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In situ x-ray diffraction measurements of MgSiO3 were performedat high pressure and temperature similar to the conditions atEarth's core-mantle boundary. Results demonstrate that MgSiO3perovskite transforms to a new high-pressure form with stackedSiO6-octahedral sheet structure above 125 gigapascals and 2500kelvin (2700-kilometer depth near the base of the mantle) withan increase in density of 1.0 to 1.2%. The origin of the D''seismic discontinuity may be attributed to this post-perovskitephase transition. The new phase may have large elastic anisotropyand develop preferred orientation with platy crystal shape inthe shear flow that can cause strong seismic anisotropy belowthe D'' discontinuity.
1 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan. 2 Institute for Frontier 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, 1-1-1 Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan.
* To whom correspondence should be addressed. E-mail: mmurakam{at}geo.titech.ac.jp (M.M.); kei{at}geo.titech.ac.jp (K.H.)
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