Boltwoodite

Boltwoodite is a hydrated uranyl silicate mineral with formula (K_0.56Na_0.42)[(UO₂)(SiO₃OH)]·1.5(H₂O), distinct in crystal structure from sodium boltwoodite, which has an orthorhombic structure rather than monoclinic.^[7] 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.^[5][8]

Boltwoodite
Acicular crystals of umber-yellow boltwoodite from Namibia (size: 1.8 × 1.7 × 1.4 cm)
General
Category	Uranium silicate mineral
Formula	(K0.56Na0.42)[(UO2)(SiO3OH)]·1.5(H2O)
IMA symbol	Bdw[1]
Strunz classification	9.AK.15
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/m
Unit cell	a = 7.0772(8) Å, b = 7.0597(8) Å, c = 6.6479(7) Å; β = 104.982(2)°; Z = 2
Identification
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	[2][3][4][5][6]

Discovery and occurrence

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.^[4] 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.^[3]