First-principles study of the effect of water on the phase transitions in Mg2SiO4 forsterite

Abstract

First-principles calculations on the phase transitions in Mg2SiO4 forsterite have been performed using density functional theory and generalized gradient approximation (GGA) implemented in the Vienna Ab-initio Simulation Package code. The effect of water incorporation was simulated by considering Mg2SiO4 crystals of three different structural phases with water contents of 0, 1.65, and 3.3 wt.%. The theoretical K 0 calculated from computed P–V curves reproduces the experimental results well. After an empirical correction to take into account the overestimation of pressure by GGA calculations, the transition pressures in anhydrous Mg2SiO4 crystals are consistent with experimental results. As water content increases, the α–β phase boundary shifts towards lower pressure, while the β–γ phase boundary towards higher pressure. Thus, the existence of water can persistently expand the region of the β phase. Further analyses reveal that the density discontinuities caused by α–β and β–γ phase transitions also vary inversely with water content.