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Science 2 June 2000:
Vol. 288. no. 5471, pp. 1632 - 1634
DOI: 10.1126/science.288.5471.1632
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Reports

A Monoclinic Post-Stishovite Polymorph of Silica in the Shergotty Meteorite

Ahmed El Goresy, 1* Leonid Dubrovinsky, 2 Thomas G. Sharp, 3 Surendra K. Saxena, 2 Ming Chen 4

A post-stishovite phase of silica was identified in the Shergotty meteorite by x-ray diffraction and field emission scanning electron microscopy. The diffraction pattern revealed a monoclinic lattice, similar to the baddeleyite-structured polymorph with the cell parameters a = 4.375(1) angstroms, b = 4.584(1) angstroms, c = 4.708(1) angstroms, beta = 99.97(3), rho  = 4.30(2) grams per cubic centimeter, where the numbers in parentheses are the maximum deviations. Transmission electron microscopy investigations indicate the presence of the alpha -lead dioxide-like polymorph, stishovite, and secondary cristobalite in the same silica grain. The mixture of high-density polymorphs suggests that several post-stishovite phases were formed during the shock event on the Shergotty parent body.

1 Max-Planck-Institut für Chemie, J.-Becher-Weg 27, D-55128 Mainz, Germany.
2 Theoretical Geochemistry Program, Institute of Earth Sciences, Uppsala University, S-75236, Uppsala, Sweden.
3 Department of Geology, Arizona State University, Tempe, AZ 85287, USA.
4 Guangzhou Institute of Geochemistry, Academia Sinica, Guangzhou 510640, China.
*   To whom correspondence should be addressed. E-mail: goresy{at}mpch-mainz.mpg.de

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