THE KINETICS OF THE TRANSITION OF THE METASTABLE PHASES OF SIO2, STISHOVITE AND COESITE TO THE AMORPHOUS STATE

The non-equilibrium phase transitions of the metastable silica phases, stishovite and coesite, were investigated at room pressure by the isochronal annealing method. The measured heat release, Q, activation energy, DELTA-G, and Avrami index, n, of the transitions stishovite-glass were Q = 41 +/- 3 kJ/mol, DELTA-G = 220-280 kJ/mol, n = 1; glass-cristabolite: Q = 5 +/- 1.5 kJ/mol, DELTA-G = 400 +/- 50 kJ/mol, n = 3 +/- 0.5. Annealing of coesite leads to a solid state transition to an amorphous state and crystallization of the stable crystalline phase in the same temperature interval, 1380-1420 K, releases Q = 8 +/- 2 kJ/mol. It was shown by annealing under high pressure that the temperature of the transition of stishovite to an amorphous state increases with pressure, and the temperature of crystallization of the amorphous state produced decreases. At P > 3.5 GPa, the direct stishovite-coesite transition takes place during heating. From the experimental results, it is assumed that the solid state transition of stishovite to an amorphous state is connected with the small value of the activation energy of seeding at the transition temperature. It is believed that the known examples of the solid state transition to an amorphous state of high pressure phases are also connected with large rate of nucleation without significant crystal growth.