Bournonite Composition, Crystallization and Structure

Composition. Sulphantimonite of lead and copper (Pb,Cu2)3Sb2S6 or 3 (Pb, Cu2)S.Sb2S3. The relative amounts of the lead and copper present vary, but in general correspond closely to the ratio Pb : Cu2 = 2 : 1.

Crystallization. Orthorhombic. Crystals usually short prismatic to tabular. Sometimes quite complex with many prism, pyramid and dome faces. Frequently twinned, giving tabular crystals with recurring reentrant angles in the prism zone (Fig. 239), where the common name of cogweel ore.

Structure. Massive; granular to compact; in crystals.

Physical Properties. H. = 2.-3. G. = 5.7-5.9. Metallic luster. Color and streak steel-gray to black.

Tests. Fusible at 1. B. B. on charcoal gives combination coating of antimony and lead lead oxides. Roasted in O. T. gives sublimate of antimony oxides. Heated con charcoal with a mixture of potassium iodide and sulfur a gives chime-yellow coating of lead iodile. Decomposed with nitric acid, solution turns blue with excess of ammonia (copper). Recognized either by characteristic crystals or abode tests.

Occurrence. A rare mineral, found in veins with galena, tetrahedrite, stibniter, chalcopyrite, shapelerite, etc. Found at Neudorf and other localities in the Harz Mountains; Kapnikbay in Rumania;

Liskeard in Cornwall; from Bolivia; etc. Has been found, also, in various places in the United States, but not in notable amount or quality.