Carnotite is a radioactive, hydrous potassium uranyl vanadate mineral, K2(UO2)2(VO4)2·3H2O. It is the principal uranium ore mineral in America, but has also been mined as a vanadium ore mineral. The water component of carnotite varies with atmospheric humidity. This mineral has a nonmetallic, dull to earthy luster, yellow color and streak, and is quite soft (H=2). Crystals are rare, but typically have flattened diamond shapes. Crystals break with one perfect cleavage (micaceous). Carnotite is typically powdery, massive, or disseminated within host rocks. The uranium content (U) makes carnotite quite radioactive.
Carnotite is one of several secondary uranium minerals. It results from the alteration of uraninite (UO2) or other U- or V-bearing minerals. Uranium-bearing minerals are soluble in water and acids. Water percolating through uraniferous rocks will end up having dissolved U compounds. These dissolved compounds precipitate elsewhere as carnotite or other secondary U minerals.
Most carnotite is found disseminated in sandstones and channel sandstones. Even <1% carnotite content can result in a distinctly yellowish-colored sandstone. Carnotite is also found associated with carbonized plant fossils, permineralized fossil tree trunks, and calcrete paleosols.
A similar mineral, tyuyamunite, is the calcium equivalent of carnotite. Tyuyamunite has a slightly greenish yellow color and fluoresces under ultraviolet (UV) light. Carnotite is typically bright yellow in color and does not fluoresce under UV. Tyuyamunite & carnotite are usually mixed together in secondary uranium mineral deposits.
Carnotite (yellow) in quartzose sandstone (uranium ore), Morrison Formation, Upper Jurassic; undisclosed locality in the Colorado Plateau, USA. (Colorado School of Mines Geology Museum, Golden, Colorado, USA)
Carnotite nodule in sandstone from an undisclosed locality in the Colorado Plateau, USA. (Colorado School of Mines Geology Museum, Golden, Colorado, USA)