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. This is due in part to the complex nucleation processes in these systems, which recent studies suggest, do not conform to classical theories of nucleation. In addition, the characterization of heteroepitaxial coatings presents unique challenges including nanometer-scale dimensions, patchiness, and similarity of structure with the substrate. Atomic Force Microscopy (AFM) is a powerful tool which is well suited to these challenges, providing nanometer scale characterization by passively monitoring and actively probing nucleation and growth in-situ. Here, we present direct evidence for non-classical nucleation of cadmium carbonate on calcite. Observations of a previously unknown metastable prenucleation phase transitioning to a heteroepitaxial crystalline phase followed by anisotropic growth will be presented. The need for and application of low force contact mode AFM, Peakforce AFM, lateral force microscopy, and tip directed particle removal, 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.

Probing dynamic heteroepitaxial nucleation processes of carbonates by atomic force microscopy

251st National Meeting (2016)

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