Effective factors in the kinetics of pressure-induced transformation.

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The kinetics of the quartz-coesite transformation were investigated at 4 GPa and 650, 750 and 850℃ using quartz single crystal and powdered samples. Optical microscopic observations for partly transformed single crystal samples indicate that the coesite heterogeneously nucleates on the surface of the quartz grain and grows into the interior. The growth rate of coesite is independent of crystallographic orientations. The mechanism of incoherent grain-boundary nucleation and interface-controlled growth seems to operate during the transformation. The thickness of the coesite rims increases linearly as a function of time and the growth rate clearly depends on temperature. The activation energy for growth of coesite is determined as 85 (±11) kJ/mol and this value is significantly small in comparison with that determined for powdered samples.Synchrotron x-ray powder diffraction studies were performed on Ca(OH)ィイD22ィエD2 and Mg(OH)ィイD22ィエD2 under room temperature pressurization to 10.4 GPa and 16.0 GPa, respectively. We used the diamond anvil cells technique and the data were collected with an imaging plate detector at BL-18C in Photon Factory at KEK, Japan. The c-axis is much softer than the a-axis and, thus, the c/a ratio decreases with pressure. There is no remarkable discontinuity in the compression curves. The atomic position of O atoms, (1/3, 2/3, z), has been successfully refined using Rietveld analysis of x-ray powder diffraction patterns. In Mg(OH)ィイD22ィエD2, a change of compression mechanism is observed at 11GPa. However, in Ca(OH)ィイD22ィエD2, it is concluded that the shortening of the interlayer spacing controls compression below the amorphization pressure and no change of compression mechanism has occurred. It has been reported that H disorder occurs at a pressure in Mg(OH)ィイD22ィエD2 and the present results clearly suggest that the H disorder has great influence on the compression behavior.