Abstracts for Annual Meeting of the Mineralogical Society of Japan

Stacking structures in pyrophyllite, Al2Si4O10(OH)2, were investigated. The specimens examined were large lath-shaped crystals (Berosovska, south Urals, Russia) and massive aggregates of fine platy crystals (Nohwa, southwest Korea). Both specimens showed powder X-ray diffraction (XRD) patterns similar to those reported previously as the 2M polytype. The common stacking sequence in the two specimens is not monoclinic with two-layer periodicity, but a uniform orientation of the 2:1 layers and near complete disorder of two alternative directions of layer offset, i.e. lateral displacement between the two tetrahedral sheets across an interlayer region. The directions of layer offset are about +-120degree from that of the intralayer shift.

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Using a hot isostatic pressing (HIP) technique, diamond/SiC composites were synthesized from diamond and Si powders. At an HIP condition of 1450 oC and 100 MPa, a pressure much lower than that of the diamond stability field, diamond powders react with molten Si to form well-sintered diamond/SiC composites. Cubes of the composites with 15 mm edge length were thereby fabricated, and an application to the second stage anvils in a Kawai-type high-pressure apparatus was attempted. The anvils were proved to be hard enough for the generation of pressures up to 30 GPa. For high-pressure and high-temperature in-situ X-ray experiments using synchrotron radiation, a hybrid anvil system using 4 cubes of the composites and 4 of conventional WC was introduced and heating experiments up to 1600 oC at 20 GPa became possible. Because the diamond/SiC composites are transparent to X rays, the present system is applicable not only to diffraction studies but also to radiographic studies that need a larger window for an X-ray image.

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Evaporation anisotropy of a synthetic single crystal of forsterite was investigated by high temperature vacuum experiments at temperatures from 1500 °C to 1750 °C. Anisotropy of evaporation rate is significant at temperatures below 1730 °C, where the rate is largest on (001), intermediate on (100), and smallest on (010) surfaces. The activation energies are 641, 592, and 570kJ/mole for (100), (010), and (001) surfaces, respectively. The anisotropy of evaporation rate is negligible at temperature above 1740 °C, where a different type of evaporation pits appears with a high density on the (010) surface.

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The pressure dependence of Raman peaks of cubic boron nitride (cBN) is determined at 100, 200 and 300C using pressure scales of ruby and gold. At pressures lower than 6 GPa, the pressure dependences of cBN Raman determined with the ruby pressure scale for transverse-optical and longitudinal-optical modes are 3.45 and 3.36 cm-1/GPa at 100C and 3.43 and 3.44 cm-1/GPa at 300 C, respectively. These values are consistent with those in a previous study conducted at room temperature using the ruby pressure scale. Synchrotron x-ray diffraction experiments using a gold pressure marker also yield 3.45 cm-1/GPa for TO mode at 200 C in a range of pressure up to 32 GPa. Under the present pressure and temperature conditions, the pressure dependence of Raman peaks of cBN seems to be independent of the temperature conditions. cBN can be used as an optical pressure marker under high temperature conditions.We also report on the Raman spectra of water under high temperature and pressure conditions and show a discontinuity in the pressure dependence of the OH stretching frequency. As pressure increases, the strength of hydrogen bonding increases rapidly in the pressure ranges up to 0.46 GPa at 25 C, 1.0 GPa at 100 C, and 1.3 GPa at 300 C and slowly above these pressures. This finding clearly demonstrates the existence of discontinuities in the pressure response of the hydrogen bonds of water, which suggests a possible structural change under these conditions.

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