-, NH 4-smectites was greater than that from TMPA- and HDTMA-clays, suggesting that the sequestrated carbamazepine molecules in smectite interlayers are more resistant to desorption compared to those sorbed by organic phases in smectite clays.

Bonding mechanisms between aflatoxin B 1 and smectite

Bentonites were used in aflatoxin-contaminated feeds and diets to reduce bioavailability of the mycotoxin. The objectives of this study were to test accessibility of smectite interlayer space to aflatoxin adsorption and to investigate bonding mechanisms between adsorbed aflatoxin B 1 molecules and smectite. The aflatoxin–smectite complex had a basal spacing greater than 1.2 nm up to 400 °C, which suggests interlayer aflatoxin adsorption. The in-phase carbonyl-stretching band shifted from 1736 cm –1 for monovalent cation (K or Na) to 1687 cm –1 for heavy-metal (Ni or Cu) complexes. At nearly 100% humidity, the cations did not distinctly affect band positions of the adsorbed aflatoxin molecules. We concluded that, under dry conditions, major bonding between adsorbed aflatoxin B 1 and smectite was ion-dipole interactions and coordination between exchangeable cations and carbonyl groups. Under humid conditions, H-bonding between carbonyl groups and exchangeable-cation hydration-shell water was the dominant bonding force.

Synthesis of Highly Reactive Subnano-Sized Zero-Valent Iron Using Smectite Clay Templates

A novel method was developed for synthesizing subnano-sized zero-valent iron (ZVI) using smectite clay layers as templates. Exchangeable Fe(III) cations compensating the structural negative charges of smectites were reduced with NaBH(4), resulting in the formation of ZVI. The unique structure of smectite clay, in which isolated exchangeable Fe(III) cations reside near the sites of structural negative charges, inhibited the agglomeration of ZVI resulting in the formation of subnanoscale ZVI particles in the smectite interlayer regions. X-ray diffraction revealed an interlayer spacing of approximately 5 A. The non-structural iron content of this clay yields a calculated ratio of two atoms of ZVI per three cation exchange sites, in full agreement with the X-ray diffraction (XRD) results since the diameter of elemental Fe is 2.5 A. The clay-templated ZVI showed superior reactivity and efficiency compared to other previously reported forms of ZVI as indicated by the reduction of nitrobenzene; structural Fe within the aluminosilicate layers was nonreactive. At a 1:3 molar ratio of nitrobenzene/non-structural Fe, a reaction efficiency of 83% was achieved, and over 80% of the nitrobenzene was reduced within one minute. These results confirm that non-structural Fe from Fe(III)-smectite was reduced predominantly to ZVI which was responsible for the reduction of nitrobenzene to aniline. This new form of subnanoscale ZVI may find utility in the development of remediation technologies for persistent environmental contaminants, for example, as components of constructed reactive domains such as reactive caps for contaminated sediments.

Glauconite formation in lacustrine/palaeosol sediments, Isle of Wight (Hampshire Basin), UK

AbstractThe clay mineralogy and chemistry of a green lacustrine marl that has been pedogenically modified in the upper part was investigated in order to better understand the formation of low-temperature Fe-rich 10 Å clay. Twelve samples in a vertical sequence have been investigated using X-ray diffraction (XRD), chemical analysis, scanning electron microscopy (SEM) and laser particle size analysis. The clay assemblage has a range of overall illite-smectite (I-S) compositions (64–100%) resulting from several I-S phases that, for the sake of modelling, have been simplified to one to three I-S phases of increasing illitic content. Where the lacustrine marl has been pedogenically modified, the smectite-rich I-S is much reduced in abundance or absent and the 10 Å -rich component is both more abundant and more illitic. These assemblages are a consequence of illitization of detrital I-S in the lake and soil, and dissolution of other clays (kaolinite and chlorite) in the hypersaline lake. Interlayer K, octahedral Fe and octahedral + interlayer Mg increase with intensity of illitization (increase range 0.32–0.63, 0.68–1.67, 0.18–0.24 per O10(OH)2, respectively), first in the increasingly saline lake, and latterly as a result of wetting and drying in a gley soil. In the soil environment, reduction of Fe(III) to Fe(II) resulted in increased layer charge but, as by this stage very few smectite interlayers remained, this did not result in an equivalent increase in illite. Laser particle-size analysis, supported by SEM observation, shows the existence of a bimodal distribution of clay particle size (maxima at 0.2 and 1.5–1.8 μm) in which the finer fraction increases largely in the pedogenically affected samples, probably due to particle break-up caused by seasonal wetting and drying. This ‘dual action’ illitization, first in a hypersaline lake and latterly through wetting and drying, may be responsible for both the intensity of illitization and exceptionally high (for the Solent Group) Fe content of the authigenic illite. The chemical characteristics of the illitic I-S and the illite end-member correspond to glauconite. Hence, this is an example of onshore, non-pelletal glauconite formation.

