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Science 5 August 2005:
Vol. 309. no. 5736, pp. 923 - 925
DOI: 10.1126/science.1114879
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Reports

The Pyrite-Type High-Pressure Form of Silica

Yasuhiro Kuwayama,1* Kei Hirose,1,2 Nagayoshi Sata,2 Yasuo Ohishi3

Silica (SiO2) exhibits extensive polymorphism at elevated pressures. X-ray diffraction measurements showed that a high-pressure form with a pyrite-type structure, denser than other known silica phases, is stable above 268 giga-pascals and 1800 kelvin. The silicon coordination number increases from 6 in the {alpha}-PbO2-type phase to 6+2 in the pyrite-type phase, leading to a large increase in density by about 5% at the phase transition.

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, Kanagawa 237-0061, Japan.
3 Japan Synchrotron Radiation Research Institute, 1-1 Koto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan.

* To whom correspondence should be addressed. E-mail: ykuwayam{at}geo.titech.ac.jp

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