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 by first principles static calculations and by 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.
Bibliographical noteFunding Information:
The authors thank David Kohlstedt, Justin Revenaugh, and George Helffrich for useful discussions. This work was supported by NSF under Grants EAR-1161023 and EAR-1348066 . Computations were performed at the Minnesota Supercomputing Institute (MSI) and in the Blue Waters system at NCSA.
- First principles
- Hydrous aluminous SiO
- Molecular dynamics
- Post-stishovite transition