Abstract:
The dissolution kinetics of features on the magnesite (104) surface were studied in aqueous solutions from pH 4.2 to 2 and at temperatures between 60 and 90°C by hydrothermal atomic force microscopy (HAFM). At pH = 4.2, HAFM images showed magnesite step orientations that are comparable to the step orientations on calcite. Similar to calcite (104), there is anisotropy in the step velocity, but the magnitude of the anisotropy is much greater for magnesite. Furthermore, below pH = 4.2, changes in the dominant step orientation were observed. These results are discussed in terms of a nearest neighbor kink dynamic model, and the associated kink dynamics were tested with kinetic Monte Carlo (KMC) simulations. The KMC results suggest that the kink dynamic model does not account for the experimental observations and that further details such as second-nearest neighbor interactions or surface/edge diffusion cannot be excluded from the model. The dominant step orientations at low pH also point toward mechanisms stabilizing steps along periodic bond chain directions.