Geochemical and mineralogical changes in compacted MX-80 bentonite submitted to heat and water gradients

A 20-cm high column of MX-80 bentonite compacted at dry density 1.72 g/cm 3 with an initial water content of 16% was submitted to heating and hydration by opposite ends for 496 days (TH test). The temperature at the bottom of the column was set at 140 °C and on top at 30 °C, and deionised water was injected on top at a pressure of 0.01 MPa. Upon dismantling, water content, dry density, mineralogy, specific surface area, cationic exchange capacity, content of exchangeable cations, and concentration of soluble salts and pH of aqueous extracts were determined in different positions along the bentonite column. In addition, the pore water composition was modelled with a geochemical software. The test tried to simulate the conditions of an engineered barrier in a deep geological repository for high-level radioactive waste. The water intake and distribution of water content and dry density along the bentonite were conditioned by the thermal gradient. Liquid water did not penetrate into the column beyond the area in which the temperature was higher than 100 °C. A convection cell was formed above this area, and liquid water loaded with ions and moving by advection evaporated towards cooler bentonite as it reached the area where the temperature was too high. In this area the precipitation of mineral phases took place. Advection, interlayer exchange and dissolution/precipitation processes conditioned the composition of the pore water along the column. In most of the column the pore water was Na–SO 4 2−, and changed to Na–Cl type near the heater. The overall changes in cation content of the pore water could be explained by changes in the smectite interlayer and mineral phases equilibrium. The TH treatment did not cause significant alteration of the smectite or the other mineral phases of the bentonite.

Clay mineral-enhanced bioremediation of marine oil pollution

Three year old laboratory mixtures of Prestige oil, Atlantic sea water and various types of clay mineral powder (Na- and Ca-smectites, palygorskite, synthetic mica-montmorillonite and a low defect kaolinite) contained significantly lower concentrations of extracted oil than the non-clay containing control. As no significant quantities of oil could be detected in the clay samples following solvent extraction, and no expansion of the viscous oil-treated smectite interlayers could be detected by X-ray diffraction, the measured hydrocarbon content is suggested to provide a reasonable estimate of the total oil present. The observed decrease in the concentration of oil over the reaction period is attributed largely to bacterial digestion rather than to hydrocarbon adsorption, with denser populations of cultured oleophilic bacteria in the more strongly digested samples. Overall, the smectites with high specific surface areas and cation exchange capacities induced the highest degree of oil breakdown, and also stimulated the digestion of the heavier aromatic and resin–asphaltene compounds. The presence of bivalent cations at interlayer sites and hydrated surfaces, including abundant Mg 2+ derived from sea water, is suggested to minimize the thickness of the diffuse ion layer and thus enhance both inorganic and organic nutrient supply from mineral surface to bacterial cell. Adding clay minerals to marine oil pollution is therefore suggested as an effective and economically interesting way of enhancing the bacterial digestion of contaminating spills.

