Termination and hydration of forsteritic olivine (0 1 0) surface

Abstract

Termination and hydration of the forsteritic (Fo90Fa10) olivine (010) surface have been investigated with high-resolution specular X-ray reflectivity and Atomic Force Microscopy. The surface was prepared by polishing a naturally grown {010} face, from which we found the polished surface in acidic (pH 3.5) alumina suspension exhibits regular steps while the basic (pH 9.5) silica polished surface is irregularly roughened, indicating there are two distinguishable mechanochemical processes for the surface dissolution. The quantitative interpretation of the regular steps from the alumina-polished surface suggests that the observed step heights correspond to multiples of crystallographic unit cell. Only this atomically terraced surface is investigated with the high-resolution X-ray reflectivity (HRXR) to determine the surface termination and hydration. The basic silica paste polished surface turned out too rough to measure with X-ray reflectivity. HRXR reveals that the alumina polished olivine (010) surface in pure water is terminated at a plane including half-occupied metal ion sites (M1), an oxygen vacancy site, and a silicate tetrahedral unit with one of its apices pointing outward with respect to the surface. An ideal termination with the oxygen vacancy would fulfill the stoichiometry of the formula unit; however, in the observation, the vacancy site is filled by an adsorbed water species and about a quarter of the remaining metal ions are further depleted. The terminating plane generates two distinct atomic layers in the laterally averaged electron density profile, on which two highly ordered adsorbed water layers are formed. The first layer formation is likely through the direct interaction with the M1 plane and the second layer is likely through the hydrogen bonding interaction with the first water layer. With this multilayered adsorbed water structure, the surface metal ion is partially hydrated by the vacancy-filling water species and adsorbed water molecules. The bulk water links to these distinct adsorbed water layers, with weak density oscillations that almost completely damp out after the first bulk water layer. The total thickness of the layered water structure including the two distinct adsorbed layers and the first layer of bulk water is slightly less than 1nm, which corresponds to roughly three molecular layers of water. These results describe the steric constraints of the surface metal ion hydration and the iron redox environment during water–olivine interactions in this particular crystallographic orientation.