Density Functional Theory Calculations of Hydrogen-Containing Defects in Forsterite, Periclase, and α-Quartz

Abstract

Electronic structure calculations using the density functional theory within the generalized-gradient approximation and ultra-soft pseudopotentials have been used to investigate the absorption of water and protons in forsterite (Mg2SiO4), periclase (MgO), and α-quartz (α-SiO2). The calculated structural parameters are found to be in good agreement with experiment. Absorption of water in the perfect lattice is calculated to be an endothermic process in all three minerals but exothermic when associatively adsorbed at the forsterite {010} surface. The introduction of cation vacancies in the lattices is energetically unfavorable, more so for the formation of a silicon vacancy than a magnesium vacancy. The process of introducing a neutral silicon vacancy in the lattice, by reaction of a silicon atom with gaseous oxygen to form SiO2, is endothermic by approximately the same amount of energy (approximately 430 kJ mol-1) in both quartz and forsterite, but introducing an equivalent magnesium vacancy in the periclase lattice is less endothermic (163 kJ mol-1) than the same process in forsterite (217 kJ mol-1). Absorption of hydrogen molecules at the vacant cation sites is a highly exothermic process, releasing on average between 213 and 273 kJ mol-1 per proton when absorbed at a magnesium vacancy and approximately 275 kJ mol-1 per proton at a vacant silicon site. When we calculate the replacement of MgO and SiO2 units by liquid water molecules, as a first step in the process of dissolution of the minerals, we find that it is an endothermic process in the bulk minerals but exothermic by 71 kJ mol-1 for replacing an MgO unit at the forsterite {010} surface.