Carnotite in sandstone from the Mesozoic of Colorado, USA. (public display, Minnesota Discovery Center, Chisholm, Minnesota, USA)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5100 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The vanadate minerals all have one or more vanadate ions (VO4 -3).

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.

The rock shown above is a sandstone from Montrose County in western Colorado that is richly infused with carnotite. The specimen is not accompanied by specific locality or stratigraphic information, but carnotitic sandstone is known from several formations in Montrose County: 1) Burro Canyon Formation (Lower Cretaceous) - carnotite occurs in conglomeratic sandstones. 2) Salt Wash Member of the Morrison Formation (Upper Jurassic) - carnotite occurs in fluvial sandstones. 3) Entrada Sandstone (Middle Jurassic) - carnotite occurs in eolian sandstones. 4) Kayenta Formation (Upper Triassic to Lower Jurassic) - carnotite occurs in fluvial sandstones.

Some info. from:

Page et al. (1956) - Contributions to the geology of uranium and thorium by the United States Geological Survey and Atomic Energy Commission for the United Nations International Conference on Peaceful Uses of Atomic Energy, Geneva, Switzerland, 1955. United States Geological Survey Professional Paper 300. 739 pp.

Photo gallery of carnotite: