Structure of cleaved surfaces of anhydrite (CaSO4) studied with atomic force microscopy

Abstract

A natural anhydrite crystal (CaSO₄, orthorhombic, space group A_mma, a₀= 7.006 Å, b₀= 6.998 Å, c₀= 6.245 Å, Z= 4, A. Kirfel and G. Will, Acta Crystallogr. Sect. B, 1990, 36, 2881) was cleaved in air in three pinacoidal directions at right angles and the surface structures were studied with atomic force microscopy (AFM). The arrangement of the outermost oxygen atoms of sulfate groups was clearly observed on the (010) and (100) faces. The (010) surface was atomically flat on the scale of 0.1 µm, although steplines with monolayer height were observed in the [101], [102] and [203] directions. On the (100) surface, a prominent feature was densely populated ridges of monolayer height running in the [011] direction. The (001) surface did not show and atomically flat surface. The reasons for the different cleavability and the step structures are discussed qualitatively. It is confirmed that smaller numbers of bonds ruptured per unit area is a condition of good cleavage. The arrangement of dipoles formed by ion pairs at the surface greatly affects stabilities of crystal faces and steplines.