The Collaborative International Dictionary
Polybasite
Polybasite \Pol`y*ba"site\, n. [See Polybasic.] (Min.) An iron-black ore of silver, consisting of silver, sulphur, and antimony, with some copper and arsenic.
Wiktionary
polybasite
n. (context mineralogy English) An iron-black ore of silver, consisting of silver, sulfur, and antimony, with some copper and arsenic.
Wikipedia
Polybasite
Polybasite is a sulfosalt mineral of silver, copper, antimony and arsenic. Its chemical formula is [(Ag,Cu)(Sb,As)S][AgCuS].
It forms black monoclinic crystals (thin, tabular, with six corners) which can show dark red internal reflections. It has a Mohs hardness of 2.5 to 3. It is found worldwide and is an ore of silver. The name comes from the number of base metals in the mineral.