The Effect of Disorder on Hydrous Mineral Stability at High Pressure

Abstract:

The water storage capacity in the Earth’s deep interior and its transport between reservoirs is in part dependent upon the stability of high-pressure phases of hydrous minerals. Prediction of mineral stability in the Earth's mantle is dependent upon accurately calculating the Gibbs free energy of each phase as a function of pressure, temperature, and composition. Contributions from crystallographic disorder to the configurational entropy can extend the stability field of minerals at high temperatures. In certain hydrous and anhydrous aluminum and magnesium silicates such as ringwoodite, some degree of disorder is favored at mantle pressure and temperature conditions. This study constrains the water storage capacity of the mantle by examining the effects of cation disorder on the structure and energetics of hydrous and anhydrous aluminum and magnesium silicates (kyanite, topaz-OH, phase egg, phase D, and δ-AlOOH) as a function of Si and Al coordination. We present the energetics of disorder at mantle pressure and temperature conditions from static lattice energy minimizations using interatomic potentials and complementary first-principles calculations coupled with modeling of configurational entropy. This work assesses the applicability of the aluminum avoidance principle in high-pressure hydrous aluminosilicates.