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

We describe the application of atomistic simulation techniques to investiga te the effect of associative and dissociative adsorption of water on the st ructures and stabilities of the low-index surfaces of forsterite. All surfa ces are amenable to associative adsorption of water, while dissociative ads orption is energetically favourable on all but the non-dipolar {100} surfac e. Often, otherwise unstable (dipolar) surfaces are stabilised to a large e xtent by hydration, e.g. the dipolar {010} surface. However, on thermodynam ic grounds we do not expect associatively adsorbed water to dissociate on a il surfaces, as the energies released for dissociative adsorption of water on the non-dipolar {010} and {100} surfaces are less than those released fo r associative adsorption. As such, there is no energetic incentive for the associatively adsorbed water molecules to dissociate. The stabilities of th e two terminations of the {010} surface, the main cleavage plane of forster ite, are reversed when hydroxylated, indicating that some dissolution of th e magnesium ions may occur upon hydration, which is shown to be an exotherm ic process for both surface terminations. The equilibrium morphology was ca lculated as a way of assessing the change in surface energies. The experime ntal morphology of forsterite is adequately reproduced, suggesting that the relative stabilities of the surfaces, both unhydrated and hydroxylated, ar e calculated correctly.