Compressional pathways of ?-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation.

ABSTRACT: In various shocked meteorites, low-pressure silica polymorph ?-cristobalite is commonly found in close spatial relation with the densest known SiO2 polymorph seifertite, which is stable above ?80?GPa. We demonstrate that under hydrostatic pressure ?-cristobalite remains untransformed up to at least 15?GPa. In quasi-hydrostatic experiments, above 11?GPa cristobalite X-I forms-a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to ?-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of ?-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures.

SUBMITTER: Cernok A 

PROVIDER: S-EPMC5467234 | biostudies-other | 2017 Jun

REPOSITORIES: biostudies-other