The Charge of Component Layers of Illite-Smectite in Bentonites and the Nature of End-Member Illite

AbstractThe nature of component layers of mixed-layer illite-smectite and their possible evolution in the course of illitization have been debated since the 1960s. The present study is a new attempt to solve these problems, using samples collected from diverse geological formations around the world. Twenty three purified illite-smectites from bentonites and hydrothermal rocks, covering the complete range of expandability, were analyzed chemically, including NH4+\$\end{document} determination, and their structural formulae were calculated. The exchangeable cations (EXCH) were plotted vs. the fixed cations (FIX) yielding the following experimental regression: EXCH=−0.43×FIX+0.41(R2=0.98)\$\end{document}FIX and EXCH depend on the charge of the illite (Qi) and smectite (Qs) interlayers, and the fractions of these interlayers in the bulk clay leading to: EXCH=−Qs/Qi×FIX+Qs\$\end{document}Analysis of these relations and independent measurements of the total specific surface area (TSSA) indicate that the layer charges of both types do not change in the course of illitization. The smectite layer charge is equal to 0.41 and the illite layer charge is equal to 0.95 per O10(OH)2. End-member illite has a well defined composition that is close to intermediate between muscovite and phengite, with fixed cations content greater than that specified in the AIPEA classification of layer silicates: FIX0.95(Si3.25Al0.75)(Al1.81Fe0.01Mg0.19)O10(OH)2\$\end{document}The established relationship allows the calculation of the mean number (N) of 2:1 layers in all fundamental particles and also the fraction of smectitic layers (fs) from FIX: N=Qi/(Qi−FIX)fs=(Qi−FIX)/Qi\$\end{document}N and fs can be used to calculate TSSA, and all three parameters can also be calculated from cation exchange capacity and from X-ray diffraction peak positions, utilizing the regressions established here.

Responses of soil clay mineralogy in the Rothamsted Classical Experiments in relation to management practice and changing land use

Changes in the clay mineralogy of soil samples collected from the Rothamsted Classical Experiments over the past ~ 150 years were examined. Comparisons were undertaken to assess the impact of (i) different fertiliser practices on the top soil and sub soil of the Park Grass Experiment and (ii) the effects of changing land use from agriculture to woodland where soil pH either remained close to pH 7 or became acidic. Analyses were undertaken on the < 0.2 µm clay fraction and measurements included cation exchange capacity (CEC), surface area and X-ray diffraction (XRD). The fine clay fraction of all the samples is composed of illite/smectite (I/S), illite and kaolinite minerals. Decomposition modelling of XRD spectra identified three I/S phases including (i) a high smectite I/S phase ( d(001) ~ 15 Å), (ii) a low smectite I/S phase ( d(001) ~ 12.5 Å) and (iii) an illite phase ( d(001) ~ 11.2 Å). The I/S phase is typically made up of ~ 60% of high smectite I/S, ~ 20% low smectite I/S and ~ 20% illite. Attempts to account for changes in clay CEC and surface area with time proved to be inconclusive, possibly because of the differing proportions of the three I/S phases in each sample. Some temporal changes in the d(001) spacing of I/S mineral phases from the decomposed XRD spectra (> 0.4 Å) are reported in both top soil (0–23 cm) and sub soil (46–69 cm) and are likely related to (i) changes in soil K + or NH 4 + status and/or (ii) increasing soil acidity. The greatest change was found in the Park Grass Experiment (NH 4) 2SO 4 plot where soil pH fell below 3.7. It is believed that solubilisation of hydroxyl-Al in smectite interlayers enabled renewed access to K + or NH 4 + ions, leading to increased collapse in d(001) spacing. However, the results show that over a diverse range of soil conditions, I/S minerals of the Batcombe series soils showed considerable resilience to major change. This is considered to be a result of competition for sorption sites by (i) the presence of competing ions added as fertiliser or liming materials, (ii) an increase in H + ions as soil acidity increases, (iii) the possible role of organic carbon in protecting I/S minerals and (iv) the role of the I/S minerals as K + reservoirs. However, the impact of decreasing soil pH should be considered when land is re-forested as this may lead to decreases in the d(001) spacing of I/S minerals, thus compromising useful clay properties such as CEC.

