Boulangerite

Formula:  Pb₅Sb₄S₁₁

Origin of name:  For C. Boulanger, French mining engineer

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:

Unit cell: a = 21.56 Ǻ, b = 23.51 Ǻ, c = 8.09 Ǻ, β = 100.8°

Colour:  Lead-gray to bluish lead-gray

Diaphaneity: Opaque

Luster:  Metallic

Habit:  Crystals long prismatic to acicular, striations parallel to elongation; compact fibrous masses, plumose or schistose

Hardness:  2.5 to 3

Specific gravity:  6.23

Cleavage:  1; {100} good

Tenacity:  Brittle, fibres flexible

Streak:  Brownish gray to brown brown

Synonyms/varieties:  Plumosite, cuproboulangerite (Cu-rich var.)

Comments: Found in hydrothermal veins formed at low to moderate temperatures