Linking high-pressure structure and density of albite liquid near the glass transition

The pressure-induced densification of NaAlSi3O8 liquid is determined following annealing immediately above the glass transition and upon quenching from superliquidus temperatures. High-field 27Al magic-angle-spinning NMR spectroscopy is used to investigate the corresponding changes in Al coordination environment that accompany the densification. We show that samples synthesized by quenching from superliquidus temperatures record lower fictive pressures (Pf) than annealed samples at the same nominal load and have lower recovered densities and average Al coordination number. Accounting for differences in Pf brings melt-quench and annealed samples into excellent agreement. The proportion of [5]Al increases from ∼3% to 29% and [6]Al from 0% to 8% between 1.8 and 7.2 GPa. The production of high-coordinated Al ([5]Al + [6]Al) with pressure is most dramatic above 3 GPa. Changes in network topology and structural disorder as revealed by the high-field 27Al NMR spectra provide new insights into the structural mechanisms of densification of the albite liquid. We posit that it is an overall weakening of the network structure on compression that is largely responsible for the anomalous pressure dependence of the transport properties observed for this liquid below ∼5 GPa.