Stabilities of crystal faces of anhydrite (CaSO4) compared by AFM observation of facet formation processes in aqueous solutions

Micro-facet formation processes were observed, with atomic force microscopy (AFM), at presumably less stable (0 1 1) and (1 1 1) faces of anhydrite (CaSO4) crystal in aqueous solutions containing either Ca2+ or SO42− ions. The order of relative stabilities of the crystal faces was (0 1 0)∼(1 0 0)>(1 1 1)>(0 1 1)≥(0 0 1). While (0 1 1), (0 1 0) and (1 1 1) faces were stabilized by either ions, (0 0 1) face was stabilized mainly by SO42− ions. Atom-resolved AFM images were observed in air for (0 1 1) and (1 1 1) faces after soaking in CaCl2 aq. (0.01 mol dm−3). At these faces, positive charges of Ca2+ at the outermost layers are partly cancelled by the negative charges of O atoms of the SO42− ions centered at lower levels. At (1 0 0) face, {1 1 1} ledges with bilayer height make growth or dissolution fronts around a screw dislocation. The relationship between natural crystal forms and chemical growth environment is discussed. Growth in sulfate-rich and Ca2+-rich conditions gives different crystal shapes.