First-principles calculations of the stability of hydrous alumino-silicates under deep-Earth conditions

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The Ohio State University

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The percent disorder affects many properties which control mineral behavior. This study used density functional theory to calculate the disorder and its effects on the energy of formation, the bulk modulus and its pressure derivative, and the minimization of the Gibbs free energy. The fully ordered structures of Phase Egg and Topaz-OH have Zero-pressure volumes (Vo = 205.4 Ǻ3 and Vo = 346.8 Ǻ3 respectively) are within 3.5% and 0.5% of reported values. Both the bulk modulus and Zero-pressure follow a linear trend with respect to the percent disorder. The Gibbs free energy for Phase Egg has a minimum at 50% disorder and Topaz-OH shows and even higher favoring of disorder at 60%. At higher pressure regimes the formation of phase Egg is energetically favorable compared to the formation of topaz-OH and stishovite.



First-principles calculations, hydrous alumino-silicates, stability of crystal structures, percent disorder, phase Egg, topaz-OH, Gibbs free energy, degrees of freedom