High-charge Smectite Research Articles

Physico-chemical properties of illite in the organic matter (OM)-rich claystone of Nankai Trough contrast with coeval OM-poor claystone: IODP Expedition 348

Naturally occurring microbially induced smectite-to-illite (Sm-I) reaction is of great interest to understand the control for the conventional Sm-I transformation, the source of bioavailable iron (Fe) in sediments, and the evolution of the microbial community in the deep biosphere. Direct comparisons of physico-chemical properties of illite-like phases including collapsed high-charge smectite and transient phase, and mature illite were performed for the organic matter (OM)-rich claystone in contrast with those of coeval OM-poor claystone (2163 m–2217 m below sea floor) buried at Nankai Trough (Integrated Ocean Discovery Program Site C0002). These clay layers are interbedded with 70°–90° dipping angle, such that they have nearly identical burial conditions of temperature, time, and fluid geochemistry except for OM content, one of the significant controls that can fuel heterotrophic microbial Fe (III) reduction. Unlike the abiotic Sm-I reaction, the local variations in the crystal structure, elemental composition, and oxidation states of Fe for the illite and illite-like phases in the OM-rich claystone suggest that illitization is promoted through the reduction of structural Fe (III) in smectite by microbial activity, however the effects of OM cannot be ruled out.

Role of clay cation exchange capacity, location of charge, and clay mineralogy on potassium availability in Indian Vertisols

Abstract Precise information on the location of charge in clay minerals and their charge density in smectite-dominant soils is rare. The present study was undertaken with three benchmark Vertisols to establish the relationship between the clay cation exchange capacity (CEC), charge density, as well as the location of charge in smectitic soil clay minerals and their relationship with potassium (K) fixation and release. Potassium fractions and their threshold levels in the Vertisols were determined by standard methods. Soils were segregated into silt, total clay and fine clay fractions for X-ray diffraction analysis and fine clay fractions were used to determine the CEC using standard methods. The Hofmann-Klemen effect (HK) and modified Greene-Kelly test was done with the fine clay to determine the CEC of the tetrahedral sheet. Subsequently, the CEC of the octahedral sheet was calculated as the difference between total CEC and the tetrahedral CEC. The results showed that ~60–64% of the total CEC is attributed to the tetrahedral layers. The tetrahedral CEC that is proportional to the tetrahedral charge density was significant and negatively correlated with K release threshold values and all fractions of K. The tetrahedral CEC contributed more toward the K fixation and release than the octahedral CEC. The study shows that the dominant presence of high-charge smectites in the fine clay fractions of these Vertisols contributed to the tetrahedral CEC and consequently to the charge density of these soils, which implied a tendency to fix K easily and release it with greater difficulty compared to soils with low-charge smectites.

New laboratory procedure for distinguishing smectites with varying permanent layer charge based on the combination of Cu-trien and ethylene glycol solvation methods and X-ray diffraction

Permanent layer charge of smectites (or smectite layers in mixed-layer minerals) is important for their properties and materials' applicationss. However, distinguishing low- and high-charge smectites by traditional laboratory methods is time-consuming and instrumentally demanding and it can also produce equivocal results. In addition, the use of structural formulae based on chemical analyses combined with knowledge on crystallochemistry is challenging in arbitrary mineral mixtures. The aim of this work was to test a novel laboratory procedure for distinguishing actual multi-mineral specimens of smectites according to their permanent layer charge. This is conducted on interlayer cation exchange by the Cu-trien complex and subsequent solvation with ethylene glycol. This procedure returned more straightforward results than the traditional routine of ethylene glycol-saturated K+-exchanged specimens by producing better-resolved X-ray diffraction (XRD) patterns. The practical application was demonstrated by analysing sauconite samples from Skorpion in Namibia, which was shown to contain more sauconite phases of distinct crystallographic properties present in variable proportions. XRD of the oriented ethylene glycol-saturated specimens was insufficient for this purpose. The procedure was tested by reference clay minerals of the smectite group with varying structural parameters. It was distinguished more smectite phases with distinct layer charges and estimated the layer charge by combining interlayer cation exchange with the Cu-trien complex and ethylene glycol solvation. This was followed by XRD analyses and full-profile diffraction pattern modeling. Further investigation is required for comparison with other methods. Possible reasons for the formation of two populations of sauconites were discussed.

