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.

Time-sequenced contact-force micrographs show that the (104) calcite cleavage surface reconstructs in humid air through pit formation and film growth. After 8 h at 80% relative humidity (RH), 50% to 80% of the surface is covered by islands that are flat-topped and 1-nm high. The lateral growth rates of individual islands are 4.2+/-0.4 nm min-1 in the 41 direction and 1.8+/-0.4 nm min-1 in the 48 direction, resulting in islands having distinct major and minor axes. On some samples, a contiguous, 1.5-nm-high film rapidly grows between the islands and the pits. The areal expansion rate of the film is 500 times faster than that of the islands. Gaps between the contiguous film and the islands expand and contract, which suggests that mass is exchanged between them and that both are loosely bound. Complementing the topographic images, polarization heights are simultaneously measured by polarization-force microscopy. The polarization heights of the islands and the contiguous film are -6 to -10 nm and -4 to -5 nm, respectively, compared to their respective topographic heights of +1.0 and +1.5 nm. Under our experimental conditions, the polarization heights are a surrogate for the local dielectric constant of the sample epsilon and arise from a convolution of the mobility and the density of surface ions. The polarization heights imply that epsilonsubstrate>epsilonfilm>epsilonisland. Changes in topographic and polarization heights at 20% and 50% RH suggest that the structures of the islands are in dynamic equilibrium with the adsorbed water. Our evidence suggests that the islands contain loosely bound water and may therefore be a hydrated calcium carbonate phase stabilized by the calcite surface.

No abstract

Nanostructures can form on mineral surfaces through reactions with H2O or O2 in the natural environment. In this study, nanostructures on the (1014) surfaces of calcite and rhodochrosite are characterized by their surface potentials using Kelvin probe force microscopy. Water-induced nanostructures on calcite have a topographic height of 1.1 (+/-0.6) nm and an excess surface potential of 126 (+/-31) mV at 45% relative humidity. The corresponding values for oxygen-induced nanostructures on rhodochrosite at the same RH are 1.3 (+/-0.7) nm and 271 (+/-14) mV, respectively. For increasing relative humidity on calcite, the topographic height of the nanostructures increases while their excess surface potential remains unchanged. In comparison, on rhodochrosite thetopographic height remains unchanged for increasing relative humidity but excess surface potential decreases. The nonzero excess surface potentials indicate that the nanostructures have compositions different from their parent substrates. The surface-potential heterogeneity associated with the distributed nanostructures has important implications for reactivity in both gaseous and aqueous environments. Taking into consideration such heterogeneities, which are not included in state-of-the-art models, should improve the accuracy of the predictions of contaminantfate and transport in natural environments.