Role of Interlayer Hydration in Lincomycin Sorption by Smectite Clays

Lincomycin, an antibiotic widely administered as a veterinary medicine, is frequently detected in water. Little is known about the soil-water distribution of lincomycin despite the fact that this is a major determinant of its environmental fate and potential for exposure. Cation exchange was found to be the primary mechanism responsible for lincomycin sorption by soil clay minerals. This was evidenced by pH-dependent sorption, and competition with inorganic cations for sorptive sites. As solution pH increased, lincomycin sorption decreased. The extent of reduction was consistent with the decrease in cationic lincomycin species in solution. The presence of Ca2+ in solution diminished lincomycin sorption. Clay interlayer hydration status strongly influenced lincomycin adsorption. Smectites with the charge deficit from isomorphic substitution in tetrahedral layers (i.e., saponite) manifest a less hydrated interlayer environment resulting in greater sorption than that by octahedrally substituted clays (i.e., montmorillonite). Strongly hydrated exchangeable cations resulted in a more hydrated clay interlayer environment reducing sorption in the order of Ca- < K- < Cs-smectite. X-ray diffraction revealed that lincomycin was intercalated in smectite clay interlayers. Sorption capacity was limited by clay surface area rather than by cation exchange capacity. Smectite interlayer hydration was shown to be a major, yet previously unrecognized, factor influencing the cation exchange process of lincomycin on aluminosilicate mineral surfaces.

Gas-hydrate promotion by smectite–bioproduct interactions

Gas hydrates accumulate on the Gulf of Mexico seafloor around hydrocarbon seeps in waters sufficiently deep to provide adequate pressure/temperature combinations. High microbial activities occur around the hydrate accumulations. To understand apparent catalytic effects of bioproducts on hydrate formation, the mechanism of sII hydrate nucleation in unconsolidated porous media was investigated in our laboratory. Because smectite clays interacting with biopolymers had been shown to promote laboratory hydrates by decreasing induction times, increasing formation rates and altering morphology, these materials commonly found in near-surface sediments were selected for study as possible nucleating agents. Dynamic light scattering (DLS) with a helium–neon laser was used to measure particle diameters down to about 2 nm. Scanning electron microscopy (SEM) was utilized to verify particle sizes and to give additional information on biopolymer–clay associations. This paper presents evidence that nanometer-sized particles of mineral–bioproduct associations of about 80 nm–450 nm diameter may act as nucleation sites for hydrate crystal initiations in sediments and then remain dispersed throughout the accumulated hydrate mass. Emulsan biopolymer was shown by SEM to apparently unfold and associate with the smectite (nontronite) in a backbone arrangement and to give multiple hydrate nucleation sites along a linear network. SEM and DLS measurements were in agreement on particle sizes and shapes. X-ray diffraction suggested that biopolymer intercalates the smectite interlayer, probably driven by clay associations with biopolymer hydrophilic groups. It is hypothesized that hydrocarbon gases attach to biopolymer hydrophobic fatty-acid branches protruding from clay interlayers and subsequently facilitate hydrate structure formation by interacting with nearby water associated with the hydrophilic segments of the biopolymer.

Surface properties of illite-smectite minerals as detected by interactions with rhodamine 6G dye

AbstractInteractions between smectite clay minerals and various organic dyes have been studied extensively, but little information has accumulated from dye interactions with mixed-layer illite-smectite (I-S) minerals, especially regarding relationships with clay layer expandability, layer charge, particle size/shape, and molecular aggregation of organic dye molecules. The purpose of this study was to investigate the surface interactions of a set of mixed-layer illite-smectites from different geological environments with Rhodamine 6G dye. The samples used have different amounts of expandable smectite interlayers, different particle size and/or shape, and different layer-charge density at the surface. Five smectites with differences in layer charge and some non-expandable layer silicates were also tested. The interactions detected by UV-vis spectroscopy show no reaction between R6G and non-expandable minerals (kaolinite, mica), and intense reactions forming H-aggregates and monomers with smectites and illite-smectites. The intensity of H-aggregate formation increases with increase in the layer charge of smectites. Mixed-layer illite-smectites interact with R6G more intensely than do smectites. H-aggregate and monomer formation increases with the illitization process for randomly ordered illite-smectites (R = 0) and decreases in the course of illitization for the ordered illite-smectites (R &gt; 0).

