Crystal faces of anhydrite (CaSO₄) and their preferential dissolution in aqueous solutions studied with AFM

Structures of cleaved surfaces of anhydrite were studied with atomic force microscopy (AFM) before and after partial dissolution in aqueous solutions of NH₄Cl and NaHSO₄. Two crystal faces showed atom-resolved images just after cleavage, (100) and (010), of which the former was roughened by the dissolution, while step structures were developed on the latter. After dissolution, steplines ran along the a- and c-axes on the (010) face, while they ran in directions inclined to these axes before. It was revealed that the arrangement of dipoles is a key factor in determining stabilities of step structures on crystal faces. On the terraces, the arrangement of oxygen atoms of the sulfate groups and calcium ions were clearly observed.