polybasite, heavy, black sulfosalt mineral, a sulfide of the elements silver, copper, and antimony ([Ag, Cu]16Sb2S11), that occurs as monoclinic crystals and masses in silver veins, sometimes in large amounts.
Polybasite is found in Chile, Peru, the Czech Republic, and many localities in Mexico and in the U.S. states of Colorado, Montana, Idaho, and Nevada. For detailed physical properties, see sulfosalt (table).