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Acicular crystals of umber-yellow boltwoodite from Namibia (size: 1.8 x 1.7 x 1.4 cm)
Category Uranium silicate mineral
(repeating unit)
Strunz classification 9.AK.15
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group P21/m
Unit cell a = 7.073(2) Å,
b = 7.064(1) Å,
c = 6.638(1) Å;
β = 105.45°; Z = 2
Color Pale yellow, orange yellow
Crystal habit Elongated crystals, acicular to fibrous
Cleavage Perfect on {010}, imperfect on {001}
Tenacity Brittle
Mohs scale hardness 3.5 - 4
Luster Silky to vitreous, dull or earthy in aggregates
Streak White
Diaphaneity Transparent to translucent
Specific gravity 4.7
Optical properties Biaxial (-)
Refractive index nα = 1.668 - 1.670 nβ = 1.695 - 1.696 nγ = 1.698 - 1.703
Birefringence δ = 0.030 - 0.033
Pleochroism Weak, X= colorless Y=Z= yellow
Ultraviolet fluorescence Fluoresces dull green in both SW and LW UV
Other characteristics Radioactive
References [1][2][3][4]

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]