Long-term geochemical evolution of the near field repository: Insights from reactive transport modelling and experimental evidences

The KBS-3 underground nuclear waste repository concept designed by the Swedish Nuclear Fuel and Waste Management Co. (SKB) includes a bentonite buffer barrier surrounding the copper canisters and the iron insert where spent nuclear fuel will be placed. Bentonite is also part of the backfill material used to seal the access and deposition tunnels of the repository. The bentonite barrier has three main safety functions: to ensure the physical stability of the canister, to retard the intrusion of groundwater to the canisters, and in case of canister failure, to retard the migration of radionuclides to the geosphere. Laboratory experiments (< 10 years long) have provided evidence of the control exerted by accessory minerals and clay surfaces on the pore water chemistry. The evolution of the pore water chemistry will be a primordial factor on the long-term stability of the bentonite barrier, which is a key issue in the safety assessments of the KBS-3 concept. In this work we aim to study the long-term geochemical evolution of bentonite and its pore water in the evolving geochemical environment due to climate change. In order to do this, reactive transport simulations are used to predict the interaction between groundwater and bentonite which is simulated following two different pathways: (1) groundwater flow through the backfill in the deposition tunnels, eventually reaching the top of the deposition hole, and (2) direct connection between groundwater and bentonite rings through fractures in the granite crosscutting the deposition hole. The influence of changes in climate has been tested using three different waters interacting with the bentonite: present-day groundwater, water derived from ice melting, and deep-seated brine. Two commercial bentonites have been considered as buffer material, MX-80 and Deponit CA-N, and one natural clay (Friedland type) for the backfill. They show differences in the composition of the exchangeable cations and in the accessory mineral content. Results from the simulations indicate that pore water chemistry is controlled by the equilibrium with the accessory minerals, especially carbonates. pH is buffered by precipitation/dissolution of calcite and dolomite, when present. The equilibrium of these minerals is deeply influenced by gypsum dissolution and cation exchange reactions in the smectite interlayer. If carbonate minerals are initially absent in bentonite, pH is then controlled by surface acidity reactions in the hydroxyl groups at the edge sites of the clay fraction, although its buffering capacity is not as strong as the equilibrium with carbonate minerals. The redox capacity of the bentonite pore water system is mainly controlled by Fe(II)-bearing minerals (pyrite and siderite). Changes in the groundwater composition lead to variations in the cation exchange occupancy, and dissolution–precipitation of carbonate minerals and gypsum. The most significant changes in the evolution of the system are predicted when ice-melting water, which is highly diluted and alkaline, enters into the system. In this case, the dissolution of carbonate minerals is enhanced, increasing pH in the bentonite pore water. Moreover, a rapid change in the population of exchange sites in the smectite is expected due to the replacement of Na for Ca.

Modeling the diffusion of Na+ in compacted water-saturated Na-bentonite as a function of pore water ionic strength

Assessments of bentonite barrier performance in waste management scenarios require an accurate description of the diffusion of water and solutes through the barrier. A two-compartment macropore/nanopore model (on which smectite interlayer nanopores are treated as a distinct compartment of the overall pore space) was applied to describe the diffusion of 22Na+ in compacted, water-saturated Na-bentonites and then compared with the well-known surface diffusion model. The two-compartment model successfully predicted the observed weak ionic strength dependence of the apparent diffusion coefficient (Da) of Na+, whereas the surface diffusion model did not, thus confirming previous research indicating the strong influence of interlayer nanopores on the properties of smectite clay barriers. Since bentonite mechanical properties and pore water chemistry have been described successfully with two-compartment models, the results in the present study represent an important contribution toward the construction of a comprehensive two-compartment model of compacted bentonite barriers.

