Epitaxy of Calcite on Mica

We report for the first time the conditions under which oriented overgrowth of calcite occurs on muscovite mica. Calcite grows in a (001) orientation parallel to the basal cleavage plane on mica surfaces that have been weathered at ambient humidity (35−40%) for periods of at least 1 h, but no epitaxy is found on freshly cleaved mica. Nanometer-thin K 2 CO 3 crystallites are known to form on mica surfaces weathered in this manner, and our experiments show that the presence of these crystallites during the initial stages of nucleation and growth is necessary for the epitaxial growth of calcite. Calcium ions do not adsorb to mica surfaces at the solution concentrations (≤20 mM) used in these experiments, and we suggest that the crystallites provide patches on the surface where the local concentration of both carbonate and calcium ions is high enough to promote epitaxy.