Post-stishovite transition in hydrous aluminous SiO2

TL;DR

This study demonstrates that hydrogen bonds and hydrogen mobility significantly lower the post-stishovite transition pressure in hydrous aluminous SiO2, linking water content to seismic features in Earth's mantle.

Contribution

It reveals the crucial role of hydrogen in reducing transition pressure in hydrous aluminous SiO2 through first principles and molecular dynamics calculations.

Findings

Hydrogen bonds lower transition pressure in hydrous aluminous SiO2.

Hydrogen mobility and redistribution facilitate the transition.

Higher hydrogen concentration further reduces transition pressure.

Abstract

Lakshtanov et al. (2007) showed that incorporation of aluminum and some water into SiO2 significantly reduces the post-stishovite transition pressure in SiO2. This discovery suggested that the ferroelastic post-stishovite transition in subducted MORB crust could be the source of reflectors/scatterers with low shear velocities observed in the mid to upper lower mantle. A few years later, a similar effect was observed in anhydrous Al-bearing silica. In this paper, we show using first principles static calculations and molecular dynamics using inter-atomic potentials that hydrogen bonds and hydrogen mobility play a crucial role in lowering the post-stishovite transition pressure. A cooperative redistribution of hydrogen atoms is the main mechanism responsible for the transition pressure reduction in hydrous aluminous stishovite. The effect is enhanced by increasing hydrogen concentration. This perspective suggests a potential relationship between the depth of seismic scatterers and the water content in stishovite.