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(named after the British crystallographer W. Miller [1801–80]), a mineral of the sulfide class; nickel sulfide, NiS, containing 64.7 percent Ni and 35.3 percent S. Millerite crystallizes in the trigonal system, forming characteristic slender brass-yellow hairlike crystals. It also forms fibrous, radiating, and other kinds of aggregates. Millerite has a hardness of 3–4 on Mohs’ scale and a density of 5,200–5,600 kg/m3. It occurs rarely in nature, usually in hydrothermal ore veins in association with other Ni and Co sulfides and arsenides, which are contained in copper-nickel ores (in Noril’sk and Monchegorsk in the USSR). Millerite is also formed during the sur-face weathering of nickel-bearing ultrabasic rocks by acidic surface waters saturated with H2S.