Changes in the state of water during phase transformations in silica exposed to water vapor

In the presence of supercritical water vapor (400 degrees C, 20-30 MPa), am orphous silica transforms to SiO2-X in a few hours, then to cristobalite in 5-20 h, and finally to quartz in 10-60 h. Changes in the state of constitu tional water were found to closely correlate with the structural transforma tions of silica. Silica structurization and water desorption were followed by mass spectrometric measurements and H-1 NMR and Si-29 NMR. The water con tent of silica passes through a maximum (2-2.5 wt %) early in the formation of SiO2-X and then decreases to 0.5 wt % in cristobalite and 0.01-0.03 wt % in quartz. The structural transformations include silica hydroxylation ac companied by the breaking of siloxane bonds and followed by dehydroxylation and structure rearrangement. The rate of water exchange between the silica matrix and supercritical water decreases with time, while the activation e nergy of water outdiffusion rises. The diffusion coefficient of water in qu artz is determined. The states of constitutional water in quartz and corund um prepared in the presence of supercritical water are compared.