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franklinite A member of the spinel group of minerals, and an end-member of the magnetite series, with composition ZnFe3+2O4, although appreciable amounts of Mn2+ and Fe2+ are also present; sp. gr. 5.0; hardness 6; black; metallic lustre; crystals cubic but normally found as octahedra or granular aggregates; occurs at Franklin, New Jersey, in association with other zinc minerals in a metamorphosed limestone.