Autunite

Formula:  Ca(UO₂)₂(PO₄)₂^.10-12H₂O

Origin of name:  After locality; Autun, Saone-et-Loire, France

Crystal system:  Tetragonal

Crystal class:  4/m 2/m 2/m

Twinning: On {110}

Unit cell: a = 6.989 Å, c = 20.63 Å

Colour:  Sulfur-yellow, lemon-yellow, green-yellow to pale green, may be dark green to greenish black

Diaphaneity: Transparent to translucent

Luster:  vitreous, pearly on {001}, masses dull

Habit:  Crystals thin to thick tabular with rectangular or octahedral shape; fan-like groups, crusts, foliate

Hardness:  2 to 2.5

Specific gravity:  3.05 to 3.20 (varies with hydration)

Cleavage:  2; {001} perfect, {100} indistinct

Tenacity:

Streak:  Pale yellow

Synonyms/varieties: Lime uranite

Comments:  Fluoresces strong yellow-green in UV light; dehydrates in air to meta-autunite; radioactive. Member of the autunite group. Formed as a secondary mineral derived from primary uranium-bearing minerals under oxidising conditions, in hydrothermal veins, granite pegmatites, etc.