Modelling the effect of water on the surface structure and stability of forsterite

We describe the application of atomistic simulation techniques to investigate the effect of associative and dissociative adsorption of water on the structures and stabilities of the low-index surfaces of forsterite. All surfaces are amenable to associative adsorption of water, while dissociative adsorption is energetically favourable on all but the non-dipolar {100} surface. Often, otherwise unstable (dipolar) surfaces are stabilised to a large extent by hydration, e.g. the dipolar {010} surface. However, on thermodynamic grounds we do not expect associatively adsorbed water to dissociate on ail surfaces, as the energies released for dissociative adsorption of water on the non-dipolar {010} and {100} surfaces are less than those released for associative adsorption. As such, there is no energetic incentive for the associatively adsorbed water molecules to dissociate. The stabilities of the two terminations of the {010} surface, the main cleavage plane of forsterite, are reversed when hydroxylated, indicating that some dissolution of the magnesium ions may occur upon hydration, which is shown to be an exothermic process for both surface terminations. The equilibrium morphology was calculated as a way of assessing the change in surface energies. The experimental morphology of forsterite is adequately reproduced, suggesting that the relative stabilities of the surfaces, both unhydrated and hydroxylated, are calculated correctly.