Density and formation of high coordinated Si in v-MgSiO3 recovered from high pressure conditions

The density and structure of decompressed vitreous MgSiO&3 were measured using the Archimedean method and ^{29}Si MAS-NMR spectroscopy. Glasses were compressed in the 6/8 multianvil apparatus to 10.0 ± 0.5 GPa at room temperature, and some were heated to 773 K for four hours, quenched, and decompressed at rates of 10.4 GPa/min and 0.08 GPa/min. Recovered samples from 10 GPa and room temperature are permanently densified by ~ 3.0% relative to the glass as-made. Heating samples to 773 K at the same pressure condition results in ~ 7.4% of permanent densification at slow decompression rates and ~11.0% for rapid decompression rates. ^{29}Si MAS-NMR spectra were obtained from these recovered glasses. At ambient pressure the ^{29}Si spectrum has one dominant peak with an isotropic chemical shift (δ-iso) of 82.3 ppm and full width half maximum (FWHM) of 24 ppm. This peak corresponds to tetrahedrally coordinated Si (^{[4]}Si) with two bridging oxygens (Q2 species). The Q2 peak for high-pressure samples is more symmetric and shifted to -81.6 ppm with a FWHM of 21 ppm. The pressure-quenched samples also possess peaks at 180 ppm and, in the case of the rapidly quench glass, an additional peak at 124 ppm. The 180 ppm peak is assigned to octahedrally coordinated Si (^{[6]}Si), based on the δ-iso measured for ^{[6]}Si species by Stebbins and Kanzaki (1991). We assign the peak at 124 ppm to 5-fold coordinated Si (^{[5]}Si), based on its intermediate position between the ^{[4]}Si and ^{[6]}Si and the results of ab initio calculations. These calculations predict a chemical shift of -120 ppm for ^{[5]}Si with three bridging oxygens (Q^{3*} species). It is noteworthy that the high-pressure samples exhibit a -0.7 ppm shift and 3 ppm reduction in the FWHM for the Q2 peak. We attribute these features to a permanent reduction in inter-tetrahedral (Si-O-Si) bond angles. The measured density differences in pressure-quenched samples and the preservation of ^{[5]}Si in the rapidly decompressed sample demonstrates the importance of quench rate on the kinetics of back-reaction to ^{[4]}Si. We conclude that densification of v-MgSiO3 at 10.0 GPa and 773 K is largely accommodated by the twisting of network chains to form ^{[5]}Si and eventually stabilization of ^{[6]}Si with significant contribution from Si-O-Si bond angle compression. These observations of structural changes accommodating densification of v-MgSiO3 aid in understanding of the properties of refractory silicate melts in the Earth's deep interior.