Autunite is a secondary uranium mineral, named for its type locality near Autun, France, with a chemical formula of Ca(UO2)2(PO4)2·10-12H2O. Autunite ranges in colour from pale yellow and yellow-green to greenish black. It dehydrates to meta-autunite on extended exposure to air, becoming yellower and less greenish.
Autunite fluoresces green-yellow under UV light, and due to its uranium content is radioactive. It may also be used as an ore. It generally occurs in the oxidation zone of uranium-bearing rocks, and is associated with other uranium minerals, such as torbernite and uranophane. Crystals may be tabular or micaceous.
Image sources: 1, 2, 3 (2 and 3 are the same sample, viewed under normal and UV light)