Rapid compression experiments on quartz and implications for transitions to metastable high-pressure phases

Impacts of meteorites and asteroids played an important role in the evolution of the Earth. The most diagnostic indications of past impacts are provided by amorphous lamellae, i.e. planar deformation features (PDFs), in the mineral quartz. Despite the importance of PDFs, the mechanisms leading to their formation are not well understood and the transitions of quartz at high pressures are unclear. In this thesis, novel experiments were performed aiming at simulating the pressure conditions during impacts of large asteroids and meteorites. Single crystals and powder samples of quartz were subjected to rapid, non-hydrostatic compression and decompression in the membrane-driven diamond anvil cell. Time-resolved synchrotron X-ray diffraction was employed to observe the crystallographic changes in-situ, while transmission electron microscopy was employed to identify the microstructural changes in recovered samples. The observations reveal that, above pressures of 15 GPa, quartz transforms to a previously unidentified metastable high-pressure phase that is isostructural to the mineral rosiaite. When pressure is released, this rosiaite-structured phase collapses to lamellae of glass, elucidating the lamellar amorphization of quartz. Under certain compression directions above 20 GPa, numerous stishovite crystals can nucleate inside rosiaite-structured silica, attesting to a previously unidentified mechanism for the formation of stishovite in the solid state. The close resemblance of the present observations with observations of shock experiments and natural, shock metamorphic quartz shows, that the present experiments can reproduce typical shock-induced structural alterations in quartz and that the identified formation processes might also be active during impacts. This has far-reaching consequences for the interpretation of shock-effects and the Hugoniot curve of quartz and provides explanations for the formation mechanisms of PDFs and stishovite in the solid state.