Molecular dynamics simulation of NH4+-smectite interlayer hydration: Influence of layer charge density and location

Classical molecular dynamics (MD) simulations were performed to quantify the possible effects of layer charge density and location on NH4+ and the water structure, energetics and dynamics of hydrated NH4+-smectites. The results showed that the layer spacing decreased with increasing charge density. Hydration occurred more easily in NH4+-smectites with tetrahedral substitution than those with octahedral substitution. Stable monolayer hydrates and hypothetical bi- and trilayer hydrates were found at a water content of 4, 10 and 15 H2O·uc–1 (H2O per unit cell), respectively. The binding affinity of the surface towards NH4+ increased as the charge density of the smectites increased. For low- and medium-charge smectites (σ = –0.5 and –1.0 e·uc–1), the affinity was also dependant on the hydration level: some of the NH4+ in inner-sphere complexes with the surface turned into outer-sphere complexes and even formed bulk water–like structures as hydration levels increased. However, for high-charge smectites (σ = –1.5 e·uc–1), NH4+ was still present in inner-sphere complexes with the oxygens of the surface hexagonal cavities regardless of hydration level. The layer charge also affected the NH4+ diffusion coefficient in the interlayer regions; NH4+ in T-substituted smectites diffused more slowly than in O-substituted ones and increasing the charge density of smectites inhibited the mobility of NH4+. Quantification of these interactions can provide a basis for understanding the swelling process of clay and the release process of NH4+ during the ion-type rare earth mining.

Surface plasmon resonance study of adhesion kinetics of smectites on the Au/water interface: Clay and organo-clay film formation

Surface plasmon resonance was employed for the in-situ investigation of film formation from aqueous dispersions of clay minerals on gold/water interface. High charge (SAz-1) and intermediate charge smectite (STx-1b) were studied for their ability to form stable clay films on gold surfaces and their adhesion kinetics at different concentrations was investigated. Layer charge and charge distribution seems to affect not only the kinetics of film formation but also the overall amount of clay mineral on the gold substrate. Subsequently to stable clay mineral film formation, the in-situ preparation of hybrid organo-clay films with the introduction of alkylammonium chains and their adsorption kinetics was examined. Alkylammonium adsorption via ion exchange in the interlayer of the clay minerals is controlled by the density of the clay mineral film in full accordance with layer charge and charge localization. Hence, a simple self-assembly procedure for the in-situ preparation of hybrid organo-clay films for novel applications is proposed.

Soil Clay Mineralogical Phase Analysis of Ganges Floodplain Soils by XRD and XRF

Soil minerals study is vital in terms of investigating the major soil forming compounds and to find out the fate of minor and trace elements in soils. It is also essential for the soil-plant interaction purpose. To identify soil mineral phases especially clay minerals, X-ray diffraction (XRD) has been a popular technique. The clay mineralogical information of soils in Bangladesh is limited, especially in Ganges flood plain region (Agro Ecological Zone (AEZ) 12 and 13). Therefore, to overcome this limitation, in this study, we performed XRD analysis of <2 mm fractions soil samples of AEX 12 and 13. However, identifying mineralogical phases by XRD in <2 mm fractions soils is not so straight-forward due to many practical problems. We fully matched only two mineralogical phases in all the soil samples which is quartz and potassium-Aluminum-Silicate. However, the full XRD peaks indicate that more minerals are also present, but due to heterogeneity of soils samples, it is difficult to find other minerals phases by only XRD peak of <2 mm fractions. Therefore, to find more information about mineralogical phases, we performed XRF analysis that provides the elemental composition of minerals phase as oxide. XRF analysis indicated the presence of secondary minerals like illite and chlorite. The presence of high percentage Fe oxide not only indicated the iron mineral phase (goethite and ferrihydrite) but also indicated iron rich high charge smectite minerals (beidellite). The presence of iron rich smectite minerals in the Ganges sediments reported in several previous studies. Thus, we concluded that only XRD in <2 mm fractions of soils is not adequate to identify the mineralogical phases of soil samples. Others analyses like XRF, XRD in <2 μm fractions will be necessary to locate an entire image of soil mineralogical phases.

