T155 High-Pressure Mineral Physics: To Honor Ho-Kwang Mao, Roebling Medalist II
Tuesday, 18 October 2005
8:00 AM - 12:00 PM
THE PYRITE-TYPE NEW HIGH-PRESSURE FORM OF SILICA
Paper number: T155-4
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM
Authors
-
Yasuhiro KuwayamaTokyo Institute of Technology, Tokyo, N/A
-
Nagayoshi SataJapan Agency for Marine-Earth Science and Technology, Yokosuka, N/A
-
Yasuo OhishiJapan Synchrotron Radiation Research Institute, Mikazuki-cho, N/A
Abstract
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.