Structure of barite (001)- and (210)-water interfaces

The structures of the barite (001) and (210) cleavage surfaces were measure d in contact with deionized water at 25 degreesC. High resolution (similar to1 Angstrom) specular X-ray reflectivity and atomic force microscopy were used to probe the step structures of the cleaved surfaces and the atomic st ructures of the barite-water interfaces including the structures of water n ear the barite-water interfaces. The barite (001) and (210) cleavage surfac es are characterized by large (> 2500 Angstrom) domains separated by unit-c ell steps (7.15 and 3.44 Angstrom steps for the (001) and (210) surfaces, r espectively). This observation was unanticipated for the (001) surface in w hich adjacent BaSO4 layers are displaced vertically by c/2 and symmetry rel ated through a 2(1) screw axis along (001). The atomic structures of the tw o cleavage surfaces derived by X-ray reflectivity reveal that near-surface sulfate groups exhibit significant (similar to0.4 Angstrom) structural disp lacements, while Ba surface ion displacements are significantly smaller (si milar to0.07 Angstrom). Water adsorbs to the barite surfaces in a manner co nsistent, in both number and height, with the saturation of broken Ba-O bon ds; no evidence was found for additional structuring of the fluid water nea r the surface.