X-RAY-ABSORPTION SPECTROSCOPY OF FE, MN, ZN, AND TI STRUCTURAL ENVIRONMENTS IN STAUROLITE

The local site geometries of Fe, Mn, Zn, and Ti in staurolite from Piz zo Fomo, Switzerland, have been determined by X-ray absorption spectro scopy (XAS) and the results used to deduce the crystallographic sites occupied by these elements. For each element, structural assignments a re based on the positions and intensities of preedge, edge, and near-e dge features, as well as on refined extended X-ray absorption fine str ucture (EXAFS) data. The least-squares refined, mean first-shell metal -O bond length, coordination number, and Debye-Waller factor for Fe (1 0.24 wt%) are 1.99 angstrom, 3.6, and 0.016 angstrom2, for Mn (0.14 wt %) they are 2.01 angstrom, 4.0, and 0.012 angstrom2, and for Zn (0.21 wt%) they are 1.95 angstrom, 5.1, and 0.018 angstrom2, respectively. T hese values, together with preedge and near-edge data, suggest that Fe and Mn are predominantly present as divalent cations in the tetrahedr ally coordinated T2 site. At least 70% of the Zn is also in the T2 sit e, but (6)Zn could also be present. Ti preedge and edge features and r efined first-shell EXAFS are consistent with Ti (0.40 wt%) being prese nt as Ti4+ in distorted octahedral coordination, most likely in the M2 site. Minor concentrations of Mn (1400 ppm) and Zn (2100 ppm) can pro vide reliable EXAFS site geometry information out to 5 angstrom (i.e., as far as for major Fe), which demonstrates the utility of the elemen t-specific XAS technique in elucidating the structures of chemically c omplex minerals such as staurolite.