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acanthite

n. (context mineral English) A mineral form of silver sulfide, Ag2S, that is a major ore of silver, that forms slender prisms.

Wikipedia
Acanthite

Acanthite, AgS, crystallizes in the monoclinic system and is the stable form of silver sulfide below . Argentite is the stable form above that temperature. As argentite cools below that temperature its cubic form is distorted to the monoclinic form of acanthite. Below 173 °C acanthite forms directly. Acanthite is the only stable form in normal air temperature.