Transmission electron microscopy and electron diffraction show that the martian meteorite Shergotty, a shocked achondrite, contains a dense orthorhombic SiO2 phase similar to post-stishovite SiO2 with the alpha-PbO2 structure. If an SiO2 mineral exists in Earth＇s Lower mantle, it would probably occur in a post-stishovite SiO2 structure, The presence of such a high-density polymorph in a shocked sample indicates that post-stishovite SiO2 structures may be used as indicators of extreme shock pressures.