X-ray absorption characterization of Cr in forsterite within the MacAlpine Hills 88<SUB>13</SUB>6 EL<SUB>3</SUB> chondritic meteorite

Abstract

Chromium K-edge X-ray absorption spectra were collected to characterize Cr in forsterite (Mg2SiO4) as well as sulfides within the MAC 88136 EL3 chondrite to determine Cr valence and to see whether forsterite within this meteorite can be used as a Cr2+-silicate standard. Spectra were measured on several areas within a nearly pure 100 x 200 mu m forsterite grain containing 0.13 wt% Cr. XANES findings indicate highly reduced Cr2+ species, with no clear evidence of Cr3+ or Cr6+. EXAFS data indicate an average 2.02 angstrom Cr-O nearest-neighbor distance, consistent with Cr-O distances found in square-planar Cr2O4 sites observed in synthetic crystalline silicates, and an average 2.69 angstrom Cr-Si second-nearest neighbor distance, consistent with Cr2+ substituting for Mg2+ in the forsterite M(1) site. Nearest-neighbor Debye-Waller factor and coordination number parameters indicate Cr2+ is likely entering forsterite in disordered sites that are possible intermediates between M(1) and square-planar Cr2+O4 configurations. Preliminary Cr XAS measurements on sulfides within this meteorite also indicate Cr2+ in CrS6 octahedra.