First principles investigation of the structural and elastic properties of hydrous wadsleyite under pressure

Abstract

In order to clarify the effect of protonation of wadsleyite under high‐pressure conditions, we determined defect structures of Mg1.875SiO4H0.25 (1Mg2+ ↔ VMg2− + 2H+, 1.65 wt % H2O), Mg1.75SiO4H0.5 (2Mg2+ ↔ 2VMg2− + 4H−, 3.3 wt % H2O) hydrous wadsleyite and their elastic properties by means of the density functional first principles method. Structural optimization calculations indicate that the most stable structures have monoclinic symmetry with magnesium M3 site vacancies. Protons are found to bond to the O1 site to align the OH dipoles along the edges of M3 vacancies. Calculated elastic constants, bulk and shear moduli, are found to decrease almost linearly with increasing water content but to increase linearly with increasing pressure. At 15 GPa and static 0 K condition, incorporation of 3.3 wt % H2O into wadsleyite, which corresponds to the maximum solubility of hydrous wadsleyite, reduces VP and VS by about 3.9 and 4.8%, respectively. This indicates that 1 wt % H2O hydration of wadsleyite corresponds to the temperature effects on bulk and shear moduli about 430 K (0 GPa) to 340 K (20 GPa) and 350 K (0 GPa) to 290 K (20 GPa), respectively. The transversely isotropic aggregates demonstrate largest positive polarization anisotropy VSH − VSV when the c axis aligns vertically in both dry and wet cases.