Kinetic studies of the .ALPHA.-quartz-coesite transformation of SiO2.

Abstract

Time resolved X-ray diffraction experiments have been performed in order to investigate the kinetics of the alpha -quartz-coesite transformation of SiO (sub 2) at around 6 GPa and 700-900 degrees C, using the "MAX80" cubic anvil type high pressure apparatus with synchrotron radiation. The correlation between a volume fraction of transformed coesite and relative intensities of reflections from alpha -quartz and coesite was experimentally determined. On the basis of the correlation, the volume fraction transformed coesite was estimated as a function of time. The Avrami rate equation was used for the determination of the kinetics. The value of the exponent n is determined as almost 1 and this result indicates that the grain growth of coesite is dominant mechanism of the transformation. The empirical activation energy, which is derived from the temperature dependency of the determined rate constant k. was estimated as 200 (+ or -40) kJ/mol at the pressure overstep of ca. 3.5 GPa from the alpha -quartz-coesite equilibrium conditions. This value is similar to a reported activation energy of oxygen diffusion in the quartz structure. This consistency suggests that the oxygen diffusion is able to control the growth rate of the coesite phase at the present experimental conditions.