Argentite
About Argentiteextended article
Argentite
Argentite is silver sulfide in its high-temperature cubic form — stable only above about 173 °C. Cooled to room temperature it inverts to monoclinic acanthite while keeping the cubic outward shape, so virtually every "argentite" crystal in a collection is actually an acanthite paramorph. Soft, sectile, and lead-grey, it has been a principal silver ore throughout mining history.