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Bornite is a copper iron sulfide mineral with the chemical formula Cu5FeS4. In its fresh state, bornite has a reddish-brown to copper-red color and a metallic luster. However, it is highly susceptible to tarnishing upon exposure to air, quickly developing an iridescent, peacock-like sheen with shades of blue, purple, red, and green. This tarnish is a thin, superficial oxidation layer. It crystallizes in the orthorhombic system, though it often appears massive or granular. It is a relatively soft mineral, making it easy to scratch.
How to Identify
- Color
- Freshly broken surfaces are reddish-brown to copper-red. Upon exposure to air, it rapidly tarnishes to iridescent shades of blue, purple, red, and green, resembling a peacock's feathers.
- Luster
- Metallic.
- Texture
- Typically massive, granular, or disseminated. Less commonly found as distinct crystals.
- Crystal Form
- Crystals are rare, usually pseudocubic or dodecahedral, but most commonly found as massive, granular, or compact aggregates.
- Cleavage
- Poor to indistinct on {111}.
- Geological Environment
- Hydrothermal veins, contact metamorphic deposits, disseminated in mafic igneous rocks, and in sedimentary copper deposits. It is often found in association with other copper sulfides like chalcopyrite, chalcocite, and covellite.
Key Facts
- Hardness: 3 on the Mohs scale.
- Specific Gravity: 4.9-5.3.
- Crystal System: Orthorhombic (pseudocubic).
- Color: Reddish-brown to copper-red (fresh); iridescent blue, purple, red, green (tarnished).
- Luster: Metallic.
- Transparency: Opaque.
- Fracture: Conchoidal to uneven.
- Cleavage: Poor to indistinct on {111}.
- Composition: Copper iron sulfide (Cu5FeS4).
Quick Check
- Color: Reddish-brown to copper-red when fresh, rapidly tarnishing to iridescent blue, purple, red, and green.
- Luster: Metallic.
- Streak: Grayish-black.
Physical Characteristics
- Crystal Habit: Typically massive, granular, or compact. Less commonly as pseudocubic or dodecahedral crystals.
- Cleavage Type: Poor to indistinct.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Metallic.
Formation
Bornite typically forms in hydrothermal veins, contact metamorphic deposits, and disseminated in igneous and metamorphic rocks. It is a primary copper sulfide mineral, often associated with chalcopyrite, pyrite, and other sulfide minerals. The characteristic iridescent tarnish, which gives it the 'Peacock Ore' moniker, is a secondary alteration product, forming when bornite is exposed to weathering and oxidation. This tarnish is a thin film of various copper oxides and sulfides that diffract light, creating the vibrant colors.
Usage
Bornite is an important ore of copper, though less significant than chalcopyrite. Its attractive iridescent tarnish makes it a popular mineral specimen for collectors and for use in decorative items, jewelry (though it is relatively soft), and metaphysical practices. It is not typically used as a building material or in industrial applications beyond copper extraction.
Age Distribution
Bornite forms in various geological settings and is not restricted to a specific geological age. It can be found in deposits ranging from Precambrian to Cenozoic.
Where to Find
Butte, Montana, USA
Historically a major copper mining district, known for significant bornite occurrences.
Cornwall, England
Famous for its copper and tin deposits, bornite is found in many of the old mines.
Mexico
Various copper deposits throughout Mexico yield bornite specimens.
Chile
A world leader in copper production, with numerous deposits containing bornite.
Zambia
Part of the 'Copperbelt' region, known for its extensive copper mineralization including bornite.
Finding Tips
Look for Copper Deposits
Bornite is a copper ore, so search in areas known for copper mineralization, particularly in sulfide-rich environments.
Identify Associated Minerals
Bornite often occurs with chalcopyrite, pyrite, chalcocite, and quartz. The presence of these minerals can indicate a bornite-rich zone.
Observe Tarnish
The most distinctive feature is the rapid and vibrant iridescent tarnish. Freshly broken surfaces will be copper-red, but within minutes to hours, the 'peacock' colors will begin to appear.
Check Hardness
Bornite is relatively soft (Mohs 3), so it can be scratched with a knife or even a copper coin. This helps differentiate it from harder sulfides like pyrite.
Consider Specific Gravity
It is a relatively heavy mineral (specific gravity 4.9-5.3), which can be a useful field indicator.
Similar Rocks
Chalcopyrite
Chalcopyrite (CuFeS2)
Also known as: Copper Pyrites, Yellow Copper Ore
Covellite
Covellite (CuS)
Also known as: Indigo Copper
Pyrite
Pyrite (FeS2)
Also known as: Fool's Gold
Scientific Classification
- Mineral Class
- Sulfides
- Group
- Bornite Group
- Crystal System
- Orthorhombic (pseudocubic)
- Chemical Formula
- Cu5FeS4
- Composition
- Copper (Cu) 63.33%, Iron (Fe) 11.16%, Sulfur (S) 25.51% (by weight)
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