bornite

n.
A brownish-bronze, lustrous copper ore with the composition Cu₅FeS₄ that tarnishes to purple when exposed to air.

[After Ignaz von Born (1742-1791), Austrian mineralogist.]

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A sulfide of composition Cu₅FeS₄, specific gravity 5.07, and hardness 3 (Mohs scale), commonly occurring as a primary mineral in many copper ore deposits. Crystals are rare; bornite is usually massive or granular. The metallic and brassy color of a fresh surface rapidly tarnishes upon exposure to air to a characteristic iridescent purple, giving rise to the name “peacock ore.” Though of lesser importance as an ore than chalcocite or chalcopyrite, masses of bornite have been mined in Chile, Peru, Bolivia, and Mexico and in the United States in Arizona and Montana. See also Chalcocite; Chalcopyrite; Copper.

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Cu
Cubic -- hexoctahedral

Environment

Commonly disseminated in igneous intrusives as a primary mineral. Also in copper ore veins, both as a primary and secondary mineral. See discussion under chalcocite.

Crystal description

Crystals rare and poor, usually in intergrown clusters and seldom over 1 in. (2-3 cm). As a rule, bornite is massive and compact.

Physical properties

Bronze. Luster metallic (but this tarnishes rapidly to purple after a freshly broken surface is exposed); hardness 3; specific gravity 4.9-5.4; fracture uneven; cleavage poor octahedral. Brittle.

Composition

Sulfide of copper and iron (63.3% Cu, 11.1% Fe, 25.6% S).

Tests

Fuses to a brittle magnetic globule on charcoal. Dissolves in nitric acid, giving a copper coloration to the solution.

Distinguishing characteristics

Could be confused with pyrrhotite (which is weakly magnetic), but it gives the copper tests, and requires roasting to become magnetic. Niccolite is similar in color, but is nonmagnetic and remains so despite roasting. The rapid development of a purple tarnish is characteristic. Bornite occurs with the other copper sulfides and as such is an important ore of copper. It has been found in fair crystals at Cornwall, England, at Charrier, near La Prugne, France, and at Bristol, Connecticut. However, at the economically important localities in Arizona it is generally massive and/or disseminated, and intimately intergrown with chalcopyrite and chalcocite.

Remarks

A characteristic and colorful tarnish has given rise to the old miner's term "peacock ore" and is responsible for its French name, erubescite. Most dealers' "peacock ore" actually is not bornite but chalcopyrite treated to produce a colorful iridescence simulating true "peacock ore."

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