millerite, a nickel sulfide mineral (NiS) found in carbonate veins, as at Keokuk, Iowa, or as an alteration product of other nickel minerals, as at Andreas-Berg, Ger. Other occurrences are in meteorites and as a sublimation product on Vesuvius. Millerite forms pale brass-yellow crystals that belong to the hexagonal system. For detailed physical properties, see sulfide mineral (table).