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

暴露在潮湿空气中的矿物质可以稳定薄水膜，这些水膜会驱动许多对水不饱和的陆地环境非常重要的化学反应。在这项研究中，我们确定了锰（氧）（氢）氧化物纳米涂层，这是由锰在菱锰矿（MnCO ₃）微粒上生长的氧化水膜中的溶解、氧化和沉淀形成的。可通过振动光谱、X 射线衍射、X 射线光电子能谱和（扫描和透射）电子显微镜鉴定的纳米涂层在含有至少 7 个等效单层（~84 H ₂ O/nm ^{2 ）的水膜中形成}）。这些薄膜是通过将微粒暴露于具有至少 50% 相对湿度 (RH) 的潮湿空气中形成的。中性 pH 值的薄膜在 90% RH 的气氛中反应 7 天，最多 14% 的 Mn ^II位于微粒的最顶部~5 nm 区域。这些反应产生低结晶度的 MnOOH、水钠锰矿 (MnO ₂ ) 和 hausmannite (Mn ₃ O ₄ ) 纳米颗粒，同时暴露在大气中 1 年。该区域仅 2% 的 Mn ^{II转化为 MnOOH。}相比之下，碱性水膜中的反应转化了高达 75% 的 Mn ^II，但仅在反应 16 天后。这些薄膜产生 MnOOH 和 MnO ₂低结晶度，以及结晶的hausmannite。这些纳米涂层的 Mn O 伸缩振动带的时间分辨生长的动力学模型揭示了两个同时发生的反应过程。1 级^过程被分配到成核事件仅在几个小时后终止，0 级过程被分配到纳米涂层在更长的反应时间内从这些核中持续生长。通过识别菱锰矿上由水膜驱动反应形成的纳米涂层，我们的研究增加了对与水不饱和环境中的缺氧-好氧边界相关的矿物学转变的新见解。