CARNOTITE
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)