Boltwoodite

Boltwoodite is a hydrated uranyl silicate mineral with formula (K0.56Na0.42)[(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 x 1.7 x 1.4 cm)
General
CategoryUranium silicate mineral
Formula
(repeating unit)		(K0.56Na0.42)[(UO2)(SiO3OH)]·1.5(H2O)
IMA symbolBdw[1]
Strunz classification9.AK.15
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/m
Unit cella = 7.0772(8) Å,
b = 7.0597(8) Å,
c = 6.6479(7) Å;
β = 104.982(2)°; Z = 2
Identification
ColorPale yellow, orange yellow
Crystal habitElongated crystals, acicular to fibrous
CleavagePerfect on {010}, imperfect on {001}
TenacityBrittle
Mohs scale hardness3.5 - 4
LusterSilky to vitreous, dull or earthy in aggregates
StreakWhite
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 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]

References
  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Mineralienatlas
  3. Handbook of Mineralogy
  4. Mindat.org
  5. Webmineral data
  6. Burns, Peter (1998). "The Structure of Boltwoodite and Implications of Solid Solution Toward Sodium Boltwoodite". The Canadian Mineralogist. 36: 1069-75.
  7. Burns, Peter (1998). "The Structure of Boltwoodite and Implications of Solid Solution Toward Sodium Boltwoodite". The Canadian Mineralogist. 36: 1069-75.
  8. "BOLTWOODITE (Hydrated Potassium Uranyl Silicate Hydroxide)".
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