Hydrophilic nanominerals exposed to air moisture host thin water films that are key drivers of reactions of interest in nature and technology. Water films can trigger irreversible mineralogical transformations, and control chemical fluxes across networks of aggregated nanomaterials. Using X-ray diffraction, vibrational spectroscopy, electron microscopy, and (micro)gravimetry we tracked water film-driven transformations of periclase (MgO) nanocubes to brucite (Mg(OH)2), as well as to amorphous magnesium carbonate (AMC) in the presence of moist CO2. We show how 3-4 monolayer-thick water films triggered the nucleation-limited growth of brucite and AMC. We also show that growth transitioned to a diffusion-limited regime as nanocoatings hindered the transport of reactive species to growth fronts. By resolving nanocoating growth on a model hydrophilic mineral, this work provides new insight into water film-driven nanomineral transformations that are important to nature and technology.