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Acicular crystals of umber-yellow boltwoodite from Namibia (size: 1.8 x 1.7 x 1.4 cm)
CategoryUranium silicate mineral
(repeating unit)
Strunz classification9.AK.15
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/m
Unit cella = 7.073(2) Å,
b = 7.064(1) Å,
c = 6.638(1) Å;
β = 105.45°; Z = 2
ColorPale yellow, orange yellow
Crystal habitElongated crystals, acicular to fibrous
CleavagePerfect on {010}, imperfect on {001}
Mohs scale hardness3.5 - 4
LusterSilky to vitreous, dull or earthy in aggregates
DiaphaneityTransparent to translucent
Specific gravity4.7
Optical propertiesBiaxial (-)
Refractive indexnα = 1.668 - 1.670 nβ = 1.695 - 1.696 nγ = 1.698 - 1.703
Birefringenceδ = 0.030 - 0.033
PleochroismWeak, X= colorless Y=Z= yellow
Ultraviolet fluorescenceFluoresces dull green in both SW and LW UV
Other characteristicsRadioactive

Boltwoodite is a hydrated potassium uranyl silicate mineral with formula HK(UO2)(SiO4)·1.5(H2O). It is formed from the oxidation and alteration of primary uranium ores. It takes the form of a crust on some sandstones that bear uranium. These crusts tend to be yellowish with a silky or vitreous luster.[4][5]

Discovery and occurrence[edit]

Orange to yellow boltwoodite on dark calcite matrix from Namibia (size: 5.2 x 3 x 2cm)

It was first described in 1956 for an occurrence in Pick's Delta Mine, Delta, San Rafael District (San Rafael Swell), Emery County, Utah, US.[3] It is named after Bertram Boltwood (1870–1927) an American pioneer of radiochemistry.

Boltwoodite occurs as secondary silicate alteration crusts surrounding uraninite and as fracture fillings. It is found in pegmatites and sandstone uranium deposits of the Colorado Plateau-type. It occurs associated with uraninite, becquerelite, fourmarierite, phosphouranylite, gypsum and fluorite.[2]