A post-stishovite SiO2 polymorph in the meteorite Shergotty: Implications for impact events

Transmission electron microscopy and electron diffraction show that the mar tian meteorite Shergotty, a shocked achondrite, contains a dense orthorhomb ic 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 occu r in a post-stishovite SiO2 structure, The presence of such a high-density polymorph in a shocked sample indicates that post-stishovite SiO2 structure s may be used as indicators of extreme shock pressures.