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

Cernok, Ana; Marquardt, Katharina; Caracas, Razvan; Bykova, Elena; Habler, Gerlinde; Liermann, Hanns-Peter; Hanfland, Michael; Mezouar, Mohamed; Bobocioiu, Ema and Dubrovinsky, Leonid (2017). Compressional pathways of α-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation. Nature Communications, 8 p. 15647.

DOI: https://doi.org/10.1038/ncomms15647

URL: http://rdcu.be/th2E

Abstract

In various shocked meteorites, low-pressure silica polymorph α-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 α-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 α-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 α-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.

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