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Bornite Mineral Facts:

Chemical Formula: Cu₅FeS₄  The Mineral is 63.3% Copper by weight. Sometimes known as Peacock ore.

Colors: Brownish-Bronze on fresh surface, quickly tarnishing to a variegated purple and blue, and eventually in time to almost black. Streak: grayish black.

Hardness: 3

Density: 5.07

Cleavage: No Cleavage. The fracture of the mineral is conchoidal.

Crystallography: Tetragonal

Luster:. Metallic.

Optics: (Refractive Index)  opaque

Composition, Structure and Associated Minerals:
Bornite is probably a basic sulpho-ferrite, though analyses yield results that vary quite widely, especially in the case of massive varieties. This variation is due to the greater or less admixture of copper sulphides, mainly chalcocite, with the bornite. The theoretical composition of the mineral is 2 5 . 5 5 S , 63 . 2 7 Cu, and 1 1 . 1 8 Fe.

Commonly in massive forms - crystals are rare. Usually in rough cubes, sometimes in penetration twins. Dodecahedron and octahedron forms at times. Crystals often form interpenetration twins. Chemically, the mineral possesses no characteristics other than those to be expected from a compound of iron, copper and sulphur. It dissolves in nitric acid with the separation of sulphur. It is easily recognized by its purplish brown color on fresh fractures and its purple tarnish. It is a good conductor of electricity.

Identification and Diagnostics
Easily fusible at 2.5. Gives odor of sulphur dioxide on charcoal or in 0. T. Yields only a very little sulphur in C. T. Becomes magnetic in R. F. If, after roasting, it is moistened with hydrochloric acid and heated, it gives an azure-blue flame (copper). Easily soluble in nitric acid with separation of sulphur; solution neutralized with ammonia gives red-brown precipitate of ferric hydroxide and blue color to filtrate. Characterized chiefly by its purple tarnish.

Occurrence, Localities and Origins:
An important and widely occurring ore of copper, but usually with other copper minerals and in subordinate amount. It has been found as a primary constituent in igneous rocks and in pegmatite veins. Bornite and chalcopyrite are the two common original copper minerals, from which other copper minerals have been derived through secondary action. It is also frequently, itself, a secondary mineral, formed in the upper, enriched zone of copper veins through the action of descending copper-bearing solutions, upon chalcopyrite. The minerals with which it is commonly associated are chalcopyrite, chalcocite, enargite, malachite, azurite, pyrite, etc. It frequently occurs in intimate mixture with chalcopyrite and chalcocite. Found in the United States at Butte, Montana; in the copper mines of Virginia and North Carolina. It was found in unusual crystals, associated with crystallized chalcocite at Bristol, Connecticut. Occurs at Acton,. Canada. Found in Cornwall; Monte Catini, Tuscany, and in various other European countries. An important ore in Chile, Peru, Bolivia and Mexico.

The crystallized mineral occurs near Redruth, Cornwall, England, and at Bristol. The massive mineral is found at many places including Norway and Sweden. It is the principal ore of some of the Bolivian, Chilian, Peruvian and Mexican mines and of the Canadian mines near Quebec. In the United States it has been mined at Bristol, Conn., and at Butte, Montana. Bornite is mined with chalcopyrite and other copper compounds as an ore of this metal.

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