Polybasite

Polybasite
Locality: Arizpe, Sonora, Mexico. Scale bar is one inch (2.5 cm.)
General
Category	Sulfosalt minerals
Formula (repeating unit)	[(Ag,Cu)6(Sb,As)2S7][Ag9CuS4]
Strunz classification	2.GB.15
Crystal system	Monoclinic and trigonal polytypes
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Identification
Colour	steel black
Cleavage	poor on (001)
Fracture	uneven
Mohs scale hardness	1.5-2
Lustre	metallic, adamantine or glimmering
Streak	black with reddish tint
Specific gravity	6-6.2
References

Polybasite is a sulfosalt mineral of silver, copper, antimony and arsenic. Its chemical formula is [(Ag,Cu)₆(Sb,As)₂S₇][Ag₉CuS₄].

It forms black monoclinic crystals (thin, tabular, with six corners) which can show dark red internal reflections. It has a Mohs hardness of 2.5 to 3. It is found worldwide and is an ore of silver. The name comes from the number of base metals in the mineral.