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

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 increase of polymerization in melts. In bridging and nonbridging oxygens and highly coordinated framework units, structurally relevant NMR parameters (e.g., the isotropic chemical shift, δiso, and quadrupolar coupling product, Pq) show a pressure dependence. Quantum chemical calculations for model silicate clusters show that 17O δiso for [n](Si,Al)−O−[4]Si mainly increases with increasing coordination number n. Although 17O δiso appears to be affected by longer-range interactions, it shows relatively moderate positive correlation with the [n](Si,Al)−O bond length and appears to decrease with the distance between Na and bridging oxygens. 17O δiso for each oxygen cluster ([n](Si,Al)−O−[4]Si) obtained from 17O 3QMAS (triple quantum magic-angle spinning) NMR also increases with coordination number n. This is consistent with quantum calculations, implying that the [n](Si,Al)−O bond length may increase while the Na−O distance decreases with pressure. Although the overall variation 17O Pq for each oxygen site is not significant, the 17O Pq value of [4]Si−O−[4]Si seems to decrease with increasing pressure, suggesting that the [4]Si−O−[4]Si angle may decrease with densification. The 27Al Pq for [n]Al increases with increasing pressure, which implies that the distortion of the network unit increases with pressure. The widths of 17O δiso distributions for [n](Si,Al)−O−[4]Si increase with increasing pressure, which implies that bond lengths and angle distributions get wider with pressure. This trend is also consistent with the increasing distortion of framework polyhedra, suggested by increases in 27Al Pq with pressure. The topological entropy was introduced to quantify the degree of randomness in the distributions of internal variables such as bond angle and length.