  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.
