Infrared spectra of nontronite reveal shifts to lower wave, number in peaks located at 3570, 1030, and 821 cm-1, which are assigned to vibrational modes for O-H stretching, Si-Ob (basal oxygens) tetrahedral vibrations, and FeO-H deformation, respectively, as the Fe2+ content increases upon reduction. A pleochroic shoulder at 1110 cm-1 resulting from Si-Oa (apical oxygens) interactions appears to be unchanged with reduction. The absorption band at 848 cm-1 shifts to higher energy with reduction of structural iron, and persists at 840 cm-1 after complete deuteration of structural hydroxyls. The band is assigned to a Fe-OH mode where the OH moves as a unit. The shift in the Si-Ob mode to lower vibrational energy with reduction may indicate a decrease in stress forces between the tetrahedral and octahedral sheets.

Nontronite and biotite were analyzed by Electron Spectroscopy for Chemical Analysis (ESCA). Both minerals yielded ESCA spectra with well-defined peaks and the elements identified by the ESCA spectra were in good agreement with the molecular formulas. High resolution scans were made of the iron (2p3/2) and oxygen (1s) electron binding energies for oxidized and reduced forms of nontronite and biotite. Binding energies for Fe3+ were observed at 711.8 eV in dithionite-reduced nontronite and in oxidized biotite. Peaks for Fe2+ occurred at 710.0 eV for unaltered biotite, at 709 eV for oxidized biotite and at 708.6 eV for hydrazine-reduced and dithionite-reduced nontronite. The oxygen (1s) peak for unaltered nontronite was skewed to the high energy side of the 530.6 eV maximum, but became more symmetrical as the Fe2+ content increased.

The effect of acid type and concentration on the reaction rate and products of dissolution of hectorite in inorganic acids was investigated. The dissolution of hectorite in hydrochloric (HCl), nitric (HNO3) and sulphuric (H2SO4) acids was characterized using quantitative chemical analysis, infrared (IR) and multinuclear MAS NMR spectroscopies. The rate of dissolution increased with acid concentration and decreased in the order HCl ≥ HNO3 = H2SO4 at the same molar concentration. No differences were found in the reaction products of hectorite treated with the three acids. The rate of Li dissolution was slightly greater than that of Mg at lesser acid concentrations (0.25 M), indicating that protons preferentially attack Li octahedra. The gradual changes in the Si-O IR bands reflects the extent of hectorite dissolution. The analysis of 29Si MAS NMR spectra relative peak intensities with dissolution time and acid concentration provided direct dissolution rates for tetrahedral (Q3) Si. After acid dissolution, most Si was bound in a three dimensional framework site (Q4), but a substantial part also occurred in the Si(OSi)3OH (Q31OH) and Si(OSi)2(OH)2 (Q220H) environments. These three sites probably occur in a hydrous amorphous silica phase. Both AlJV and AlVt rapidly disappeared from 27Al MAS NMR spectra of the dissolution products with acid treatment. The changes in IR and MAS NMR spectra of hectorite due to acid dissolution are similar to those of montmorillonite.

The uniformity of clay films affects the transport of an anionic electroactive probe through the film. When cations other than Na+ are present in the initial swelling of the film (such as native K+ and Ca2+), or when the native clay is highly charged, internal dislocations of the film, caused by variable stacking domains, enhance the diffusive transport of Fe(CN)63− across the film. This effect is investigated using clay-modified electrodes in which the clays are of variable exchange forms (Na+, Ca2+, or K+) and variable charge. The charge on the clay is determined both by the native form of the clay and by reduction of structural Fe3+. The results suggest that swelling of clays increases in a linear fashion with CEC, at least for minimally charged clays.

