PNNL

Abstract

Molecular dynamics simulations of water in contact with the (001) and (010) surfaces of orthoclase (KAlSi3O8) were carried out to investigate the structure and dynamics of the feldspar-water interface, contrast the intrinsic structural properties of the two surfaces, and provide a basis for future work on the diffusion of ions and molecules in microscopic mineral fractures. Electron density profiles were computed from the molecular dynamics trajectories and compared with those derived experimentally from high-resolution X-ray reflectivity measurements by Fenter and co-workers (Fenter et al., 2003a). For each surface, three scenarios were considered whereby the interfacial species is potassium, water, or a hydronium ion. Excellent agreement was obtained for the (001) surface when potassium is the predominant interfacial species. Good agreement was found for the (010) surface with some discrepancies which could be due in part to the fact that our model does not take into account the increased roughness of the (010) surface compared to the (001) surface. The two surfaces showed similarities in the extent of water ordering at the interface, the activation energies for water and potassium desorption, and the adsorption localization of interfacial species. However, there are also important differences between the two surfaces in the coordination of a given adsorbed species, adsorption sites density, and the propensity for water molecules in the adsorbed and first hydration layers to coordinate to surface bridging oxygen atoms. These differences may have implications for the extent of dissolution in the proton-promoted regime since hydrolysis of Si(Al)-O-Si(Al) bonds is thought to be the major dissolution mechanism.