Elastic properties of hydrous forsterites under high pressure: First-principle calculations

Elastic and electronic properties of anhydrous (Mg2SiO4) and hydrous forsterite crystals with 3.2 wt% (Mg1.75H0.5SiO4) and 1.6 wt% water (Mg1.875H0.25SiO4) under high pressure were investigated using first-principle calculations within local density approximation. Different hydrogen locations around the Mg cation vacancy (as structurally bound OH) have been compared in terms of total energy and elastic properties. Comparison of the anhydrous and hydrous forsterite crystals shows that hydration reduces the mass density, elastic moduli, and sound velocities. Adding 3.2 wt% and 1.6 wt% water leads to reductions of compressional and shear wave velocities (VP and VS) of forsterite by 3.1–7.1% and 3.6–9.7%, and by 2.4–4.1% and 3.3–6.6% at pressures from 0 to 14 GPa, respectively. The reduction of sound velocity of hydrous forsterite decreases with water content. This may be the main reason for formation of the low velocity zones in the Earth mantle. However, substitution of hydrogen with Mg cation in forsterite has little effect on its electronic state. The band gap of hydrous forsterite crystals is comparable to the gap of anhydrous one and slightly increases with increasing pressure.