Hydrogen incorporation in stishovite at high pressure and symmetric hydrogen bonding in δ-AlOOH

First-principles calculations of stishovite (SiO2), δ-AlOOH, and stishovite–δ-AlOOH solid solutions reveal that coupled substitutions of Al3+ and H+ for Si4+ provide a means for dissolving significant amounts of hydrogen in stishovite under lower-mantle conditions. The enthalpy of solution is positive, decreasing in magnitude with increasing pressure. Combining our results with a model of the configurational entropy we find that the solubility of water in stishovite exceeds 0.3 wt% H2O at 25 GPa and 1500 K, realistic temperatures for subducting slabs, and that the solubility increases with increasing pressure and temperature. We predict asymmetric hydrogen bonding in the stishovite–δ-AlOOH solid solution that becomes increasingly symmetric with increasing pressure. Our results support the recently predicted symmetric hydrogen bonding in δ-AlOOH, and indicate that symmetric hydrogen bonding may be stable at ambient conditions in this material.