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pyrrhotite Mineral, FeS; sp. gr. 4.6–4.7; hardness 3.5–4.5; hexagonal; bronzeyellow but darkens rapidly to reddishbrown on exposure; greyish-black streak; metallic lustre; crystals rare, and can be platy or tabular, but the mineral is usually massive or granular; no cleavage; magnetic; occurs in igneous rocks, e.g. gabbro or norite, as disseminated grains, and also in metamorphic contact zones in association with chalcopyrite and pyrite. Pyrrhotite was used formerly for the production of sulphuric acid.