Mineral nucleation and growth in natural environments often occur in mixed cation systems, where minor and trace elements either co-precipitate with or nucleate on abundant phases. As a result, particles of major mineral phases are coated by minor mineral phases – a phenomenon that significantly contributes to the complexity of geochemical systems. No current theoretical framework can reliably predict the outcomes of heterogeneous nucleation and growth reactions in complex geochemical systems. Recent work indicates heterogenous nucleation of otavite (CdCO3) on calcite (CaCO3) can proceed through non-classical pathways, whereby intermediate amorphous phases first form in solution before undergoing transformation to a heterogeneous crystalline product. Using in situ Pair Distribution Fuction (PDF) and Small-Angle X-ray Scattering (SAXS), along with ICP-MS and SEM, this complimentary study 1) determines a novel short-range ordered Cd-ACC phase en route to vaterite formation and 2) finds amorphous carbonate is stabilized by Cd<sup>2+</sup>. Results are highly dependent on ACC supersaturation, suggesting Cd-induced stability of Cd-ACC are dependent primarily on amorphous phase kinetics rather than thermodynamics. Findings provide a quantitative understanding of precursor structure and aggregation that impact the chemical heterogeneities frequently observed in mineral coatings.

Nucleation of mixed-cation amorphous carbonate precursors: Example of cadmium in ACC and vaterite

ACS Spring Meeting 2021

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