Boltwoodite

Hydrated potassium uranyl silicate mineral
(repeating unit)(K0.56Na0.42)[(UO2)(SiO3OH)]·1.5(H2O)IMA symbolBdw[1]Strunz classification9.AK.15Crystal systemMonoclinicCrystal classPrismatic (2/m)
(same H-M symbol)Space groupP21/mUnit cella = 7.0772(8) Å,
b = 7.0597(8) Å,
c = 6.6479(7) Å;
β = 104.982(2)°; Z = 2IdentificationColorPale yellow, orange yellowCrystal habitElongated crystals, acicular to fibrousCleavagePerfect on {010}, imperfect on {001}TenacityBrittleMohs scale hardness3.5–4LusterSilky to vitreous, dull or earthy in aggregatesStreakWhiteDiaphaneityTransparent to translucentSpecific gravity4.7Optical propertiesBiaxial (−)Refractive indexnα = 1.668 – 1.670 nβ = 1.695 – 1.696 nγ = 1.698 – 1.703Birefringenceδ = 0.030 – 0.033PleochroismWeak, X = colorless, Y = Z = yellowUltraviolet fluorescenceFluoresces dull green in both SW and LW UVOther characteristics RadioactiveReferences[2][3][4][5][6]

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]

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. ^ a b Handbook of Mineralogy
  4. ^ a b Mindat.org
  5. ^ a b 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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