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Coffinite

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Coffinite
Blenda smolista + coffinit.jpg
Pitchblende and coffinite in a sample from a Czech mine
General
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]

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