Species of uranyl ions as well as secondary uranium minerals are affected by the geochemical environments at the surface of the Earth. In other words, the evolution of secondary uranium minerals can provide us various information on the geochemical environments for a better understanding of geochemical processes involving migration behavior of elements. The author reviews geochemistry of uranyl ions and crystal chemistry of uranium minerals, and shows the evolution of secondary uranium minerals at some uranium deposits including Oklo, Cigar Lake, Shinkolobwe and Koongarra. The primary uraninites are altered by oxidizing conditions at the latter two deposits, and by reducing conditions at the former two deposits, which result in different types of evolution of secondary uranium minerals.
