Heteroepitaxial nucleation and oriented growth of manganese oxide islands on carbonate minerals under aqueous conditions.

Manganese redox cycling and the accompanying dissolution and precipitation reactions are important processes in natural waters. In the present study, Mn2+ (aq) is reacted with O2(aq) at circumneutral pH to form manganese oxide islands on the (1014) surface of MnCO3. The islands grow heteroepitaxially. The effects of the substrate surface morphology, the substrate atomic structure, and the aqueous concentration of Mn2+ are investigated. On terraces, rhombohedral oxide islands form with 90 degrees rotation relative to the crystallographic axis of the underlying carbonate substrate. Although the island heights self-limit between 2 and 3 nm depending on reaction conditions, the islands grow laterally to several square microns before separate islands collide and coalesce. The islands do not grow over substrate steps or down dissolution-pit edges. Comparison studies done with MgCO3 and CaCO3 show that the former also promotes heteroepitaxial growth whereas the latter does not. This difference is explained by the relative bond length mismatch between the structures of the carbonate substrates and the atomic structures of manganese oxides. A free energy model is also presented to explain why the heights of the manganese oxide islands self-limit. Our results provide an improved basis both for the development of predictive models of contaminant fate and transport and for the modeling of hydraulic flow through carbonate aquifers.