2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 4
Presentation Time: 8:45 AM


HIROSE, Kei1, KUWAYAMA, Yasuhiro1, SATA, Nagayoshi2 and OHISHI, Yasuo3, (1)Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan, (2)Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan, (3)Japan Synchrotron Radiation Research Institute, 1-1-1 Koto, Mikazuki-cho, 679-5198, Japan, kei@geo.titech.ac.jp

Silica (SiO2) exhibits extensive polymorphism at elevated pressures. Previous theoretical calculations predicted the phase transition from α-PbO2-type to pyrite-type structure around 200 GPa. However, no experimental studies of silica have been made at such multimegabar pressures. We performed in-situ X-ray diffraction measurements of silica up to 291 GPa using the laser-heated diamond-anvil cell (LHDAC) techniques at the synchrotron facility of SPring-8. Results show that the pyrite-type high-pressure form is stable above 268 GPa and 1800 K. Rietveld analysis showed that the pyrite-type silica has six Si-O bonds with 1.608 Å distance and additional two interpolyhedral Si-O bonds with 2.372 Å distance at 271 GPa and 300 K. Such interpolyhedral Si-O distance in pyrite-type structure is much shorter than that in α-PbO2-type structure. The increase in coordination number of Si from 6 to 6+2 results in a large increase in density. The density of pyrite-type silica is 6.576(1) g/cm3 at 271 GPa and 300 K, which is larger by 4.7% than that of α-PbO2-type phase when compared at equivalent pressure. The new phase may be an important constituent in a rocky core of Uranus and Neptune. It may be also included in the terrestrial extrasolor planets.