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

Minerals exposed to moist air stabilize thin water films that drive a score of chemical reactions great importance water-unsaturated terrestrial environments. In this study, we identified Mn (oxy)(hydr)oxide nanocoatings formed by the dissolution, oxidation and precipitation in oxygenated grown on rhodochrosite (MnCO3) microparticles. Nanocoatings could be vibrational spectroscopy, X-ray diffraction, photoelectron (scanning transmission) electron microscopy containing equivalent at least 7 monolayers (∼84 H2O/nm2). These were exposing microparticles with 50% relative humidity (RH). Films neutral pH reacted up 14% MnII located topmost ∼5 nm region atmospheres 90% RH for d. produced MnOOH, birnessite (MnO2) hausmannite (Mn3O4) nanoparticles low crystallinity, while exposure atmospheric 1 yr. converted only 2% MnOOH. contrast, alkaline ∼75% but after 16 d reaction. MnOOH MnO2 as well crystalline hausmannite. Kinetic modeling time-resolved growth MnO stretching bands these revealed two concurrent reaction processes. A 1rst-order process was assigned nucleation events terminating few hours, 0-order sustained from nuclei over longer time. By identifying film-driven rhodochrosite, our study adds new insight into mineralogical transformations relevant anoxic–oxic boundaries