Intercalation of Ethylene Glycol in Smectites: Several Molecular Simulation Models Verified by X-Ray Diffraction Data

AbstractOrgano-clays represent a special challenge for molecular simulations because they require accurate representation of the clay and the organic/aqueous sections of the model system and accurate representation of the interactions between them. Due to the broad range of force-field models available, an important question to ask is which sets of parameters will best suit the molecular modeling of the organo-intercalated smectites? To answer this question, the structure of the ethylene glycol (EG)-smectite complex is used here as a testing model because the intercalation of EG in smectites provides a stable interlayer complex with relatively constant basal spacing.Three smectite samples with substantially different layer charge and charge localization were selected for X-ray diffraction (XRD) measurements. Their molecular models were built and molecular-dynamics simulations performed using various combinations of the organic force fields (CGenFF, GAFF, CVFF, and OPLS-aa) with ClayFF and INTERFACE force fields used to describe smectites. The simulations covered a range of different EG and water contents. For every structure, the density distribution of interlayer species along the direction perpendicular to the layer plane was calculated and then used to optimize the XRD patterns for these simulated models.A comparison of these results with experimental XRD patterns shows very large discrepancies in the structures and basal spacings obtained for different layer charges as well as for different force fields and their combinations. The most significant factor affecting the accuracy of the calculated XRD patterns was the selection of the clay-mineral force-field parameters. The second important conclusion is that a slight modification of the basal oxygen parameters for non-electrostatic interactions (increase of their effective atomic diameters) may be a simple and straightforward way to improve significantly the agreement between the modeled XRD patterns with experiments, especially for high-charge smectites. Generally, among organic force fields, the least accurate results were obtained with CGenFF. For unmodified ClayFF, its combination with GAFF gave the best results, while the two other sets (OPLS-aa and CVFF) gave the best results in combination with ClayFFmod. The INTERFACE and INTERFACEmod produced much better results for low-charge montmorillonite than for high-charge smectites.

Expansion Behavior of Octadecylammonium-Exchanged Low- to High-Charge Reference Smectite-Group Minerals as Revealed by High-Resolution Transmission Electron Microscopy on Ultrathin Sections

AbstractUltrathin sections of reference 2:1 layer silicates treated with octadecylammonium cations were examined using high-resolution transmission electron microscopy (HRTEM) to establish the layer structure. Hitherto, few HRTEM ultrathin-section data existed on the expansion behavior of smectite-group minerals with different interlayer-charge values. Without such information, the expansion behavior of both low-charge and high-charge smectite minerals cannot be characterized and the structures observed in HRTEM images of clay-mineral mixtures cannot be interpreted reliably. Reference smectite-group minerals (Upton, Wyoming low-charge montmorillonite; Otay, California high-charge montmorillonite; a synthetic fluorohectorite; and a Jeanne d’Arc Basin offshore Newfoundland clay sample) with a range of layer charge values were examined. To prevent possible intrusion of epoxy resin into interlayers during embedding, the clay samples were first embedded in epoxy, sectioned with an ultra microtome, and then treated with octadecylammonium cations before examination using HRTEM. Lattice-fringe images showed that lower-charge (&lt;0.38 eq/O10(OH)2) 2:1 layers had 13–14 Å spacings, whereas higher-charge (&gt;0.38 eq/O10(OH)2) 2:1 layers had 21 and 45 Å spacings. These differently expanded silicate layers can occur within the same crystal and an alternation of these layer types can generate rectorite-like structures. For comparison, clay samples were also treated with octadecylammonium before epoxy embedding and sectioning and then examined with HRTEM. These samples mostly had highly expanded interlayers due to epoxy intrusion in the interlayer space. The reference clay minerals embedded in epoxy resin, sectioned, and treated with octadecylammonium cations were used to characterize smectite-group minerals in a natural clay sample from the Jeanne d’Arc Basin, Eastern Canada. Smectite-group minerals in this sample revealed similar structures in lattice-fringe images to those observed in the pure reference clay samples. Rectorite-like structures observed in lattice-fringe images were in fact smectite crystals with short, alternating sequences of low-charge and high-charge smectite layers rather than illite-smectite (I-S) phases with expanded smectite layers and non-expanded 10 Å illite layers.

