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Andersonite crystals from the Repete Mine, Blanding, San Juan County, Utah (size: 5.2 x 3.6 x 1.5 cm)
CategoryCarbonate minerals
(repeating unit)
Strunz classification5.ED.30
Crystal systemTrigonal
Crystal classHexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space groupR3m
Unit cella = 18, c = 23.83 [Å]; Z = 18
ColorBright green to yellow-green
Crystal habitRhombohedra, often flattened, pseudocubic; crystalline crusts, granular
Mohs scale hardness2.5
DiaphaneityTransparent to translucent
Specific gravity2.8
Optical propertiesUniaxial (+)
Refractive indexnω = 1.520 nε = 1.540
Birefringenceδ = 0.020
PleochroismVisible O = colorless E = Light yellow
Ultraviolet fluorescencebright pale green to yellow-green
SolubilitySoluble in water

Andersonite, Na2Ca(UO2)(CO3)3·6H2O, or hydrated sodium calcium uranyl carbonate is a rare uranium carbonate mineral that was first described in 1948. Named after Charles Alfred Anderson (1902–1990) of the United States Geological Survey, who first described the mineral species, it is found in sandstone-hosted uranium deposits. It has a high vitreous to pearly luster and is fluorescent. Andersonite specimens will usually glow a bright lemon yellow (or green with blue hints depending on the deposit) in ultraviolet light. It is commonly found as translucent small rhombohedral crystals that have angles close to 90 degrees although its crystal system is nominally trigonal. Its Mohs hardness is 2.5, with an average specific gravity of 2.8.

It occurs in the oxidized zone of uranium-bearing polymetallic ore deposits. It also may occur as an efflorescent crust on the walls and timbers of uranium mines. As this mineral is water-soluble, samples must be stored in dry conditions. It occurs with schrockingerite, bayleyite, shwartzite, boltwoodite, liebigite and gypsum.[1]

It was first described in 1948 for an occurrence in the Hillside Mine near Bagdad, Eureka District, Yavapai County, Arizona.[2]