Abstract:
This paper reports on our use of scanning force microscopy (SFM) to study calcite dissolution rates. Until now, calculation of rates has been limited to surfaces on which steps retreat at an isotropic velocity. More commonly, on surfaces where steps in different directions retreat at different velocities, the rate depends on the velocities and the densities of the differently oriented steps. Here, we show that calculating rates from SFM image sequences is possible for anisotropic surfaces if the ratio of step lengths in different directions is constant in the analyzed images (e.g., by considering steps at nonintersecting pits exclusively). In contrast to nonintersecting pits, high velocity steps are formed at intersecting pits on the calcite (1014) surface. At these steps, material can be removed without the slow nucleation of kink sites. The morphology of these steps is not straight and they become easily pinned by particles or impurities. Therefore, measuring the velocity of step retreat directly in the images fails, but calculating the dissolution rate of surface regions with high velocity steps is still possible. Dissolution rate is roughly similar in both the deep etch pits and the areas with high velocity steps at intersecting pits. Consequently, we suggest that the formation of high velocity steps contributes considerably to the weak enhancement of the rate with increasing dislocation density because additional etch pits within the intersectional regions do not significantly increase the rate. The calculated dissolution rate of ~1.5 x 10-6 mol m-2 s-1 on the calcite (1014) surface in water at 24°C and pCO2 = 10-3.5 atm corresponds well to the rates obtained from batch experiments. Thus, SFM can be regarded as an instrument capable of acquiring rates even on surfaces with anisotropic step velocities.