| Abstract
| - Manganese redox cycling and the accompanyingdissolution and precipitation reactions are importantprocesses in natural waters. In the present study,Mn2+(aq) is reacted with O2(aq) at circumneutralpH toform manganese oxide islands on the (101̄4) surface ofMnCO3. The islands grow heteroepitaxially. The effectsofthe substrate surface morphology, the substrate atomicstructure, and the aqueous concentration of Mn2+ areinvestigated. On terraces, rhombohedral oxide islands formwith 90° rotation relative to the crystallographic axis ofthe underlying carbonate substrate. Although the islandheights self-limit between 2 and 3 nm depending on reactionconditions, the islands grow laterally to several squaremicrons before separate islands collide and coalesce. Theislands do not grow over substrate steps or down dissolution-pit edges.Comparison studies done with MgCO3 andCaCO3 show that the former also promotes heteroepitaxialgrowth whereas the latter does not. This difference isexplained by the relative bond length mismatch betweenthe structures of the carbonate substrates and the atomicstructures of manganese oxides. A free energy model isalso presented to explain why the heights of the manganeseoxide islands self-limit. Our results provide an improvedbasis both for the development of predictive modelsof contaminant fate and transport and for the modeling ofhydraulic flow through carbonate aquifers.
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