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Twelve nontronites and two ferruginous smectites have been characterized with respect to Fe3+ occupancy of tetrahedral sites. The techniques used were near infrared, Fe-K X-ray absorption near-edge and X-ray absorption fine-structure spectroscopies, along with two X-ray diffraction techniques. The results show that calculations of the structural formulae of many nontronites should be adjusted to include Fe3+ in tetrahedral sites. The nontronite from Spokane County, Washington, (∼44% Fe2O3) is essentially an end-member with its non-siliceous tetrahedral sites occupied by Fe3+. Samples with chemical compositions similar to Garfield nontronite (∼36.5% Fe2O3) may have small amounts (<5% of total Fe3+) of tetrahedral Fe3+. Tetrahedral Fe3+ is unlikely to be present in samples containing less than ∼34% Fe2O3.

This paper reports on the new application of polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy to fine-grained layer silicates taking the Garfield nontronite as a case study. Up to now application of P-EXAFS to structural studies of layer silicates has been restricted to single phyllosilicate crystals Manceau et al. 1990), but we show here that P-EXAFS can rigorously be applied to self-supporting clay films without loss of spatial resolution. The quantitative analysis of P-EXAFS requires however the preparation of highly oriented clay films, the orientation distribution of which can be assessed by texture goniometry. The Fe K-edge linear dichroism measurements were simulated by ab initio EXAFS modeling performed on a nontronite cluster whose structure was refined by distance-valence least-squares calculations. It is shown that ab initio modeling quantitatively accounts for the angular dependence of experimental EXAFS spectra. These calculations allowed for the identification of the fundamental character of single-and multiple-scattering paths of the photoelectron, and the structural interpretation of all spectral features observed up to 6.5 for the in-plane and outof-plane radial structure functions of nontronite. In practice, P-EXAFS measurements allow the determination of the flattening angle of Fe(O,OH) 6 octahedra, cations distribution in the octahedral sheet with an enhanced sensitivity, and differentiation between dioctahedral and trioctahedral structures.

It has long been realized that cations play a critical role in the readsorption of water into the interlayer region in clay minerals. To explore possible differences in the water dynamics related to the presence of cations in clays, and to examine the dynamics of its surface water, which plays a prominent role in diffusion of water in clay barriers a comparative study was carried out to highlight differences between water dynamics in montmorillonite and halloysite. Whereas montmorillonite has interlayer cations that interact with interlayer water, and which can rehydrate after dehydration at temperature, halloysite has no interlayer cations. Water is found in both interlayers and on the surface of these clay particles. In this study we show that by combining incoherent inelastic neutron scattering (quasi-elastic and elastic fixed window) and neutron spin echo, it was possible to discriminate the dynamics of surface water (by collapsing the interlayer region by heating and rehydrating the surface layer) from interlayer water. The analysis of the elastic fixed window scans in the temperature range 5−300 K revealed an extension of water dynamics in montmorillonite to lower temperatures than in halloysite. These differences suggested mechanisms that cations (Na+ in this case) in the interlayer regions facilitate water mobility allowing interlayer water to be readmitted to montmorillonite. Finally it was shown that the occurrence of magnetic fluctuations, caused by the presence of paramagnetic Fe3+ ions in the crystalline clay lattice, gave rise to a quasi-elastic contribution that disrupted the evaluation of water diffusion computed from such measurements. Therefore previous estimates of water diffusion coefficients might have been overestimated in recent literature.

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Abstract--A structural model for the geometry of Fe(III) octahe-dra near the surface of finely divided ferrihydrite was elaborated based on the bond-valence theory and by considering the interaction of water molecules in the 2 nearest hydration spheres. In contrast to bulk Fe atoms, which are bonded to bridging oxo (O) and hydroxo (OH) ligands, surface Fe atoms are also octahedrally coordinated to HzO ligands forming the 1st hydration shell ((H20)0. In the wet state, external water molecules of the 2nd hydration shell ((H20)n) are singly H-bonded to (H20) 1, while they are doubly coordinated in the dry state. Accordingly, wet ferrihydrite contains twice as many sorbed water molecules as dry ferrihydrite, and the structural difference due to the 2nd hydration shell accounts quantatively for the 15% increase of ferrihydrite weight experimentally measured in moist atmosphere. The interaction of surface Fe atoms with their 2 nearest hydration spheres modifies the geometry of surface Fe octahedra as compared to bulk octahedra, and idealized Fe-OH and Fe-H20 bond lengths in the wet and dry state were evaluated by the bond-valence theory. Our structural model provides a sound crystal-chemical basis to describe many apparent incongruities of Fe X-ray absorption near edge structure (K-XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic data that have led to differing interpretations of the coordination environment of Fe in ferrihydrite by various investigators.