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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)
Category Carbonate minerals
(repeating unit)
Strunz classification 5.ED.30
Crystal system Trigonal
Crystal class Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space group R3m
Unit cell a = 18, c = 23.83 [Å]; Z = 18
Color Bright green to yellow-green
Crystal habit Rhombohedra, often flattened, pseudocubic; crystalline crusts, granular
Mohs scale hardness 2.5
Luster Vitreous
Diaphaneity Transparent to translucent
Specific gravity 2.8
Optical properties Uniaxial (+)
Refractive index nω = 1.520 nε = 1.540
Birefringence δ = 0.020
Pleochroism Visible O = colorless E = Light yellow
Ultraviolet fluorescence bright pale green to yellow-green
Solubility Soluble in water
References [1][2][3]

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]