Atomic force microscopic observation of directional layer growth and dissolution on surfaces of sulfate minerals

Abstract Crystal faces of sulfate minerals such as CaSO 4 (1 0 0) and SrSO 4 (0 0 1) show directionality due to the orientations and arrangements of sulfate ions. Atomic force microscopic (AFM) observation of monatomic pits during dissolution of the crystals in aqueous solutions clearly shows the directionality. The shapes of the pits reflect relative stabilities of various step structures depending upon the solution contents and concentration. A complex spiral etch pit was observed at a screw dislocation on SrSO 4 (0 0 1). The surface was divided into two parts having opposite directionality. In the solution growth of SrSO 4 (0 0 1) cleaved in air, multi-nucleation was at first observed all over the surface due to the remaining ad-ions. After completion of a full monolayer, well-controlled and directional layer-by-layer growth was observed. Island growth was observed at the highest terraces and the shape of the islands resembled monatomic pits formed in the dissolution experiment.