An orthorhombic mineral, Cu₃AsS₄; dimorphous with luzonite, metallic gray-black; in vein and replacement copper deposits as small crystals or granular masses; an important ore of copper and arsenic; may contain up to 7% antimony; localities include Butte, MT; Chuquicamata, Chile; Cerro de Pasco, Colquijirca, Peru; Tsumeb, Namibia; and Bor, Serbia.