Tetrahedrite – Tennantite,
Gray Copper, Fahlore

Composition. Tetrahedriti, Cu8Sb2S7 or 4Cu2S.Sb2s3 = Sulfur 23.,1, antimony 24.8, copper 52.1. Tennantite, Cu8As2S7 or 4Cu2S.As2S3 = sulfur 25.5, arsenic 17.0 copper 57.5. Antimony and arsenic are usually both present and the two species graduate into each other, so that no sharp line can be drawn between them. The copper is often replaced in varying amounts by iron zinc, silver, mercury, lead, etc

Crystallization. Isometric; tetrahedral. Habit tetrahedral.

Tetrahedron tristehedron, dodecahedron and cube the common forms.

Structure. Frequently in crystals. Also massive, coarse or fine granular.

Physical Properties. H. = 3.4. G. = 4.7-5. Metallic luster, often splendent. Color grayish black to black. Streak black.

Tests. Easily fusible at 1.5. on charcoal or in O. T. gives test for antimony or arsenic, or both, After roasting, and moistening with hydrochloric acid, gives azure-blue flame. Decomposed by nitric acid with separation of sulfur and antimony trioxide; solution made alkaline with ammonia turns blue. The two species are only to be told apart by testing for the presence of antimony and arsenic, and as both are often present in the same specimen a quantitative analysis may be necessary in order to positively determinate to which end of he series it belongs. Recognized by its tetrahedral crystals, or when massive y its fine-grained structure and by its gray color.

Occurrence. Tetrahedritec is the most common member of sulphosalt group and is widespread in occurrence and varied in association. Commonly found in copper or silver veins, usually as a primary constituent but at times it may be secondary in origin. Usually associated with chalcopyrite, pyrite, sphalerite, galena and various other silver, lead and copper ores. May carry sufficient silver to become an important ore of that metal (the highly argentiferous variety is known as freibergite). Is  found in the United States in various silver and copper mines in Colorado, Nevada, Arizona etc. Found in Cornwall, England; the; Harz Mountains, Germany; Freiber, Saxony; Pribran in Bohemia, various  places in Rumania; in the silver mines of Mexico, Peru and Bolivia.

Use. An ore silver and copper.