Structures of the CaSO4(001) surface studied with atomic force microscopy in air and in solution

Abstract

Surface structures of the less cleavable (001) surface of a natural anhydrite (CaSO₄) crystal were studied with optical microscopy and atomic force microscopy (AFM). Macro-steps, as observed with an optical microscope, changed directions together, at surfaces of discontinuity in the (010) direction, which acted as twist and/or tilt boundaries between thin crystal plates. AFM observation revealed that the cleaved surface had mostly bilayer steps, and no monolayer steps. The cleavage occurs solely at one of the two interlayers, where much smaller numbers of S–O bonds need to be broken. The step signs are determined by the local tilt of the crystal with respect to the cleavage plane. The terraces are atom-flat and gave well-resolved AFM images. Observed step directions suggest a contribution to the step stabilities, by ‘electric double rows’ due to the alignment of positive and negative charges along the steps. Dissolution of the surface in a half-saturated aqueous solution of calcium sulfate was followed in situ with AFM. While showing instability compared to the other low index surfaces, well controlled line-by-line dissolution of bilayer steps along the [100] and [010] directions were observed at the surface. Much higher stability of the [100] step is also explained by a smooth alignment of positive and negative charges. The stability of higher steps and instability of curved bilayer steps are also discussed. In the dissolution process, the surface profile at the lower parts of the crystal is quickly copied along the [100] direction.