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Coffinite

Chemical compound
Coffinite is a uranium-bearing silicate mineral with formula: 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... Wikipedia
Crystal class: Ditetragonal dipyramidal (4/mmm); H-M symbol: (4/m 2/m 2/m)
Crystal habit: Rarely as crystals, commonly as colloform to botryoidal incrustations, fibrous, pulverulent masses
Crystal system: Tetragonal
Formula (repeating unit): U(SiO4)1−x(OH)4x
Optical properties: Uniaxial (+/−)
Other characteristics: Radioactive 72.63% (U)