Reactions at the mineral-water interface are complex and key to understanding and predicting geochemical processes.  The nucleation and growth of divalent metal carbonates on calcite is of particular interest due to its role in carbon sequestration and determining the fate of toxic and nuclear waste.  However, a robust molecular scale understanding of heteroepitaxial nucleation and growth of carbonate systems has not been established.  While mounting evidence suggests non-classical nucleation mechanisms are involved in many homogeneous systems, direct evidence for similar processes in heteroepitaxial systems are less prevalent.  In addition, the characterization of heteroepitaxial coatings presents unique challenges including nanometer-scale dimensions, patchiness, and similarity of structure with the substrate.  Utilizing Atomic Force Microscopy (AFM) heteroepitaxial nucleation and growth of Otavite (CdCO3) on a Calcite (CaCO3) substrate was monitored in-situ at the nanometer scale.  Through the application of low force contact mode AFM, Peakforce AFM, lateral force microscopy, and tip directed particle removal, direct evidence for non-classical nucleation of cadmium carbonate on calcite was observed.  The formation of a previously unknown prenucleation phase transitioning to a heteroepitaxial crystalline phase followed by anisotropic growth will be presented. In addition, preliminary experiments providing evidence for the formation of an amorphous precursor phase in solution will be discussed.  These findings have important implications for our understanding of carbonate nucleation and will help to establish a more accurate and robust model for the nucleation and growth processes of carbonate systems and heteroepitaxial nucleation in general.

Nucleation and epitaxy-mediated phase transformation of a precursor cadmium carbonate phase at the calcite/water interface

253rd National Meeting (2017)

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