Benjaminite

Formula:  (Ag, Cu)₃ (Bi, Pb)₇ S₁₂

Origin of name:  For Marcus Benjamin, American chemist (1857-1932) of the U.S. National Museum

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning: Polysynthetic

Unit cell: a = 13.25 Å, b = 4.05 Å, c = 20.25 Å, b = 103.14°

Colour:  Gray on fracture tarnishing to dull or yellow or copper

Diaphaneity:

Luster:  Metallic to greasy

Habit:  Massive

Hardness:  3.3 to 3.5

Specific gravity:  6.7

Cleavage:  Fair parallel to elongation

Tenacity:

Streak:

Synonyms/varieties:

Comments: Found in quartz vein near the contact of a soda-granite and an intrusive rhyolite; with arsenides; in a veinlet in calcite and in a molybdenite pipe deposit