Rheological properties of palygorskite–bentonite and sepiolite–bentonite mixed clay suspensions

This study examines the influence of layer charge and smectite content on the rheological properties of mixed bentonite–palygorskite suspensions. Eight bentonite samples containing smectite with layer charge ranging between 0.38 and 0.54e/huc were used to prepare 5% w/v mixed palygorskite–bentonite aqueous suspension at pH7. The influence of smectite on the apparent viscosity and yield point of palygorskite suspensions depends on the proportion of the smectite in the mixture and the layer charge and charge localization of the smectite. The interaction between the two end member clay minerals is maximum at low percentage (10 to 20%) of bentonite in the mixture. Low charge and high charge smectite tended to improve and deteriorate, respectively, the yield point and apparent viscosity of the suspension. This behaviour seemed to be related to the influence of layer charge of smectite on the degree of delamination and the thickness of smectite quasicrystals, which consequently determine the number of particles involved in the interaction and the strength of particle–particle linkages.

Microbial reduction of Fe(III) in smectite minerals by thermophilic methanogen Methanothermobacter thermautotrophicus

Clay minerals and thermophilic methanogens can co-exist in hot anoxic environments, including the continental subsurface, geysers, terrestrial hot springs, and deep-sea hydrothermal vent systems. However, it is unclear whether thermophilic methanogens are able to reduce structural Fe(III) in clay minerals. In this study, the ability of a thermophilic methanogen Methanothermobacter thermautotrophicus to reduce structural Fe(III) in iron-rich and iron-poor smectites, (nontronite NAu-2 and Wyoming montmorillonite SWy-2) and the relationship between iron reduction and methanogenesis were investigated. M. thermautotrophicus reduced Fe(III) in nontronite NAu-2 and montmorillonite SWy-2 with H2/CO2 as substrate. The extent of bioreduction was 27% for nontronite and 13–15% for montmorillonite. Anthraquinone-2,6-disulfonate (AQDS) did not enhance the extent of bioreduction, but accelerated the rate. When methanogenesis was inhibited via addition of 2-bromoethane sulfonate (BES), the extent of bioreduction decreased to 16% for NAu-2 and 9% for SWy-2. These data suggest that Fe(III) bioreduction and methanogenesis were mutually beneficial. The likely mechanism was that Fe(III) bioreduction lowered the reduction potential of the system so that methanogenesis became favorable, and methanogenesis in turn stimulated the growth of the methanogen, which enhanced Fe(III) bioreduction. NAu-2 was partly dissolved and high charge smectite and biogenic silica formed as a result of bioreduction.

Reaction of bentonites with pyrite concentrate after wetting and drying cycles at 80°C: relevance to radioactive waste (Radwaste) storage

AbstractThe mineral stability of two bentonites was studied in the presence of pyrite concentrate to simulate the possible reactions between the bentonite barrier used in a high-level nuclear waste (HLW) repository and host rock containing up to 5 wt.% of admixed pyrite. Smectite was the only bentonite mineral affected by pyrite treatment under the experimental conditions used. Bentonites reacted differently with pyrite due to the different nature of the smectites. The distinct crystal chemistry of the smectites controlled by the composition of the parent rocks influenced the smectite surface properties (cation exchange capacity and layer charge distribution) which resulted in a different response of the bentonites to pyrite treatment. A partial transformation of the original smectites into H-smectites represented the initial stage of smectite destabilization on acid attack. Rising non-equivalent isomorphous substitution in the octahedral sheets of the smectites enhanced smectite reactivity and thus the reaction between bentonite and pyrite, causing lower stability of the bentonite containing high-charge smectite.

Characterization of smectite to NH4-illite conversion series in the fossil hydrothermal system of Harghita Bai, East Carpathians, Romania

