Phase transformation and rheology of minerals in the Earth's crust and mantle

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Three following subjects were intensively studied and the following results were obtained, respectively.1. Aluminous garnet-perovskite phase transformationLaser-heated diamond anvil cell experiments using natural garnets as starting materials, and analytical electron microscopy of the recovered samples showed the following results ; 1) although the main post-garnet phase under the lower mantle condition is orthorhombic perovskite, some Al-rich phase (s) and stishovite coexist with orthorhombic perovskite depending on temperature and prssure, 2) the silicate LiNbO_3 phase was found for the first time as a metastable phase transformed from the orthorhombic perovskite, 3) there are positive correlations between Fe, Al and Ca contents in perovskite, and these contents tend to increase with pressure.2. Cubic-tetragonal transition in (Mg, Fe) SiO_3 majoriteAnalytical electron microscopy of the synthetic majorite by multi anvil cell and the natural meteorite majorite showed that the synthetic majorite is tetragonal, while natural majorite is cubic. Microstructures of synthetic majorites indicate the wide stability range of cubic phase at higher temperatures thah-1900ﾟC at 20 GPa. The boundary temperature between cubic and tetragonal phases increases with Fe content.3. Phase transition of enstatite under high pressure and hihg temperatureHigh temperature and high pressure in situ X-ray diffraction experiments using synchrotron radiation showed that there exists a wide stability range of monoclinic C2/c phase on the high pressure side of orthorhombic Pbca. The beta anble of this C2/c is signficantly smaller than that of high temperature C2/c, indicating that these two phases are the different structures with each other. There is a possibility that monoclinic P2_1/c has a stability field between orthorhombic Pbca and monoclinic C2/c at lower temperatures.