PNNL

Abstract

Minerals exposed to atmospheric moisture stabilize thin water films that drive a score of chemical reactions of great importance to water-unsaturated terrestrial environments. Here, we identified Mn (oxy)(hydr)oxide nanocoatings formed in water films hosted by micrometer-sized rhodochrosite (MnCO3). These mineralogical transformations were detected in ~3-16 nm thick films (~120-700 H2O/nm2), formed upon exposure a stream of 101 kPa N2(g) bearing 1.53-2.75 kPa H2O (50-90 % relative humidity) and ~20 kPa O2. These films offered a three-dimensional solvation environment that triggered and mediated Mn2+ dissolution, hydration, oxidation and precipitation reactions. Films of neutral pH reacted 4-10% of the Mn-bearing phases in the topmost ~5 nm region of the rhodochrosite surface, even after 1 yr of reaction time under ambient air. In contrast, reactions in strongly alkaline water films converted up to ~75% of the Mn in this region after 16d of reaction time, a result of the faster oxidation rates of hydrolyzed Mn(II) species.