Ultrahigh pressure structural changes in a 60 mol. % Al2O3-40 mol. % SiO2 glass

Abstract

Structure of an Al-containing silicate glass (60 mol. % Al2O3-40 mol. % SiO2, A40S) is investigated up to 131 GPa, a pressure close to that of the Earth's core-mantle boundary, by using our recently developed double stage large volume cell. The first peak (r1) of the pair distribution function, which corresponds to T-O distance (T = Al, Si), rapidly increases below 16 GPa, indicating an increase of average coordination number (CN) of T-O from ~4 to 6. The r1 linearly decreases in the pressure range of 25-110 GPa, but it displays a slope change and becomes nearly constant above 110 GPa. The slope change may imply a structural change in the A40S glass around 110 GPa, and may be explained by the change in Al-O distance associated with the Al-O CN increase from 6 to >6 as predicted by molecular dynamics simulations (Ghosh and Karki, 2018). Our observations suggest an important role for aluminum in densification of aluminosilicate at the deep lower mantle, which might imply a dense aluminosilicate magma with negative buoyancy.