THE DISTRIBUTION OF DISSOLUTION RATES ON THE CALCITE CLEAVAGE SURFACE
On new cleavage surfaces, smooth, low relief terraces exhibit a characteristic pattern of etch pit nucleation and development. After initial nucleation, small and widely separated etch pits coalesce and cannibalize one another after several hours of reaction, ultimately resulting in large (diameter ≥ ~102 mm), intersecting etch pits. VSI rates of surface normal retreat in these areas range from 10-11.4 to 10-11.6 mol/cm2/sec. In contrast, areas of high macrostep density (high relief formed during cleaving) are often densely populated by small etch pits (presumably reflecting locally high defect densities) that persist even after tens of hours of reaction. These areas dissolve at VSI rates up to almost an order of magnitude faster up to 10-10.7 mol/cm2/sec. Although these higher rates of surface normal retreat are still slower than the slowest rates observed in powder experiments, this pattern is consistent with the notion that grain boundaries may be an important source of the overall difference in powder (bulk) versus VSI rates. It has been suggested elsewhere that the discrepancy in the rates derived from AFM step velocities versus bulk powder measurements reflects the relationship of step density and reactivity (e.g., Dove and Platt 1996). Grain edges or boundaries, along which cleavage step density must reach a maximum, would thus be sites of accelerated dissolution. Because the ratio of edge length to surface area in a free grain varies as the reciprocal of grain diameter, this leads to the expectation of a nonlinear relationship between grain size and specific dissolution rate.