One of the more common lead-bismuth sulphides, along withgalenobismutite.
Appears to contain Cu and Ag as apparently essential constituents, according to Clark (1993) ("Hey's Mineral Index") and Zak et al. (1992) and the detailed study of Topa & Makovicky (2010). The latter propose the general formula CuxAgi+sPb8-2s-0.5(x+i)Bi8+sS20; they state: cosalite is "a unique case of solid-solution based on a combined omission-interstitial insertion substitution".