Ab Initio Structures and Energetics of Hydrated Flat and Terrace-Step Surfaces of Forsterite (Mg2SiO4)

Abstract

Forsterite (Mg₂SiO₄), a model divalent metal silicate mineral, has been extensively studied in the context of mineral carbonation. Although dissolution is a key step in this process, the mechanisms by which forsterite dissolves under high CO₂ conditions remain poorly understood. Atomistic simulations could aid in exploring these mechanisms, but it is essential first to understand the structures and energetics of the relevant forsterite surfaces. We present an ab initio study of the structure and surface energy at 0 K of the flat (010), (110), (001), (111), (021), (101), and (120) faces of forsterite using the density functional PBE Hamiltonian and a plane-wave basis set. Dry surfaces became stabilized upon hydration through the formation of bonds between surface Mg and O from water, as well as by the formation of hydrogen bonds. According to surface energy values, the stability order of the hydrated forsterite faces was found to be (120) < (101) < (021) < (111) < (001) < (110) < (010). We also investigated the energetics of the terrace-step (04̅1) surface as a model site for forsterite dissolution. Among all the facets, the (04̅1) surface is the least stable termination in water. Hydration of Mg atoms on the (04̅1) surface increases their susceptibility to dissolution. The presence of a step and its hydration destabilize the terraces, making the step retreat more likely than a dissolution front advancing along the [010] direction. This research will support future simulations to investigate forsterite dissolution in water under CO₂-rich conditions.