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 un-twinned 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 twinning 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 Rädelerz (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 2, and the specific gravity 5.8.
Fig. 1. - Crystal of Bournonite. Fig. 2. - Twinned Crystal
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-Bánya near Nagy-Bánya in Hungary. A few other localities are known for this mineral.
(L. J. S.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)