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.
Highlights
In various shocked meteorites, low-pressure silica polymorph a-cristobalite is commonly found in close spatial relation with the densest known SiO2 polymorph seifertite, which is stable above B80 GPa
After performing high-pressure experiments, recovered samples were investigated by powder X-ray diffraction (PXRD) and transmission electron microscopy (TEM)
Most of cristobalite found in shocked materials is believed to form after pressure release due to high post-impact temperatures
Summary
Low-pressure silica polymorph a-cristobalite is commonly found in close spatial relation with the densest known SiO2 polymorph seifertite, which is stable above B80 GPa. It is documented alongside all the natural occurrences of seifertite, a post-stishovite high-pressure polymorph of SiO2 only found in heavily shocked (25 GPa or higher) meteorites[5,6,7,8] According to these observations, a-cristobalite seems to be stable at variable pressure conditions (from ambient to more than 25 GPa), thereby not recording the peak transient pressure as the other associated phases. A-cristobalite seems to be stable at variable pressure conditions (from ambient to more than 25 GPa), thereby not recording the peak transient pressure as the other associated phases Triggered by this curious behaviour and later by interesting petrographic observations, by the coexistence of seifertite and a-cristobalite, the behaviour of a-cristobalite at high pressures was examined by numerous experimental[9,10,11,12,13,14,15,16,17,18] and theoretical studies[19,20,21] for more than two decades. This explains the seemingly controversial observations regarding its natural coexistence with a-cristobalite
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