Acanthite
Chemical compound
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Acanthite is a form of silver sulfide with the chemical formula Ag₂S. It crystallizes in the monoclinic system and is the stable form of silver sulfide below 173 °C. Argentite is the stable form above that temperature. Wikipedia
IMA symbol: Aca
Color: Iron-black
Crystal system: Monoclinic
Cleavage: Indistinct
Crystal class: Prismatic (2/m); (same H-M symbol)
Fracture: Uneven
Luster: Metallic