Relation between etch-pit morphology and step retreat velocity on a calcite surface in aspartic acid solution

Effects of l-aspartic acid (l-Asp) on dissolution of calcite were investigated. The step retreat velocity and dissolution rate of calcite were measured simultaneously using an AFM flow-through system. The etch-pit morphology of calcite was observed using confocal laser scanning microscopy.Results show that the etch-pit morphologies changed drastically depending on the l-Asp concentration ([l-Asp]) in the order of rhomboidal, pentagonal, and triangular (not perfectly, but retaining an extra step). The change in obtuse step directions and appearance of the [0 1 0] step triggered these morphological changes. Addition of l-Asp accelerated all step retreats at [l-Asp]<0.01 M, which implied the effect of l-Asp on the diffusive barrier. In contrast, at [l-Asp]>0.01 M, l-Asp inhibited the retreats of obtuse steps and [0 1 0] step, although the retreat velocities of acute steps were constant irrespective of [l-Asp].These results suggest that the directional changes and the inhibition of retreat velocities of obtuse steps were attributed to the generation of [4 1 1] and [4 5 1] steps caused by l-Asp. Moreover, we confirmed the preferential effects of l-Asp on the [4 8 1]+ to [4 4 1]± step edge, and proposed the preferential effects of l-Asp on the [4 1 1] to [4 5 1] step edge.