Carnotite

Carnotite is a hydrated potassium uranyl vanadate, K2(UO2)2(VO4)2 3H2O, instantly recognizable by its vivid canary-yellow to greenish-yellow color. It crystallizes in the monoclinic system but is most often seen as soft, earthy, powdery coatings and crusts.

Because it contains uranium, carnotite is radioactive, and it is one of the most important ores of both uranium and vanadium, especially in the sandstone deposits of the Colorado Plateau in the United States.

Its striking yellow color makes it an obvious indicator of uranium mineralization, and it played a central role in the early uranium and radium mining industries.

Carnotite is a secondary mineral that forms in sedimentary environments, chiefly in sandstones, where uranium- and vanadium-bearing groundwater precipitates the mineral around organic material, fossil wood, and permeable beds.

It typically impregnates porous sandstone as disseminated yellow coatings, cement, and concentrations, often associated with tyuyamunite and other secondary uranium minerals.

The classic deposits occur on the Colorado Plateau (Colorado, Utah, Arizona, New Mexico) in the Morrison Formation and related sandstones. Other occurrences are known in Australia, Kazakhstan, the Congo, and elsewhere.

Carnotite is identified by its intense canary-yellow to greenish-yellow color, dull to earthy luster, and very soft, powdery, often friable habit (hardness around 2) that can rub off as a yellow dust.

The decisive test is radioactivity: carnotite registers strongly on a Geiger counter due to its uranium content, distinguishing it from non-radioactive yellow minerals like sulfur or autunite (though autunite is also radioactive and fluoresces, while carnotite generally does not fluoresce).

Its occurrence as yellow coatings in sandstone, often near fossil wood, is a strong contextual clue.

Carnotite is a major ore of uranium and vanadium. Uranium is used as nuclear reactor fuel and historically as a source of radium, while vanadium is used to strengthen steel alloys.

The Colorado Plateau carnotite deposits were economically vital during the early 20th-century radium boom and again during the mid-century uranium rush.

Because it is radioactive, carnotite must be handled and stored with appropriate care: limit handling, avoid inhaling dust, store in ventilated conditions, and keep away from living areas. It is not used in jewelry and is collected mainly as a scientific or ore specimen.