Polybasite-pearceite

Metallic polybasite crystal in matrix. From the Big Seven Mine, Neihart District, Cascade County, Montana. Field of view is about 0.5 mm. *(Photo courtesy Jesse Crawford; unauthorized reproduction is prohibited.)*

Formula: [(Ag,Cu)₆(Sb,As)₂S₇][Ag₉CuS₄] Monoclinic

Description:

Polybasite-pearceite are a series of black minerals formed in moderate to low temperature silver-rich hydrothermal deposits.

FOREST COUNTY: Polybasite-pearceite are very rare in the Crandon massive sulfide deposit, near Little Sand Lake. The mineral occurs with stephanite and tetrahedrite-tennantite as very tiny grains intergrown with galena (Lambe and Rowe, 1989).