Uraninite

Formula: UO₂

Origin of name: from composition

Crystal system: Cubic/Isometric

Crystal class: 4/m 3 2/m

Twinning: Rare on {111}

Unit cell: a = 5.468 Å

Colour: Black to brownish or grayish black, steel-black to valvet-black, pale gray, pale green

Diaphaneity: Nearly opaque, transparent in thinnest fragments

Luster: Submetallic, pitchy, greasy, dull

Habit: Crystals uncommon, typically cubic, cubo-octahedral or modified octahedrons or dodecahedrons, massive, dense to granular, botryoidal to reniform, colloform, banded, columnar, may be dendritic

Hardness: 5 to 6

Specific gravity: 10.63 to 10.95, decreasing markedly with oxidation to as low as 6.5

Cleavage: 1; octahedral

Tenacity: Brittle, conchoidal to uneven fracture

Streak: Black to brownish black, grya, olive-green, shining

Synonyms/varieties: bröggerite (Th-rich variety), nasturan, pitchblende (cryptocrystalline variety), gummite

Comments: series with thorianite, usually partially oxidised, radioactive