The structures of the barite (001) and (210) cleavage surfaces were measured 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 structures of the barite-water interfaces including the structures of water near the barite-water interfaces. The barite (001) and (210) cleavage surfaces are characterized by large (> 2500 Angstrom) domains separated by unit-cell steps (7.15 and 3.44 Angstrom steps for the (001) and (210) surfaces, respectively). This observation was unanticipated for the (001) surface in which adjacent BaSO4 layers are displaced vertically by c/2 and symmetry related through a 2(1) screw axis along (001). The atomic structures of the two cleavage surfaces derived by X-ray reflectivity reveal that near-surface sulfate groups exhibit significant (similar to0.4 Angstrom) structural displacements, while Ba surface ion displacements are significantly smaller (similar to0.07 Angstrom). Water adsorbs to the barite surfaces in a manner consistent, in both number and height, with the saturation of broken Ba-O bonds; no evidence was found for additional structuring of the fluid water near the surface.