IN-SITU INVESTIGATION OF GROWTH AND DISSOLUTION ON THE (010) SURFACE OF GYPSUM BY SCANNING FORCE MICROSCOPY

The kinetics of crystal growth and dissolution on the (010) surface of gypsum were investigated in situ by Scanning Force Microscopy (SFM). It could be shown that growth and dissolution on the (010) surface in aqueous solution is a layer-by-layer process. Monolayer steps parallel to [001], [100], and [101], move with a distinctive anisotropy in vel ocity. During dissolution experiments, etch pits develop at surface re gions, where local structural defects may occur. Therefore, etch pits are getting deeper at the same lateral position. As a consequence of t he anisotropy in velocity of step movement, the etch pits are elongate d in [001]. Isolated holes remain stable over a long period during gro wth experiments. Such holes can occur at the same position in several overgrowing monolayers. This kind of memory effect might also be expla ined by local structural defects. Local surface topography has a stron g influence on the velocity of step movement. At surface regions with a high step density, the velocity of monolayer step movement is reduce d compared to isolated monolayer steps. Isolated [100] monolayer steps move with a velocity of up to 30.0 nm s(-1), whereas [010] steps move with up to 9.5 nm s(-1), and [001] steps 2.5 nm s(-1), in an undersat urated aqueous solution (9.8 mmol L(-1)). Imaging monolayer steps with lateral molecular resolution reveals the molecular arrangement at mon olayer steps. Individual kink sites can be observed. The kink site for mation energy along [001] monolayer steps is 4.1 +/- 0.7 KJ mol(-1) in saturated aqueous solution. Observed kink site density agrees with pr edicted values from Monte Carlo simulations.