Clay mineral reactions caused by frictional heating during an earthquake: An example from the Taiwan Chelungpu fault

To understand the chemical reactions of clay minerals in a fault zone during an earthquake, we analyzed the clay minerals in the Chelungpu fault, which slipped during the 1999 Chi‐Chi earthquake. X‐ray diffraction spectroscopy showed that kaolinite and smectite contents were lower in the black gouge zone than in the surrounding gray gouge, breccia, or fracture‐damaged zones. We applied a chemical kinetics approach to examine whether dehydroxylation of kaolinite and dehydration of interlayer water, dehydroxylation, and illitization of smectite occurred during coseismic frictional heating, and found that the first two reactions could complete under the temperature‐time profile of the Chi‐Chi earthquake, reconstructed by a previous study. Because dehydration of smectite interlayer water and dehydroxylation of kaolinite would have completed 3.6 and 8.2 s after the beginning of slip, the resulting release of water might have affected the frictional mechanism during the earthquake.

Comparison of saponite and montmorillonite behaviour during static and stirring maturation with seawater for pelotherapy

Two different bentonites (saponite and montmorillonite) have been used for maturation with seawater. These bentonites are very rich in phyllosilicates (> 90%), with calcite, silica minerals (quartz, cristobalite) and feldspars (plagioclase) as subordinated minerals. Maduration tests have been realized both in static and stirring conditions and sampling points at 0, 15, 30, 60, 90 and 120 days have been established. Several mineralogical (XRD, SEM-EDX) chemical and physico-chemical (CEC, pH, granulometry, plasticity, conductivity) analytical techniques have been used. Results indicate no mineralogical changes during maturation process but differences in the smectite crystallinity (crystallite size) and basal spacing d (001). The last related with the variation of exchangeable cations (mainly Na + for Ca 2+) within the smectite interlayer. Variations in granulometry, plasticity index and conductivity have also been observed. Similar results are reported for static and stirring procedures, and therefore the former is preferred. Maturation time of between 60 and 90 days would be enough for the bentonite to achieve the optimum physico-chemical properties for its use as a peloid. Between the two bentonites under study, saponite is clearly better than montmorillonite for its application as peloids in pelotherapy. This fact is mainly due to differences in grain size distribution and consequently in the cationic exchange capacity, which enables in saponite a higher retention of water ions making possible their transference to patients, and therefore reaching a better influence in their health.

A Thermodynamic Understanding of Clay‐Swelling Inhibition by Potassium Ions

Caged: Potassium ions are trapped in coordination cages composed of surface hexagons and water molecules in a smectite interlayer (see picture; blue K, red O, white H). This microstructure leads to a striking stability of the single-layer K-smectite hydrate and inhibits further swelling of the clay.

Modeling of smectite illitization in burial diagenesis environments

The kinetics of smectite illitization in shale samples from eight wells have been modeled using the equation −d S/d t = kK 0.25 S 5 [Cuadros, J., Linares J., 1996. Experimental kinetic study of the smectite-to-illite transformation. Geochim. Cosmochim. Acta 60, 439–453], where S is the proportion of smectite layers in illite-smectite (I–S), t is time, k is the rate constant and K is the concentration of potassium. The I–S compositions and the physical parameters were obtained from the literature. Potassium concentration is the only unknown parameter and it was chosen to fit the experimental data. I propose that this variable is termed “effective K concentration,” as it is affected not by the actual K concentration alone but also by those of other cations that compete with K for the smectite interlayer sites, especially Ca, by ion mobility in the sediments and possibly by other variables. The kinetic equation allows an accurate fit to the experimental data in all the wells, reproducing both the illitization onset and the patterns of the plots of % smectite vs. depth. The relative changes of the values of “effective K concentration” compare well with K/Ca equivalent ratios in the whole rock. Three of the modeled wells, located in near proximity, produced similar “effective K concentration” patterns. These results support the validity of the equation used in the models. They also suggest that this equation can be used to assess K concentrations in sediments and K mobility within basins.