Ferrous saponite is a secondary mineral that can be used to reveal the redox state of past aqueous environments on Mars. In mineralogical analyses for ferrous saponite formed in laboratory simulations or contained in future returned samples from Mars, its oxidation by the Earth’s air could be problematic due to the high redox sensitivity. Here, we performed micro X-ray diffraction and scanning transmission X-ray microscopy analyses for a single particle of synthesized ferrous saponite without any exposure to air. The sample was reanalyzed after air exposure for 10–18 h to assess the adequacy of our anoxic preparation/measurement methods and the impacts of air on the sample. We found that the crystal structures agreed with ferrous saponite, both before and after air exposure; however, ferrous iron in saponite was partially oxidized, at least until 0.1–1 μm from the surface, after air exposure at the submicron scale, forming micro-vein-like Fe(III)-rich features. Together with our results of infrared spectroscopy of ferrous saponite, we showed that oxidation of octahedral iron occurred rapidly and heterogeneously, even in a short time of air exposure without any structural rearrangement. Since ferrous saponite is expected to exist on carbonaceous asteroids and icy dwarf planets, our methodology is also applicable to mineralogical studies of samples returned from these bodies.