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Bornite is a copper iron sulfide mineral with the chemical formula Cu5FeS4. It is renowned for its striking iridescent tarnish, which develops upon exposure to air, displaying shades of blue, purple, red, and green, resembling a peacock's feathers, hence the common name 'Peacock Ore'. When freshly broken, bornite has a reddish-brown to copper-red color, but this quickly oxidizes to the characteristic iridescent hues. It is an important ore of copper and is often found in association with other copper sulfides.
How to Identify
- Color
- Freshly broken surfaces are reddish-brown to copper-red, rapidly tarnishing to iridescent shades of blue, purple, red, and green.
- Luster
- Metallic.
- Texture
- Typically massive, granular, or disseminated. Less commonly found as distinct crystals.
- Crystal Form
- Crystals are rare, usually pseudocubic dodecahedra or octahedra, but most commonly found as massive aggregates or disseminated grains.
- Cleavage
- Poor or indistinct cleavage on {111}.
- Geological Environment
- Hydrothermal veins, porphyry copper deposits, skarn deposits, massive sulfide deposits, and supergene enrichment zones.
Key Facts
- Hardness: 3.0 on the Mohs scale.
- Specific Gravity: 4.9-5.3 g/cm³.
- Crystal System: Tetragonal (pseudocubic) at room temperature, but cubic above 228°C.
- Color: Reddish-brown to copper-red, tarnishing to iridescent blue, purple, red, green.
- Luster: Metallic.
- Transparency: Opaque.
- Fracture: Conchoidal to uneven.
- Cleavage: Poor or 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: Massive, granular, disseminated; rarely as pseudocubic dodecahedral or octahedral crystals.
- Cleavage Type: Poor/indistinct on {111}.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Metallic.
Formation
Bornite typically forms in hypogene (primary) copper deposits, often associated with chalcopyrite, pyrite, and other sulfide minerals. It can also occur in supergene (secondary) enrichment zones where it forms from the alteration of primary copper sulfides. Hydrothermal processes are the primary mechanism for its formation, where hot, mineral-rich fluids circulate through crustal rocks, depositing sulfide minerals as they cool and react with the host rock. It is commonly found in porphyry copper deposits, skarn deposits, and massive sulfide deposits.
Usage
Bornite is an important ore mineral for copper, though less significant than chalcopyrite. Its distinctive iridescent tarnish makes it a popular mineral specimen for collectors. It has no significant industrial uses beyond its role as a copper ore.
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, including bornite.
Mexico
Various copper deposits throughout the country yield bornite.
Chile
A leading copper producer, with numerous deposits containing bornite.
Peru
Significant copper mining operations often encounter bornite.
Zambia
Part of the 'Copperbelt' region, known for its rich copper sulfide deposits.
Finding Tips
Look for Copper Deposits
Bornite is almost exclusively found in copper ore deposits. Areas with historical or active copper mining are prime locations.
Examine Sulfide Veins
Search for bornite within hydrothermal veins or disseminated in altered host rocks, often alongside other sulfide minerals like chalcopyrite and pyrite.
Observe Tarnish
The distinctive iridescent tarnish is the most reliable visual identifier. Freshly broken surfaces will be copper-red, but exposure to air quickly brings out the 'peacock' colors.
Check for Associated Minerals
Bornite is commonly found with chalcopyrite, pyrite, chalcocite, covellite, and native copper.
Safety Precautions
When collecting in old mine workings or dumps, be aware of unstable ground, hazardous materials, and potential exposure to heavy metals. Always wear appropriate personal protective equipment (PPE), including gloves and eye protection. Wash hands thoroughly after handling mineral specimens.
Similar Rocks
Chalcopyrite
Chalcopyrite
Also known as: Copper Pyrites, Yellow Copper Ore
Covellite
Covellite
Also known as: Indigo Copper
Chalcocite
Chalcocite
Also known as: Copper Glance
Scientific Classification
- Mineral Class
- Sulfides
- Group
- Sulfide minerals
- Crystal System
- Tetragonal (pseudocubic)
- Chemical Formula
- Cu5FeS4
- Composition
- Copper (Cu) 63.3%, Iron (Fe) 11.2%, Sulfur (S) 25.5% by weight.
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