BOURNONITE  , a

mineral species, a sulphantimonite of lead and copper with the formula PbCuSbS3 . 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 name given by Bournon himself (in 1813) was endellione, since used in the form endellionite, after the locality in Cornwall where the mineral was first found . The crystals are orthorhombic, and are generally tabular in habit owing to the predominance of the basal pinacoid (c); numerous smooth bright faces are often developed on the edges and corners of the crystals . An untwinned crystal is represented in fig . 1 . Usually, however, the crystals are twinned, the twin-plane being a face of the prism (m) ; the angle between the faces of this prism being nearly a right angle (86° 20'), the twinnihg gives rise to cruciform groups (fig . 2), and when it is often repeated the group has the appearance of a cog-wheel, hence the name Radelerz (wheel-ore) of the Kapnik miners . The repeated twinning gives rise to twin-lamellae, which may be detected on the fractured surfaces, even of the massive material . The mineral is opaque, and has a brilliant metallic lustre with a lead-grey colour . The hardness is 22, and the specific gravity 5.8 . At the original locality, Wheal Boys in the parish of Endellion in Cornwall, it was found associated with jamesonite, blende and chalybite .

Later, still better crystals were found in another Cornish mine, namely, Herodsfoot mine near

Liskeard, which was worked for argentiferous galena . Fine crystals of large size have been found with quartz and chalybite in the mines at Neudorf in the Harz, and with blende and tetrahedrite at Kapnik-Banya near Nagy-Banya in Hungary . A few other localities are known for this mineral . (L . J .