Al³⁺ coordination changes in liquid aluminosilicates under pressure

Geochemists often debate the conditions in which Al³⁺ may become 6-coordinated in molten silicates and how such liquid structures are related to those of coexisting crystal phases^1-4. To date no experimental evidence for the occurrence of Al³⁺ in this coordination state has been presented and computer simulation studies have suggested that pressures exceeding 100 kbar may be required for the 4→6 conversion^5,6. To examine this question in the laboratory, we have taken advantage of the ability of the glass transition to freeze the structural equilibrium of a high-pressure melt and preserve it for subsequent examination in ambient conditions. Glasses of albite composition have been prepared by quenching melts under pressures of 0-80 kbar (0-8 GPa). The Al³⁺ coordination has been determined by ²⁷Al solid-state NMR spectrometry. We find that the 4-coordinated state is retained to pressures well beyond 30 kbar. A new peak at -16 p.p.m. relative to Al(H₂O)³⁺₆, which we associate with octahedral Al³⁺ appears weakly at 60 kbar and becomes a prominent feature of the spectrum at 80 kbar.