- Coffinite
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Coffinite 
Pitchblende and coffinite in a sample from a Czech mineGeneral Category Nesosilicate Chemical formula U(SiO4)1-x(OH)4x Crystal symmetry Tetragonal 4/m 2/m 2/m ditetragonal dipyramidal Unit cell a = 6.97 Å, c = 6.25 Å; Z = 3 Identification Color Black (from organic inclusions; pale to dark brown in thin section Crystal habit Rarely as crystals, commonly as colloform to botryoidal incrustations, fibrous, pulverulent masses Crystal system Tetragonal Fracture Irregular to subconchoidal Tenacity Brittle to friable Mohs scale hardness 5-6 Luster Dull to adamantine Streak Grayish black Diaphaneity Opaque, transparent on thin edges Specific gravity 5.1 Optical properties Uniaxial (+/-) Refractive index nα = 1.730 - 1.750 nβ = 1.730 - 1.750 Birefringence δ = 1.730 Pleochroism Moderate; pale yellow-brown parallel to and medium brown perpendicular to long axis Alters to Metamict Other characteristics Radioactive References [1][2][3] Coffinite is a uranium-bearing silicate mineral: U(SiO4)1-x(OH)4x.
It occurs as black incrustations, dark to pale-brown in thin section. It has a grayish black streak. It has a brittle to conchoidal fracture. The hardness of coffinite is between 5 and 6.
It was first described in 1954 for an occurrence at the La Sal No. 2 Mine, Beaver Mesa, Mesa County, Colorado, USA,[3] and named for American geologist Reuben Clare Coffin (1886-1972).[1] It has widespread global occurrence in Colorado Plateau-type uranium ore deposits of uranium and vanadium. It replaces organic matter in sandstone and in hydrothermal vein type deposits.[1] It occurs in association with uraninite, thorite, pyrite, marcasite, roscoelite, clay minerals and amorphous organic matter.[1]
References
- ^ a b c d Mineral Handbook
- ^ Webmineral data
- ^ a b Mindat
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