Freibergite

Freibergite is a complex sulfosalt mineral of silver, copper, iron, antimony and arsenic with formula (Ag,Cu,Fe)12(Sb,As)4S13. It has cubic crystals and is formed in hydrothermal deposits. It forms one solid solution series with tetrahedrite and another with argentotennantite. Freibergite is an opaque, metallic steel grey to black and leaves a red...
Found on http://en.wikipedia.org/wiki/Freibergite

Freibergite

Freibergite is a variety of terahedrite containing between 28 and 36 percent silver.
Found on http://www.probertencyclopaedia.com/browse/HF.HTM
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