BOURNONITE, a mineral species, a sulphantimonite of lead and copper with the formula It is of some interest on account of the twinning and the beautiful development of its crystals. It was first mentioned by Philip Rashleigh in 1797 as "an ore of antimony," and was more completely described by the comte de Bournon in 1804, after whom it was named. The crys tals are orthorhombic, and are generally tabular in habit owing to the predominance of the basal pinacoid; numerous smooth bright faces are often developed on the edges and corners of the crystals. Usually the crystals are twinned, the twin-plane being a face of the prism ; the angle between the faces of this prism being nearly a right angle, the twinning gives rise to cruciform groups, and when it is often repeated the group has the appear ance of a cog-wheel, hence the name Rddelerz (wheel-ore). The mineral is opaque, and has a brilliant metallic lustre with a lead-grey colour. The hardness is 21, and the specific gravity 5.8.
(L. J. S.)