Adsorption is believed to be a dominant mechanism of uranium distribution between solid and solution, and thus, to play a major role in uranium transport. Because iron oxides and hydroxides are abundant at the Earth's surface and are great adsorbents of uranium, we have examined natural rocks that contain iron minerals along with uranium, and also carried out Fe-U coprecipitation and aging experiments to find how uranium is distributed between Fe minerals. Transmission and scanning electron microscopy reveals that microcrystals (10-50 nm) of metatorbernite (Cu(UO2)2(PO4)28H2O) are scattered within nodules consisting of fine-grained (2-50 nm) goethite and hematite, where the ground water is undersaturated with respect to metatorbernite, for the natural rocks from the Koongarra ore deposit, Australia. The microscopy also reveals that microcrystals (a few nm) of dehydrated schoepite ((UO2)O0.25(OH)1.5) are formed among fine-grained hematite after aging coprecipitated Fe-U in the laboratory, and the solution is undersaturated with respect to schoepite. The beam size of microscopes is found to be important for the chemical analysis of such microcrystals. We detect a strong signal of uranium for a beam size < 40 nm; whereas a weak uranium signal is obtained for a beam size > 150 nm. Our results indicate that such a weak uranium signal should not be taken as a result of homogeneously distributed uranium over goethite and hematite surfaces by, for instance, adsorption. The micrcrystallization observed in both the field and laboratory suggests that fine grained uranyl minerals play a major role in uranium transport and migration.