Ammoniumillite (NH4-I) is one of the alteration products present in a breccia structure in the fossil hydrothermal system from Harghita Bãi (East Carpathians), Romania. A series from smectite (S) via ordered interstratified structures to NH4-I (40 to 5%S) was characterized by Xray diffraction (XRD), Fourier transforminfrared spectroscopy (FTIR), scanning and transmission electron microscopy (SEM and TEM), and chemical analyses. Calculation of one-dimensional Xray patterns was simulated with the NEWMOD code. Transition from two- to one-water smectite interlayer was identified by XRD. Selected samples were saturated with K+, Mg2+, and Li+cations to differentiate low- to high-charge smectite or beidelite layers. X-ray patterns of random powders of K+saturated samples, heated at 300 °C show a transition from 1Md to cis- and trans- vacant 1M polytype. The cell parameters of the cisvacant and transvacant 1M polytype were calculated by oblique texture electron diffraction. The vibration frequencies at 1430 cm-1 of the NH bond were identified in the samples analyzed. Scanning and transmission electron microscopy images show morphological changes from flaky to lathlike shapes. The mean shape ratio of lath crystals ranges from 6 to 5.42 nm and the mean area from 7.8 to 24 × 104 nm2. The mean thickness of the NH4-I layers ranges from 4.62 to 7.89 nm. The calculated structural formula of end-member NH4-I (5%S) is: [(NH4+)0.66K+0.10Na+0.01Sr2+0.02]0.81 (Al3+1.85Fe3+0.01Mg2+0.15)2.01(Si4+3.30 Al3+0.70)4.00O10(OH)2. The fixed NH4+ content quantified ranges from 0.39 to 0.66 atoms per half unit cell [O10(OH)2]. Tetrahedral and octahedral substitutions took place as the %S decreases. The NH4-I-S series formed via direct precipitation from solution at different temperatures.

Transformation of mica to high-charge smectite, depending on the particle size, in a yellow-brown soil of China.

Clay mineralogical composition of a Yellow-brown soil of China was examined. Common to the A and C horizons, micas and their transformation products dominated the clay fraction. The transformation products of micas were, however, different between the fine (<0.2 μm) and coarse (0.2-2 μm) clay fractions. Irregularly interstratified mica/smectite or mica/vermiculite/smectite of the smectite-rich phase was found in the fine clay fraction, while smectite and regularly interstratified mica/smectite were detected in the coarse clay fraction. The endo-type and exo-type K-replacement models were proposed to explain the different transformation products between the two clay fractions

Clay minerals record from Late Quaternary drill cores of the Ganga Plains and their implications for provenance and climate change in the Himalayan foreland

This study documents the coupling of provenance and climate change over the last 100ka manifested in clay mineralogy of sediments from two cores (~50m deep) in the Ganga–Yamuna interfluve in the Himalayan Foreland Basin, India. Depth distribution of the texture and clay mineral assemblage in the two cores show notable differences on account of pedogenesis and sediment supply over the last 100ka. Core sediments from the northern part of the interfluve (IITK core) are micaceous and dominated by hydroxyl-interlayered dioctahedral low-charge smectitea (LCS) in fine clay fraction but by trioctahedral high-charge smectite (HCS) in silt and coarse clay fractions. In contrast, core sediments from the southern part of the interfluve (Bhognipur core) are poor in mica and both LCS and HCS are recorded in the upper 28m of the core while the lower part is dominantly LCS in all size fractions. The paleosols in the two cores formed in the sub-humid to semi-arid climatic conditions resulting in clay minerals such as 1.0–1.4nm minerals, vermiculite, HCS and also preserved the LCS, hydroxyl-interlayered vermiculite (HIV) and pseudo-chlorite (PCh), and kaolin that formed earlier in a humid climate. The preservation of LCS, HIV, kaolin and PCh is a clear indicator of climate shift from humid to semi-arid in the Ganga Plains as their formation does not represent contemporary pedogenesis in the alkaline chemical environment induced by the semi-arid climate. As the simultaneous formation of both HCS and LCS is not possible at the expense of mica, the abundance of LCS sediments from both the cores suggests the role of plagioclase weathering in the formation of LCS. In the upper 28m of the Bhognipur core, the presence of both HCS and LCS in the fine clays suggests a change in sediment provenance from cratonic to a dominantly Himalayan source during Holocene. The climatic records inferred from the typical clay mineral assemblages of the two interfluve cores are consistent with the Marine Isotope Stages (MIS). The humid interglacial stages (MIS 5, 3, and 1) are marked by dominance of HIV, PCh, and LCS whereas the dominance of HCS together with pedogenic carbonate (PC) is noted in semi-arid stages (MIS 4 and 2).

