Mineral nucleation and growth can be catalyzed by the presence of mineral substrates and heteroepitaxial coatings can form as a result, if similarities in atomic structure between the substrate and the growing phase allows for lattice registry. However, the mechanisms of heteroepitaxial mineral nucleation and growth in aqueous media remain poorly understood. 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. The nucleation and growth of divalent metal carbonates on calcite (CaCO3) is of particular interest due to its role in carbon sequestration and determining the fate of toxic and nuclear waste. By utilizing in situ atomic force microscopy (AFM), the heteroepitaxial nucleation and growth of otavite (CdCO3) or rhodochrosite (MnCO3) on calcite single crystals were monitored in-situ at the nanometer scale. By careful non-invasive imaging and nano-mechanical probing, 3D islands of otavite were shown to form via an amorphous precursor phase, which transitioned to a heteroepitaxial crystalline phase, while layer-by-layer (2D) growth exhibited layer-dependent anisotropic growth behaviors. Moreover, the growth rate of rhodochrosite 3D islands was found to be commensurate with that of otavite despite a 2.5 times greater lattice mismatch with respect to the calcite substrate. This greater than expected growth rate was shown to be facilitated by Mn-Ca intermixing within the first few nanometers of growth by X-ray spectroscopy and electron microscopy techniques and may also play a role in the observed layer dependent anisotropy of 2D growth in the otavite system. These findings highlight the strong influence of the substrate on the composition and growth mechanisms of surface precipitates in aqueous conditions and will help to establish more accurate and robust models of heteroepitaxy in natural systems.

Molecular scale mechanisms of heteroepitaxial nucleation and growth at mineral-water interfaces

255th National Meeting (2018)

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