Wed 5th July @ noon, Rm 312.222
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 a-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11GPa 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.
Short bio:
I am currently studying rocks from the Moon! My project RESOLVE (Reconstructing the History of Lunar Volatiles) was funded by the European Commission in the H2020-MSCA-IF-2015 call.
It aims at understanding how the abundance and the distribution of volatile components, as well as their isotopic composition are influenced by the crystal structure of the host mineral in lunar samples. The volatile containing minerals – primarily phosphates – respond to pressure increase caused by the impact events by accommodating i.e. compressing their crystal structures, causing redistribution of the volatile, but also of the stable (e.g., H, Cl) and radiogenic isotopes (e.g., Pb). Therefore they not only provide a unique opportunity for precise age-determination of an impact event, but also simultaneous determination of the isotopic signature of the volatiles, a fingerprint to the source of the volatiles. This is a powerful approach for discriminating between indigenous and externally-derived volatile sources (solar wind, cosmic radiation, etc.), which is one of the remaining puzzles in studying the origin of water in the inner Solar System.