Structural Fe in ferruginous smectite (sample SWa-1, Source Clays Repository of the Clay Minerals Society) was reduced by a mixture of five Pseudomonas species of bacteria in a defined Fe-free medium to determine the effect of microbial reduction on clay swelling. Iron(II), total Fe, and gravimetric water content (mw/mc) were determined in clay gels equilibrated at applied pressures of 0.1, 0.3, and 0.5 MPa. The water content of microbially reduced SWa-1 decreased at all three applied pressures as the Fe(II) content approached about 0.8 mmol Fe(II)/g-clay. As Fe(II) increased from 0.8 mmol/g-clay, however, further change in mw/mc was negligible. Concurrent with microbial reduction of structural Fe was a significant decrease in the swelling pressure (PI) of SWa-1: for example, when mw/mc = 1.2 (g/g), PI changed from 0.47 MPa at Fe(II) = 0.2, to 0.19 MPa at Fe(II) = 0.9 mmol/g-clay. Both biologically and chemically reduced smectites displayed lower values of mw/mc and a concurrent decrease in II as Fe(II) content increased, but the effect of Fe(II) on mw/mc was greater for the microbially reduced smectites at all applied pressures.

The magnetic susceptibilities of seven different nontronites in their natural oxidation states were measured between 5 and 100 K. Results revealed that the magnetic exchange interaction in all samples was antiferromagnetic, except no clear minimum occurred at the Néel temperature. Possible explanations for this phenomenon which are discussed include magnetic dilution due to isomorphous substitution, and antiferromagnetic frustration due to either non-centrosymmetric distribution of octahedral Fe3+ or tetrahedral Fe3+ substitution. A computer simulation model was developed to demonstrate the effects of these variables on long-range magnetic ordering. Magnetic dilution and tetrahedral Fe3+ content could explain the anomalous antiferromagnetic behaviour in some, but not all, samples. The non-centrosymmetric model is the only one which explains the behaviour of all samples. In this model, at least 13% of the octahedral Fe3+ would occupy trans-dihydroxide sites, with the balance in cis sites. Magnetic frustration occurs because two Fe3+ neighbours of a third Fe3+ ion are also neighbours to each other, making the simultaneous satisfaction of all antiferromagnetic exchange interactions impossible.

Kogure and co-workers have published several papers reporting the stacking structures of various 2:1 layer phyllosilicates (e.g. aspidolite, Kogure et al. (2004); wonesite, Kogure et al. (2005); pyrophyllite, Kogure et al. (2006)), mainly by using high-resolution transmission electron microscopy (HR-TEM). However, there is ambiguity in wording that refers to the lateral displacement between two tetrahedral sheets across the interlayer regions. Zvyagin et al. (1969) used the symbol “τ” for this displacement, which describes the structures of pyrophyllite and talc, but no universally accepted wording was used for other 2:1 phyllosilicates or 1:1 phyllosilicates.

Griffithite is a trioctahedral smectite with dioctahedral domains, found in the <2 μm fraction of weathered basic rock from Griffith Park, California, USA. Crystalline admixtures (albite, calcite, quartz and maghemite) are concentrated in the 0.2 – 2 μm fraction, while the <0.06 μm fraction contains only trace amounts of other minerals. Griffithite is primarily an Fe–rich saponite with negative charge located in the tetrahedral sheets. The octahedral occupancy is ∼91%, and ∼26% of the octahedra contain trivalent atoms imparting a net positive charge to the octahedral sheet. Medium levels of Fe(III) reduction in griffithite, up to 60% of total Fe, can be achieved by adding solid sodium dithionite to clay dispersions in a citrate–bicarbonate buffer. By contrast >90% reduction of Fe(III) to Fe(II) is achieved in nontronites using the same method. The lower reducibility of Fe(III) in griffithite relative to nontronites may be due to structural differences between griffithite and nontronites, such as a more negative tetrahedral charge and a positive octahedral charge.

Virtually all 2:1 clay minerals contain some Fe in their crystal structure, which may undergo redox reaction with surrounding redox-active species causing potentially significant changes in the chemical and physical properties of the clay mineral and its surrounding matrix. This phenomenon was originally of interest mostly as a laboratory experiment using strong inorganic reduction agents, but the discovery that the structural Fe could be reduced by microorganisms in natural soils and sediments opened the way for this to become a practical method for altering the chemical and physical properties of soils and sediments in situ. The purpose of this report was to review the body of literature that has been published since the inception of this field of inquiry and to complement, update, and complete three other reviews that have been published during the intervening years. Studies of microbial reduction of structural Fe in smectites have revealed the extent of reduction, effects on chemical and physical properties, reversibility (or lack thereof) of microbial reduction, stoichiometry, possible reaction mechanism, and types of organisms involved. Some organisms are also capable of oxidizing structural Fe, such as in biotite or reduced smectite, while one appears to be able to do both. Illitic layers resist reduction by microorganisms, but this can be partially overcome by the presence of an electron shuttle compound such as anthraquinone-2,6-disulfonate, which also enhances the extent of reduction in smectites. Microorganisms may be employed as an in situ reducing agent to drive redox cycles for structural Fe in constituent clay minerals of soils and sediments, which in turn can serve as an abiotic source for redox-mediated remediation of environmental contaminants.

