Rhodochrosite (MnCO3) microparticles exposed to oxygen-rich humid air acquire nanometric water films that mediate in the formation of Mn (oxy)(hydr)oxide nanocoatings. As Mn is the second most abundant heavy metal in Earth’s crust, and Mn-bearing minerals are highly redox-active, the Mn (oxy)(hydr)oxide formation affects many (bio)geochemical processes, including heavy metal transport, Mn bioavailability to plants, and microbial respiration. Studies on nanocoating formation and composition contribute to a growing literature on mineral transformation reactions at anoxic-oxic boundaries of water-unsaturated environments. These are settings of widespread occurrence and importance in nature, especially in porous systems (e.g. vadose zones of soils, alkali soils, dry lakes) of Earth’s crust exposed to moist atmospheric air. In this study, vibrational spectroscopic, X-ray diffraction, surface compositional and imaging methods were used to identify Mn (oxy)(hydr)oxide nanocoatings grown by Mn dissolution, hydrolysis, oxidation, and precipitation within 7-8 monolayers of oxygenated water films. We find that MnOOH, Mn3O4 and MnO2 were the main reaction products in neutral and alkaline water films. While films of neutral pH required 7 days for converting only 14% of MnII on the topmost 5 nm of MnCO3 surfaces, reactions within alkaline water films took only 40 h to convert up to 68% of MnII in this region. From the time-resolved formation of Mn-O bonds, we find that nanocoating growth began through a 1st-order (possibly topotactic) process, followed by a 0-order process from the sustained growth of nanoparticles in the thin three-dimensional network of water films. By identifying nanocoatings formed by water film-driven reactions on rhodochrosite, our study adds new insight into mineralogical transformations relevant to anoxic–oxic boundaries in water-unsaturated environments.

Water film-driven Mn (oxy)(hydr)oxide nanocoating growth on rhodochrosite

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