Kinetic Studies of Transitions from Amorphous Silica and Quartz to Coesite

The process of formation of coesite was investigated kinetically at 550° to 900°C under 23 to 55 kbars pressure using amorphous silica and quartz as starting materials. When amorphous silica transformed to coesite, strained quartz was observed as an intermediate phase. The rate process was concluded to be consecutive through quartz. When quartz transformed to coesite, a similar strained quartz was recognized as an intermediate stage from the broadness of the diffraction lines of the quartz phase. In the transition from amorphous silica, the formation rate of coesite increases with increasing pressure up to 40 kbars at 550°C but then decreases under pressures as high as 55 kbars. At 900°C, the formation of coesite from quartz was faster than that from amorphous silica under 23 and 31 kbars but slower under 40 kbars. These results are explained by assuming that the rate constant for each step depends on temperature and pressure.