Polybasite, (Ag,Cu)16Sb2S11 (silver copper antimony sulfide), is a somewhat uncommon silver bearing mineral. It forms interesting crystals that have a pseudohexagonal outline with rhombohedral striations. — Photo: Darla Sondrol · CC0
Polybasite
A black silver-copper-antimony sulfosalt forming six-sided tabular crystals with dark-red internal reflections.
Intro adapted from Wikipedia (CC BY-SA).
Polybasite is an IMA-recognized mineral species in the sulfosalt class, with the chemical formula [Ag₉CuS₄][(Ag,Cu)₆(Sb,As)₂S₇].
It crystallizes in the monoclinic crystal system (space group P-3m1). It is a member of the Polybasite structural group.
Key properties: Mohs hardness 2–3; specific gravity 6.1; Black streak; Metallic luster; optically biaxial.
IMA symbol Plb, IMA status: approved, named in 1829.
Found in silver veins of low to medium temperature of formation.
It is found associated with Pyrargyrite, tetrahedrite, stephanite, other silver sulfosalts, acanthite, gold, quartz, calcite, dolomite, barite.
Its name comes from the Greek for many and base, in allusion to the many metallic bases present.