POLARIZED EXAFS, DISTANCE-VALENCE LEAST-SQUARES MODELING (DVLS), AND QUANTITATIVE TEXTURE ANALYSIS APPROACHES TO THE STRUCTURAL REFINEMENT OF GARFIELD NONTRONITE

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 a pplication of P-EXAFS to structural studies of layer silicates has bee n restricted to single phyllosilicate crystals (Manceau et al. 1988; M anceau et al. 1990), but we show here that P-EXAFS can rigorously be a pplied to self-supporting clay films without loss of spatial resolutio n. The quantitative analysis of P-EXAFS requires however the preparati on of highly oriented clay films, the orientation distribution of whic h can be assessed by texture goniometry. The Fe K-edge linear dichrois m 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 nb initio modeling quanti tatively accounts for the angular dependence of experimental EXAFS spe ctra. These calculations allowed for the identification of the fundame ntal character of single-and multiple-scattering paths of the photoele ctron, and the structural interpretation of all spectral features obse rved up to 6.5 Angstrom for the in-plane and out-of-plane radial struc ture functions of nontronite. In practice, P-EXAFS measurements allow the determination of the flattening angle of Fe(O,OH)(6) octahedra, ca tions distribution in the octahedral sheet with an enhanced sensitivit y, and differentiation between dioctahedral and trioctahedral structur es.