Reduction of structural Fe(III) in nontronite by methanogen Methanosarcina barkeri

Clay minerals and methanogens are ubiquitous and co-exist in anoxic environments, yet it is unclear whether methanogens are able to reduce structural Fe(III) in clay minerals. In this study, the ability of methanogen Methanosarcina barkeri to reduce structural Fe(III) in iron-rich smectite (nontronite NAu-2) and the relationship between iron reduction and methanogenesis were investigated. Bioreduction experiments were conducted in growth medium using three types of substrate: H 2/CO 2, methanol, and acetate. Time course methane production and hydrogen consumption were measured by gas chromatography. M. barkeri was able to reduce structural Fe(III) in NAu-2 with H 2/CO 2 and methanol as substrate, but not with acetate. The extent of bioreduction, as measured by the 1,10-phenanthroline method, was 7–13% with H 2/CO 2 as substrate, depending on nontronite concentration (5–10 g/L). The extent was higher when methanol was used as a substrate, reaching 25–33%. Methanogenesis was inhibited by Fe(III) reduction in the H 2/CO 2 culture, but enhanced when methanol was used. High charge smectite and biogenic silica formed as a result of bioreduction. Our results suggest that methanogens may play an important role in biogeochemical cycling of iron in clay minerals and may have important implications for the global methane budget.

Mixed-layer illite-smectite in the Kinnekulle K-bentonite, northern Baltic Basin

AbstractThe composition and particle morphology of diagenetic mixed-layer illite-smectite (I-S) in the shallow buried Ordovician Kinnekulle K-bentonite were studied to understand the process of illitization in the Baltic Basin. The same K-bentonite bed from 12 different locations in the Basin was sampled and analysed by means of X-ray diffraction (XRD), atomic-force microscopy (AFM) and K-Ar dating. Illite-smectite in the samples was identified as a highly illitic R1 type illite-smectite vermiculite (high-charge smectite) mixed-layer mineral with 63–78% illitic layers. Illite-smectite was characterized by log-normally distributed thin particles with an area-weighted mean thickness varying from 1.9 to 3.6 nm and 2.1 to 3.8 nm by XRD-PVP and AFM analysis, respectively. The K-Ar diagenetic ages of the mixed-layer minerals suggest an illitization age of 370 to 420 Ma that agrees with the latest phase of the Caledonian orogeny. Illitization of the Kinnekulle bentonite was probably driven by the intrusion of K-rich fluids.

Thermodynamic modelling of clay dehydration, stability and compositional evolution with temperature, pressure and H 2O activity

We propose a thermodynamic approach to model the stepwise dehydration with increasing temperature or decreasing H 2O activity of K, Na, Ca and Mg-smectite. The approach relies on the relative stability of the different solid-solutions that describe the hydration of di- or trioctahedral-smectites containing 0, 1, 2 or 3 interlayer water layers. The inclusion of anhydrous mica end-members makes it possible to cover, with the same solid-solution model, the entire range of composition from low-charge smectite to mica, through high-charge smectite and illite. Non-ideal Margules parameters were used to describe the non-ideality of the solid solutions between the hydrated and dehydrated smectite end-members. Standard state properties of all smectite end-members as well as Ca- and Mg-muscovite and -phlogopite were initially estimated by oxide summation. These values were then refined and the other non-ideal interactions were estimated on the basis of different experimental data. The stepwise dehydration of smectite, and its stability and compatibility relations were calculated by Gibbs free energy minimising. Our results account for the progressive evolution of smectite to interlayered illite/smectite and then to mica, as observed in nature and experiments, and our model provides an explanation for the thermodynamic stability of smectite and illite/smectite compared to mica + kaolinite or pyrophyllite assemblages. The results suggest that the enthalpic contribution of interlayer water is a function of the ionic potential of the interlayer cation and the number of interlayer water molecules. This evolution makes possible to estimate the standard-state thermodynamic parameters and hydration-temperature behaviour of smectite of virtually all possible compositions. For the four-interlayer cations considered in the study, our model reproduces the 3 → 2 → 1 water-layer transitions that accompany a reduction of water activity or an increase of temperature at ambient pressure. The range of water content and interlayer distance calculated for the 3w, 2w and 1w states are also in fair agreement with the experimental values at ambient pressure.

