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millerite Mineral, NiS; sp. gr. 5.2–5.6; hardness 3.0–3.5; trigonal; brass-yellow and opaque; greenish-black streak; metallic lustre; crystals usually long, slender, acicular, and in radiating groups; cleavage perfect, rhombohedral; occurs as tufts of radiating fibres in cavities, and as replacement for other nickel minerals, also in veins carrying nickel minerals and other sulphides, and around some volcanoes as sublimation products. It is a minor ore of nickel and named after the British mineralogist W. H. Miller.