argentite

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argentite (silver glance) Ag2S, ore mineral for silver; sp. gr. 7.2–7.4; hardness 2.0–2.5 (can be cut with a knife); cubic; black and opaque; slimy black streak; metallic lustre; crystals commonly cubes or octahedra, but can be massive; cleavage poor, cubic; occurs in hydrothermal veins in association with native silver and as a weathering product of primary silver sulphides; unstable below 179°C and replaced by the monoclinic form, acanthite.