Mapping Three-dimensional Dissolution Rates of Calcite Microcrystals: Effects of Surface Curvature and Dissolved Metal Ions

The morphological evolution of micron-sized calcite crystals dissolved in static acidic solutions, with and without dissolved Pb2+ ions, was imaged using transmission X-ray microscopy (TXM). The area-normalized dissolution rates measured by TXM increased with time in both Pb-free and Pb-rich solutions but with distinct morphological evolution. Calcite reacted in Pb-free solutions exhibited rounding at corners and edges with faster dissolution at acute corners/edges than obtuse corners/edges. Numerical simulations indicate that this is controlled primarily by solution mass transport that is faster near the acute corners/edges. In comparison, dissolution of calcite in Pb-rich solutions was 50% slower than that in Pb-free solutions and exhibited less rounding at corners. Faces of the calcite rhombs exhibited increased surface roughness and the subsequent development of surface micropyramids that formed preferentially near the acute edges of the calcite rhombs. Spatially resolved dissolution rates reveal that pyramid formation is associated with reduced dissolution rates near the pyramid apex. The results demonstrate the role of impurity metal ions in controlling the dissolution rate and the associated complexities in the morphological evolution of dissolving mineral surfaces