Nanostructures can form on mineral surfaces through reactions with H 2O or O2 in the natural environment. In this study, nanostructures on the (1014) surfaces of calcite and rhodochrosite are characterized by their surface potentials using Kelvin probe force microscopy. Water-induced nanostructures on calcite have a topographic height of 1.1 (±0.6) nm and an excess surface potential of 126 (±31) mV at 45% relative humidity. The corresponding values for oxygen-induced nanostructures on rhodochrosite at the same RH are 1.3 (±0.7) nm and 271 (±14) mV, respectively. For increasing relative humidity on calcite, the topographic height of the nanostructures increases while their excess surface potential remains unchanged. In comparison, on rhodochrosite thetopographic height remains unchanged for increasing relative humidity but excess surface potential decreases. The nonzero excess surface potentials indicate that the nanostructures have compositions different from their parent substrates. The surface-potential heterogeneity associated with the distributed nanostructures has important implications for reactivity in both gaseous and aqueous environments. Taking into consideration such heterogeneities, which are not included in state-of-the-art models, should improve the accuracy of the predictions of contaminantfate and transport in natural environments.