The clay-mineral fraction of samples from a bentonite deposit in the Zvolenská kotlina Basin (Slovakia), formed by the alteration of andesitic pyroclastics, was dominated by Fe-bearing montmorillonite of relatively low charge. The total Fe content of the samples ranged from 3.58 to 5.81 wt.% Fe. Variable-temperature Mössbauer spectroscopy revealed that structural Fe(III) in smectite accounts for 70–90% of the total Fe in the unfractionated samples. No structural Fe(II) was observed by Mössbauer spectroscopy, but a small amount was detected by wet chemical analysis. The remainder of the Fe is present in oxide and/or oxyhydroxide phases dominated by poorly ordered goethite and hematite. The hyperfine parameters of the Mössbauer spectra for the raw bentonite are consistent with structural Fe(III) in an aluminosilicate phase mixed with varying quantities of hematite, goethite and possibly maghemite. Except in the fine fraction of sample L28, the hematite failed to undergo the Morin transition, as shown by the negative values for quadrupole splitting at 298 K and 6 K.

Redox activation (reduction of structural Fe) of smectites greatly alters their chemical reactivity and physical properties, which may be exploited for various environmental, agricultural or industrial purposes. Their re-oxidation during preparation, characterization, and use is, however, a significant risk to their utility. In this study, methods and apparatus were developed and described which mitigated reoxidation. Ferruginous smectite (sample SWa-1, Na saturated) was used as the model smectite. It was reduced with sodium dithionite in a citrate-bicarbonate buffer solution at 70°C for 4 h, which achieved a maximum Fe(II)/total Fe ratio of 0.9113 ± 0.0048. The first step in rendering reduced samples useful is to remove from them the reducing agents and other solutes present during reduction. This was accomplished in the present study by reducing the sample in an inert-atmosphere reaction tube (IRT) (a 50 mL centrifuge tube equipped with a removable septum cap), then removing solutes from the suspension by centrifuge washing. The washing steps were performed with the aid of a controlled-atmosphere liquid exchanger (CALE) which provided connections between the sample suspension and deoxygenated solutions. The reduced state was measured by 1, 10-phenanthroline or by Mössbauer spectroscopy at 77 K to give Fe(II)/total Fe ratios. Some samples were freeze dried after washing. Results revealed that if reduced smectites are washed without protection from atmospheric O2, the extent of reoxidation is on the order of 40 to 60%. If the sample is subsequently dried, reoxidation increases to more than 76%. If the sample is protected using the IRT and the CALE, however, reoxidation is decreased to less than 2%. Freeze drying in a glove box allowed reoxidaton to increase to slightly more than 10%. These results indicate that more reoxidation occurred during the drying stage than during the washing stage. These observations lead to the conclusions that (1) protection of reduced samples from atmospheric O2 is essential if extensive reoxidation is to be prevented, and (2) the methods and apparatus described herein are effective for accomplishing that purpose in abiotically reduced smectites. They may also be effective if applied to microbially reduced smectites.

Multi-temperature (15, 100, 150, 200, 300, 450, 600, 900 K) single crystal neutron diffraction data on the type I clathrate Ba8Ga16Si30 are analyzed with the maximum entropy method to obtain direct space nuclear densities. The nuclear densities suggest that the guest atoms are structurally disordered, and the disorder appears to be temperature dependent with increased host-guest interaction at high temperatures.

Solid-state chemistry has been used to control both the size and the interconnection distance of small II-VI semiconductor moeities incorporated in zeolitic hosts with the sodalite-type structure. Structural characterization was carried out using X-ray Rietveld powder methods, and optical properties of these materials were also measured. These novel materials show quantum superlattice effects as evidenced by blue shifts in their optical absorption spectra.