EFFECT OF PRESSURE ON THE CRYSTAL STRUCTURE OF HYDROUS RINGWOODITE,MG1.97SIH0.06O4 UP TP 7.9 GPA

The specimen used in this study was a single crystal of hydrous ringwoodite synthesized by Ohtani and Mizobata (1998) using a multi-anvil apparatus at conditions of 1680C and 22 GPa. Electron microprobe analysis showed a chemical formula of Mg1.97SiH0.06O4. Sets of X-ray diffraction intensities up to 7.9 GPa were measured with a single crystal of 35x35x24 micron using synchrotron radiation at the beam line BL-10A, Photon Factory, High Energy Accelerator Reasearch Organization, Tukuba, Japan. The modified Merrill-Bassett type diamond anvil pressure cell was used. The 4:1 fluid mixture of methanol and ethanol was used for pressure medium and SUS301 stainless steel plate was used for gasket. The pressure was calibrated using the ruby fluorescence method. The single crystal X-ray diffraction data were measured at ambient pressure and at high pressures, 3.2 GPa, 5.0 GPa, 6.2 GPa and 7.9 GPa. The atomic parameters were refined by SHELEX-93. The compression of T-O distance was 0.25 % while the compression of M-O was 1.8 % at 7.9 GPa. The compression of the unit cell axial length was 1.3 % at 7.9 GPa. The compressibility of the unit cell is close to the compressibility of the MO6 octahedron. This fact indicate that the compression of the crystal structure is governed by the compression of MO6 octahedron.