MILLERITE, a mineral consisting of nickel sulphide, NiS. Crystals belong to the rhombohedral system and have the form of slender needles arranged in divergent groups or of delicate fibres loosely matted together. The colour is brass-yellow and the lustre metallic. Before the chemical composition of the mineral had been determined it had been known as “capillary pyrites” or “hair pyrites” (Ger. Haarkies), and was not distinguished from the capillary forms of pyrites and marcasite: the name millerite was given by W. Haidinger in 1845, in honour of W. H. Miller. The hardness is 3–312 and the specific gravity 5·65. There are perfect cleavages parallel to the faces of the rhombohedron (100); and gliding planes parallel to the faces of the rhombohedron (110), on which secondary twinning may be readily produced artificially by pressure.
Typical specimens of millerite are found in the coal measures in the neighbourhood of Merthyr Tydvil in South Wales, where the delicate needles and fibres occur with crystals of quartz and pearl-spar in the fissures of septarian nodules of clay-ironstone. Radiating groups of needles are found with ankerite in cavities in haematite in the Sterling mine at Antwerp in Jefferson county, New York. At the Gap mine in Lancaster county, Pennsylvania, the mineral occurs as fibrous encrusting masses with a velvety lustre. The most perfect crystals are those formerly found with calcite, diopside and a bright green chrome-garnet in a nickel mine at Orford in Sherbrooke county, Quebec. (L. J. S.)