Pressure Induced Transformations in Low Density Silica Polymorphs

Properties and behaviour of silicon dioxide SiO2 at high pressures and temperatures are of great interest due to its wide ranging implications in fundamental physics, geophysics and material sciences. Last decade studies revealed number of enigmatic phenomenon associated with high-pressure silica polymorphs - formation of unidentified phases on the compression of *-cristobalite, controversial theoretical and experimental information on the possible post-stishovite phases, discovery of new dense natural silica polymorph in the Shergotty meteorite in the mineralogical environment which is unlikely for post-stishovite phase. New experiments on compression of synthetic a-cristobalite and natural o-tridymite shows that at pressure above 40 GPa both materials transforms to a-PbO2-like silica phase. We demonstrate that Cristobalite XII and Cristobalite XIII phases discovered by Tsuchida and Yagi (1990), and natural silica polymorph recently found in the SNC meteorite Shergotty (Sharp et al., 1999) are a-PbO2-like silica phase. Experiments with electrically and laser heated DAC show that at 80 GPa a-PbO2-like silica stable up to at least 2000 K, but at 60 GPa it transforms to stishovite even at 1000 K. Depending on the pressure-temperature path in experiments with cristobalite and tridymite new phases of silica were synthesized. Their possible structures and nature will be discussed.