Thermodynamic properties of anhydrous and hydrous wadsleyite, β-Mg2SiO4

Wadsleyite, -Mg2SiO4, is an important high pressure mineral that may act as a sink of water in the Earth's transition zone. In this study, we first determine the heat capacity and entropy of anhydrous wadsleyite at ambient pressure both experimentally and by first principles simulations. The measured standard entropy S-0 at 298K is 86.7(11)J/(molK). Then, extended simulations are performed to explore the effect of pressure and hydration on these thermodynamic properties. The stability of several structural models of hydrogen incorporation is investigated and the lowest energy structures are used to derive the vibrational entropy and the heat capacity of hydrous wadsleyite with a nominal content of 1.6 and 3.3wt% H2O at an ambient pressure and at 15GPa. Considering an estimation of the configurational entropy, S-0 of hydrous wadsleyite with 1.6 and 3.3wt% H2O is predicted to be about 3.5 and 5J/(molK) higher than that of anhydrous wadsleyite at the ambient pressure.