Microbial effects in promoting the smectite to illite reaction: Role of organic matter intercalated in the interlayer

Cysteine and toluene as model organic molecules were intercalated into Fe-rich smectite (nontronite, NAu-2). The illitization of these intercalated smectites as induced by microbial reduction of structural Fe 3+ was investigated. Iron-reducing bacterium Shewanella putrefaciens CN32 was incubated with lactate as the sole electron donor and structural Fe 3+ in cysteine- and toluene-intercalated NAu-2 (referred to as cysteine-NAu-2 and toluene-NAu-2 hereafter) as the sole electron acceptor. Anthraquinone- 2, 6-disulfonate (AQDS) was used as an electron shuttle in bicarbonate buffer. The extent of Fe 3+ reduction in cysteine-NAu-2 and toluene-NAu-2 was 15.7 and 5.4%, respectively, compared to 20.5% in NAu-2 without organic matter intercalation. In the bioreduced NAu-2, X-ray diffraction, and scanning and transmission electron microscopy did not detect any discrete illite, although illite/ smectite mixed layer or high charge smectite phases were observed. In bioreduced cysteine-NAu-2, discrete illite and siderite formed. In contrast, bioreduction of toluene-NAu-2 did not result in any mineralogical changes. The contrasting bioreduction results between cysteine- and toluene-intercalated nontronite may be ascribed to the nature of organic matter-bacteria interactions. Whereas cysteine is an essential amino acid for bacteria and can also serve as an electron shuttle, thus enhancing the extent of Fe 3+ bioreduction and illitization, toluene is toxic and inhibits Fe 3+ -reducing activity. This study, therefore, highlights the significant role of organic matter in promoting the smectite to illite reaction under conditions typical of natural environments (i.e., non-growth condition for bacteria).

Influence of layer charge and charge distribution of smectites on the flow behaviour and swelling of bentonites

The influence of layer charge and charge distribution of dioctahedral smectites on the rheological and swelling properties of bentonites is examined. Layer charge and charge distribution were determined by XRD using the LayerCharge program [Christidis, G.E., Eberl, D.D., 2003. Determination of layer charge characteristics of smectites. Clays Clay Miner. 51, 644–655.]. The rheological properties were determined, after sodium exchange using the optimum amount of Na 2CO 3, from free swelling tests. Rheological properties were determined using 6.42% suspensions according to industrial practice. In smectites with layer charges of − 0.425 to − 0.470 per half formula unit (phfu), layer charge is inversely correlated with free swelling, viscosity, gel strength, yield strength and thixotropic behaviour. In these smectites, the rheological properties are directly associated with the proportion of low charge layers. By contrast, in low charge and high charge smectites there is no systematic relation between layer charge or the proportion of low charge layers and rheological properties. However, low charge smectites yield more viscous suspensions and swell more than high charge smectites. The rheological properties of bentonites also are affected by the proportion of tetrahedral charge (i.e. beidellitic charge), by the existence of fine-grained minerals having clay size, such as opal-CT and to a lesser degree by the ionic strength and the pH of the suspension. A new method for classification of smectites according to the layer charge based on the XRD characteristics of smecites is proposed, that also is consistent with variations in rheological properties. In this classification scheme the term smectites with intermediate layer charge is proposed.

Effects of Ethylene Glycol Saturation Protocols on XRD Patterns: A Critical Review and Discussion

AbstractStudy of the transformation of smectite to illite, chlorite or vermiculite via interstratified clay minerals needs precise qualitative and quantitative determinations of the different layers in the mixed-layer clays and is generally based on X-ray diffraction (XRD) patterns after specific treatments of the clay samples. Saturation with K or Mg followed by ethylene glycol (EG) solvation are classical methods used to identify high-charge smectite and vermiculite. These procedures have been applied to two experimental clays, one composed of smectite layers and the second, a mixture of vermiculite and smectite layers. Different methods of glycolation (EG vapor or liquid EG) produce significant differences in the XRD patterns. Comparison with literature data indicates that K-saturated, high-charge smectite (≈0.8 &lt; total charge &lt;1/unit-cell) and Mg-vermiculite (whatever its charge) do not expand in ethylene glycol vapor (d values ≈14–15 Å). Expansion to 17 Å in liquid ethylene glycol occurs for Mg-vermiculite with a total charge of &lt;~1.2/unit-cell and for K-saturated, high-charge smectite, when the tetrahedral charge is &lt;≈0.7/unit-cell. This study shows that: (1) glycolation procedures need to be standardized; (2) the use of saturation protocols using both liquid ethylene glycol and ethylene glycol vapor yields useful additional information about the distribution of charges in clay minerals.