Characterization of a high-pressure phase of silica from the Martian meteorite Shergotty

Recently, there has been substantial interest in post-stishovite high-pressure polymorphs of SiO 2 , discovered as extraterrestrial minerals, or synthesized in the laboratory. Previous investigators reported the presence of “α-PbO 2 -like” and “baddeleyite-like” SiO 2 in the Martian meteorite Shergotty, and also the synthesis of an a-PbO 2 -like phase at pressures of 60-80 GPa in a laser-heated diamond anvil cell. To provide definitive information about the nature of the natural “a-PbO 2 phase,” we recovered a small sample from the Shergotty meteorite, obtained powder X-ray diffraction patterns, and performed a Rietveld refinement of the structure. The resulting cell parameters and space group are a = 4.097(1) Å, b = 5.0462(9) Å, c = 4.4946(8) Å, and Pbcn. The structure refinement confirms that this sample does have the α-PbO 2 structure