XAS OF TRACE-ELEMENT COORDINATION IN NATURAL SEDIMENTS AT AMBIENT ANDCRYOGENIC TEMPERATURES

The molecular coordination of trace levels of Zn (1-2 wt%) in natural sediments from an acid mine drainage and of V, Cr, and Mn (10-300 ppm) from weathered hydrothermal soils was examined by synchrotron X-ray a bsorption spectroscopy (XAS) at ambient and cryogenic (approximate to 10 K) temperatures. XAFS analyses of mine drainage sample spectra show that, with increased weathering, Zn coordination changes from a sulfi de phase in its host mineral, sphalerite (ZnS), to coordination indica tive of hydroxide phases. In samples with silica, carbonate, and iron- hydroxide phases (3-54 wt% Fe(OH)(3)), the local atomic Zn environment changes with the total amount of bulk iron hydroxide, but Zn is not i ncorporated into carbonate or silica phases. Analysis of pre-edge and K-edge spectral features of very low concentrations of V, Cr, and Mn i n oxidized, kaolinite-rich soils from hydrothermal areas indicates red uced valence states for these ions (V-3+,V-4+, Cr3+, Mn2+) and suggest s trapping of metals under paleo-reducing conditions during clay forma tion.