The presence of mineral surfaces leads to geochemical reaction outcomes that are often not predicted by bulk thermodynamics and classical nucleation and growth mechanisms. Alternative nucleation and growth pathways at mineral-water interfaces are difficult to elucidate due to the number of competing molecular-scale controls. For instance, heterogeneous nucleation and growth can lead to mineral coatings that control the chemistry of the aqueous phase. Previously, cation concentration was shown to control the transition from non-classical to classical nucleation pathways. Specifically, AFM, X-ray pair distribution function analysis, and AIMD revealed that, at low [CdCl2], a (Cd,Ca)CO3 solid solution formed on CaCO3 through amorphous precursor particles that nucleated in solution, deposited on the surface, and underwent an epitaxy-mediated phase transformation. In contrast, at higher [CdCl2], a uniform CdCO3 coating grew from step edges or 2D crystalline nuclei. Recently, we have utilized the competitive heterogeneous growth between CoCO3 and Co(OH)2 to parse out factors which favor the formation of one or the other phase. XPS and SEM/EDS were employed to determine the influence of lattice mismatch by comparing growth on CaCO3 and MgCO3 substrates, the effect of thermodynamic driving force by varying pH and ion concentration, and the kinetic effects of water exchange rates by elevating reaction temperatures. Overall, a higher degree of epitaxial growth of CoCO3 was observed on MgCO3 relative to CaCO3, which was greatly increased on both substrates by a decrease in pH, while a shift from uniform CoCO3 coatings to large Co(OH)2 outgrowths was observed at elevated temperatures on CaCO3.

Competitive heterogeneous nucleation and growth of cobalt hydroxide and cobalt carbonate provides insights into the molecular-scale controls of interfacial reactions

ACS Spring 2022

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