Experimental study on kinetics of the olivine-spinel phase transition.

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In the present study, we aimed to propose the preliminary kinetics data for the olivine*spinel transition by using the Mg_2GeO_4 analogue, Isothermal experiments had already been carried out as a function of H_2O content at 0.07GPa to examine the effect of the amount of H_2O on the Mg_2GeO_4 olivine-spinel transition. In the present study, the additional kinetic experiments were also tried to carry out at the higher pressure(0.15GPa)by using a newly-introduced high-pressure type reactor, but the most of these runs met with failure simply because of the machine trouble (mainly a pressure leak). In a series of these experiments, small amounts of Mg(OH)_2 was enclosed to control the content of H_2O (X) by varying its amount, and was heated hydrothermally as a function of time. By fitting the experimental data at each set of temperatures and X's to the Avrami equation, a specific time (ts) was derived at which the olivine-spinel phase transition is first detectable. The ts had been obtained at each set of temperatures and X's at 0.07GPa. However, since only a few runs were successfully performed in each set of temperatures and X's at 0.15GPa, the specific times could not be determined. Consequently, the effect of the pressure (between at 0.07GPa and 0.15GPa) on the kinetics of the Mg_2GeO_4 olivine*spinel transition could not be systematically discussed. However, it may safely be said that the effect of the pressure seems to be considerably smaller than the effect of the amount of H_2O on the transition by considering the fact that even a small amount of H_2O drastically activates the nucleation rate in the Mg_2GeO_4 olivine*spinel transition.