Argentite - Wikipedia
In mineralogy, argentite (from the Latin argentum, silver) is cubic silver sulfide (Ag 2 S), which can only exist at temperatures above 173 °C, 177 °C or 179 °C. When it cools to ordinary temperatures it turns into its monoclinic polymorph, acanthite. Argentite belongs to the galena group.. The name "argentite" sometimes also refers to pseudomorphs of argentite: specimens of acanthite which ...