Structure and elasticity of hydrous ringwoodite: A first principle investigation

First principle calculations were performed to investigate structural, IR, and elastic properties of hydrous ringwoodite and their evolution with pressure up to 36 GPa. Hydrogen defects are introduced by creating Mg- or Si-vacancies Mg1.875H0.25SiO4, Mg1.75H0.5SiO4 and Mg2Si0.875H0.5O4. Energy considerations imply that the Mg-vacancy coupled substitution will be the easiest to form, but, in the Earth, both vacancies will participate in the process. Calculated IR spectra, when compared with reported observations, suggest that both types of defects are abundant in synthetic samples. We find that (d ln VS/d ln VP) for lateral variations in the H content of ringwoodite will be quite small, suggesting that this quantity will be a sensitive metric for identifying the presence of dissolved water in the transition zone. lculated bulk modulus decreases linearly with increasing water content with d K / d ( C H 2 O ) = − 7.1 ( GPa / wt % ) at room pressure, decreasing to −6.0 (GPa/wt%) at 20 GPa. The shear modulus similarly demonstrates a decrease with increased water content given, averaged over the substitution models, by d G / d ( C H 2 O ) = − 3.0 ( GPa / wt % ) at room pressure, decreasing to −1.8 (GPa/wt%) at 20 GPa. Over this pressured range, the water induce variation of d ln(VS)/d ln(VP) is 0.62 at 0 GPa to 0.2 at 20 GPa.