To better understand the relationship between atomic-scale structures and densities of aluminosilicate glasses and liquids, we used 27Al MAS NMR to determine the speciation of aluminum ions in K3AlSi3O9, Na3AlSi3O9, and Ca3Al2Si6O18 glasses quenched from melts at 3 to 10 GPa. These data are a first approximation of high-pressure melt structure and illustrate the effects of the type of modifier cation. High field strength modifier cations (e.g., Ca) clearly induce more high-coordinated Al than lower field strength cations (e.g., Na and K). Measured glass densities show that, especially with rapid decompression, a significant portion of the total densification observed in-situ in melts is retained on return to ambient temperature and pressure. Observed increases in Al coordination are well correlated with decreased volume, which suggests that this structural change is a major part of the mechanism for recovered densification of high-pressure melts. Additionally, 23Na MAS NMR, combined with the 27Al MAS spectra and density determinations, reveal that other changes, such as the compression of modifier cation sites and/or decreased network bond angles, must also be significant, especially at low pressure.

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