Nucleation and Epitaxy-Mediated Phase Transformation of a Precursor Cadmium Carbonate Phase at the Calcite/Water Interface

Abstract

Mineral nucleation can be catalyzed by the presence of mineral substrates; however, the mechanisms of heterogeneous nucleation remain poorly understood. A combination of in situ time-sequenced measurements and nanomanipulation experiments were performed using atomic force microscopy (AFM) to probe the mechanisms of heteroepitaxial nucleation of otavite (CdCO 3 ) on calcite (CaCO 3 ) single crystals that exposed the (101̅4) surface. Otavite and calcite are isostructural carbonates that display a 4% lattice mismatch, based on their (101̅4) surface areas. AFM observations revealed a two-stage process in the nucleation of cadmium carbonate surface precipitates. As evidenced by changes in the height, shape, growth behavior, and friction signal of the precipitates, a precursor phase was observed to initially form on the surface and subsequently undergo an epitaxy-mediated phase transformation to otavite, which then grew epitaxially. Nanomanipulation experiments, in which the applied force was increased progressively until precipitates were removed from the surface, showed that adhesion of the precursor phase to the substrate was distinctively weaker than that of the epitaxial phase, consistent with that of an amorphous phase. These findings demonstrate that heterogeneous mineral nucleation can follow a nonclassical pathway like that found in homogeneous aqueous conditions.