Acanthite, Ag
2S, crystallizes in the
monoclinic system and is the stable form of
silver sulfide below 173 °C.
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.