In situ observation and kinetics of the phase transformation under high pressure and stress

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Present investigation aims to discuss the kinetics of the phase transformation of silicates and hydrates constituting earth crust and mantle. It has been known that the phase change often stays in the metastable or even unstable state by the Ostwald step rule, when the thermal energy is not large enough to supply the proper activation energy for the phase transformation. For the purpose of understanding the above mentioned mechanical process, quantitative kinetic investigations have been undertaken by the simulation of physico-chemical process found in the earth interiors as a function of temperature, pressure and stress. The following research projects have been performed :(1) X-ray diffraction study of the phase transition of Ca(OH)2, which is an analog material to mafic hydrates such as serpentine, has been carried out using the single crystal under hydrostatic pressure condition at 300K with diamond-avil cell (DAC). The compression induced a high density amorphous state but never b … More rought a CaCl2 type high pressure stable phase. This pressure-induced amorphization was caused by elastic deformation of the lattice and it is a reversible transformation.(2) An internal Pt heating system installed in the DAC enable to simulate the more precise phase transformation in the earth mantle in consideration of the geothermal gradient. Kinetics study of the structure change from quartz to coesite under various pressures and temperatures was undertakek by the time resolved diffraction study, Their physical properties such as thermal expansibity, bulk moduli and elastic constants were observed. It has clarified that the various compression paths and heating processes bring a different threshold transition critical pressure and temperature.(3) The present Raman spectroscopic study and EXAFS experiment accounts for the elastic or plastic lattice deformation causes the amorphization under high pressure. The investigation of the local structure of glass and high density amorphous phase of GeO2 could explain the different mechanism of the pressure-induced amorphization.(4) Simulation of structure change of SiO2, GeO2, SnO2 and PbO2 having a rutile structure has been executed under variable stress field through the molecular dynamical calculation using optimized pair potential and molecular orbital calculation based on DX-Xalpha method. Less