Densification of MgSiO3 glass with pressure and temperature

The density and structure of MgSiO3 glass (v-En) recovered from a series of annealing experiments up to 1000°C at 2.0, 5.5 and 8.5 GPa have been investigated using Archimedes' method and Raman spectroscopy, respectively. The densities of recovered glasses are found to be a complex function of pressure and temperature. At room temperature, compression up to 8.5 GPa, followed by decompression, yields a glass with a density within 0.6 % of the 1-atm value. Likewise, the 1-atm density is fully recovered in glass heated up to ~500°C at 2.0 GPa at higher pressures. A sharp increase in recovered density is observed between 500°C and 800°C at 2.0 GPa, 200°C and 500°C at 5.5 GPa and from room-T and 300°C at 8.5 GPa. At higher annealing temperatures the changes in density are more modest. This break in slope occurs for a glass density of 2.89 g/cm3 at 2.0 GPa and 2.95 g/cm3 at 5.5 and 8.5 GPa. Above ~900°C v-En crystallizes at all pressures. Raman spectra for annealed glasses show a progressive shift in the Si-O-Si bending mode to higher wave number with recovered density. From these data we estimate a ~4.5 % decrease in the Si-O-Si bond angle with densification from 2.75 g/cm3 (1-atm, room T) to 3.0 g/cm3.