Water Solubility in Octahedrally Coordinated Silicates

The incorporation of water into nominally anhydrous silicates, associated with mechanical weakening and lowered melting points, can serve as a significant volatile reservoir in the planet. Using density functional theory, we performed a series of first-principles, calculations to address the substitution mechanism of hydrogen coupled with aluminum in octahedrally coordinated silicates (SiO2 stishovite and MgSiO3 perovskite). We additionally consider the anhydrous dissolution of Al2O3 as a competing solution mechanism [Panero et al., 2006]. The solution mechanisms under consideration are constrained by charge balance. In both minerals, we find the stable H solution mechanism to be Al+H = Si with asymmetrically bonded OH...O. In stishovite, the hydrogen bond becomes increasingly symmetric with pressure (OH distance of 1.04 Å at 0 GPa increases to 1.11 Å at 60 GPa). Our results indicate that symmetric hydrogen bonding may be stable in hydrous stishovite at deep mantle conditions. In Mg-perovskite, the hydrogen bonds along the octahedral edge with an OH distance increasing from 1.02 Å at 0 GPa to 1.08 Å at 150 GPa and OHO bond angle decreasing from 136° to 129°. Hydrogen incorporation has a negligible effect on density in stishovite and perovskite and decreases the bulk modulus by 1.2 % and 0.4 % per mole % alumina in stishovite and perovskite, respectively. The effect on bulk modulus is identical to anhydrous aluminum incorporation through oxygen vacancies (2Al+VO=2Si). The Margules parameters for Al2O3 and δ-AlOOH solution in stishovite are 95 kJ/mol and 52 kJ/mol, respectively; while in perovskite they are are 12 kJ/mol and 44 kJ/mol. The positive enthalpies of solution indicate that solubility of water is an entropy-driven process. Assuming ideal configurational entropy of solution, the solubility of water in stishovite exceeds 0.3 wt% H2O at 25 GPa and 1500 K, applicable to conditions of subducting slabs. The solubility increases with both pressure and temperature. With stishovite making up as much as 20 mol% of a basaltic layer subducting into the lower mantle, this solubility can account for the transport of the mass of the oceans over the age of the Earth. In Mg-perovskite, however, the solubility of H2O is significantly lower: less than 100 ppm H2O at pressures and temperatures equivalent to the top of the lower mantle, with only slight increases with pressure. A lower mantle with just 50 ppm (wt) H2O, however, can account for a mass of water equivalent to about 1% of the Earth's oceans, or about 200 times the mass of water in the Earth's atmosphere.