David A. McKeown^1,*, Andrew C. Buechele¹, Ryan Tappero², Timothy J. McCoy³ and Kathryn G. Gardner-Vandy³

¹Vitreous State Laboratory, The Catholic University of America, 620 Michigan Avenue NE, Washington, D.C. 20064, U.S.A.
²Photon Sciences Department, Brookhaven National Laboratory, Upton, New York 11793, U.S.A.
³Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560-0119, U.S.A.

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 × 200 μm 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 Å Cr-O nearest-neighbor distance, consistent with Cr-O distances found in square-planar Cr²⁺O₄ sites observed in synthetic crystalline silicates, and an average 2.69 Å 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 CrS₆octahedra.

Reference
McKeown DA, Buechele AC, Tappero R, McCoy TJ and Gardner-Vandy KG (2014) X-ray absorption characterization of Cr in forsterite within the MacAlpine Hills 88136 EL3 chondritic meteorite. American Mineralogist 99:190-197.
[doi:10.2138/am.2014.4508]
Copyright: The Mineralogical Society of America

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