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 velocity. During dissolution experiments, etch pits develop at surface regions, where local structural defects may occur. Therefore, etch pits are getting deeper at the same lateral position. As a consequence of the anisotropy in velocity of step movement, the etch pits are elongated in [001]. Isolated holes remain stable over a long period during growth experiments. Such holes can occur at the same position in several overgrowing monolayers. This kind of memory effect might also be explained by local structural defects. Local surface topography has a strong influence on the velocity of step movement. At surface regions with a high step density, the velocity of monolayer step movement is reduced 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 undersaturated aqueous solution (9.8 mmol L(-1)). Imaging monolayer steps with lateral molecular resolution reveals the molecular arrangement at monolayer steps. Individual kink sites can be observed. The kink site formation energy along [001] monolayer steps is 4.1 +/- 0.7 KJ mol(-1) in saturated aqueous solution. Observed kink site density agrees with predicted values from Monte Carlo simulations.