Structure of Silicate Glasses and Melts at High Pressure: Quantum Chemical Calculations and Solid‐State NMR.

Abstract

Despite their strong implications for magmatic processes in the earth's interior and pressure-induced structural changes in other amorphous, covalent oxide materials, little is known, beyond the coordination numbers of the framework cations, about the structures of silicate melts and glasses at high pressure. Here, we use multinuclear (27Al and 17O) solid-state NMR and quantum chemical calculations to study the structures of silicate glasses quenched from melts at pressures up to 10 GPa in a multianvil apparatus. The pressure-induced changes in atomic configurations in the silicate melts include the formation of highly coordinated framework units such as [5,6]Al and [5,6]Si and bridging oxygens linking these framework units and the presence of nonbridging oxygens coordinated by network-modifying cations and [5,6]Si. The proportion of high-pressure configurations such as [5,6]Al−O−[4]Si increases with pressure, but the fraction of nonbridging oxygen (Na−O−[4]Si) decreases with pressure, leading to an incre...