AUTUNITE (Uranium mineral)
Autunite is a hydrated calcium uranyl phosphate mineral with the formula of Ca(UO2)2(PO4)2 · 10-12H2O. Due to the uranium content of 48.27%, it is radioactive and used as uranium ore. Autunite fluoresces bright yellow-green under LW & SW ultraviolet light. Autunite was named after Autun, France, where the mineral was found in 1852. The type locality is Saint-Symphorien-de-Marmagne uranium deposit, Autun, Saône-et-Loire, Bourgogne-Franche-Comté, France.
Structure and Occurrence
Autunite often occurs as tabular square crystals in small crusts or fan-like masses. Autunite dehydrates in the air to meta-autunite. Autunite is a product of the alteration of uraninite. It is the most common surface uranium mineral, abundant in the oxidation zones of uranium deposits. It is also found more rarely in pegmatitic veins containing uranium and in some hydrothermal veins. Associate minerals include meta-autunite, torbernite, phosphuranylite, saleeite, uranophane, and sabugalite.
Autunite belongs to the Orthorhombic Crystal System, Dipyramidal Class. It is a soft mineral with a hardness of 2.0 – 2.5 on the Moh’s Scale. Luster can be Sub-Vitreous, Resinous, Waxy, or Pearly. Fracture is micaceous. Autunite is fragile and may easily split with low pressure.
The color of Autunite may be yellow, greenish-yellow, pale green, dark green, or greenish-black. Color may become yellower and less greenish with even slight dehydration.
Autunite was discovered inside the Daybreak Mine on Mount Kit Carson, Spokane, Washington, in granitic rock’s vugs, fractures, and shear zones. Another phosphate mineral, Apatite, was found in the mine, which may have helped form autunite by providing a source of phosphate and lime. The formation may have occurred with the interaction of uranium that leached from a separate deposit. Some 90,000 lbs. of U3O8 were produced from nine properties, although most ore came from the Daybreak Mine.
Localities:
Autunite has been found on every continent, including Antarctica. Notable occurrences include Autun, France; Cornwall, England; Democratic Republic of the Congo; Bergen, Germany; and Portugal.
In the United States, Autunite is notable in Mitchell County, North Carolina, and Mt. Spokane, Washington, as well as in North and South Dakota.
For more information about this mineral, visit mindat.org or join our Facebook group to network with others of like minds. Visit us on Instagram and/or join us at our Gem and Mineral shows.
By Bill Jones, Sidewinder Minerals
