Structural iron in dioctahedral and trioctahedral smectites: a polarized XAS study

The chemical form of structural Fe in smectites influences many physicochemical properties of these clay minerals. Powder EXAFS data for structural Fe in smectites have been reported; however, the preferred orientation of clay platelets with respect to the X-ray beam may lead to erroneous conclusions on the local chemical environment. Dioctahedral montmorillonite and for the first time trioctahedral hectorite were prepared as textured samples, and the Fe local environment was probed by analysis of the X-ray absorption pre-edge peaks at the magic angle and by polarized EXAFS (P-EXAFS) spectroscopy. Compared to powder measurements, overlapping contributions from shells with distinct orientations can be filtered more easily by P-EXAFS, thus decreasing uncertainties on structural parameters. The pre-edge spectrum of montmorillonite is similar to spectra commonly reported for dioctahedral smectites. In contrast, the pre-edge spectrum of hectorite is notably distinct and hints to either differences in the site symmetry and/or in covalence. In both smectites, Fe is surrounded by a first O shell at a distance consistent with sixfold-coordinated Fe(III), suggesting that Fe(III) is located in the smectite octahedral sheet. This is corroborated by the distances and orientations of neighboring cationic shells, such as in-plane (Mg, Al) and out-of-plane Si shells. For montmorillonite, the results indicate Fe substitution for Al/Mg in the octahedral sheet, and a number of Fe neighbors consistent with random distribution in the octahedral sheet. For hectorite, results indicate a slight tendency for Fe atoms to form pairs in octahedral sheets; however, low numbers of neighboring cations were obtained, presumably a consequence of the presence of vacancies and/or Li in the vicinity of Fe, or of the coexistence of Fe and Mg neighbors with mutually canceling EXAFS waves. Consistent with pre-edge data, the coordination numbers can also indicate some incoherency in Fe-cation interatomic distances in hectorite as a consequence of site distortion. These results suggest that Fe3+ in hectorite locally distorts the structure of the trioctahedral phyllosilicate and tends to aggregate charge-deficient (i.e., vacant or Li+-containing) octahedral sites.