Bornite Mineral Overview

Mineral name: Bornite
Mineral class: Sulfide

Known Facts

• Chemical formula: Cu₅FeS₄
• Crystal system: Orthorhombic (commonly appears massive)
• Mohs hardness: 3
• Color range: Copper-red, bronze, tarnishes to purple, blue, iridescent
• Luster: Metallic
• Streak: Grayish-black
• Cleavage: Poor
• Fracture: Uneven to subconchoidal
• Specific gravity: Approximately 5.0–5.1
• Common locations: United States, Peru, Chile, Australia, Kazakhstan
• Uses: Copper ore, collector specimens
• Similar minerals: Chalcopyrite, chalcocite, covellite

Physical and Optical Characteristics

Bornite is easily recognized by its:

• Metallic luster
• Iridescent tarnish, often called “peacock ore”

Fresh surfaces are typically:

• Copper-red to bronze

However, exposure to air causes rapid tarnishing, producing:

• Purple, blue, and iridescent colors

It typically occurs as:

• Massive or granular aggregates
• Rarely as distinct crystals

Bornite is generally opaque and relatively soft, with a hardness of about 3.

Chemical Composition and Structure

Bornite has the formula:

• Cu₅FeS₄

It is a copper iron sulfide, containing:

• Copper (Cu)
• Iron (Fe)
• Sulfur (S)

It has a complex crystal structure that changes slightly with temperature, though it is commonly described as orthorhombic at lower temperatures.

Its high copper content makes it an important copper-bearing mineral.

Formation and Geological Occurrence

Bornite forms in a variety of ore-forming environments, including:

Hydrothermal deposits

• Common in:
  • Veins
  • Porphyry copper deposits

Magmatic segregation

• Found in:
  • Igneous sulfide deposits

Secondary enrichment zones

• Forms through:
  • Alteration of primary copper minerals

It is commonly associated with:

• Chalcopyrite
• Chalcocite
• Pyrite
• Quartz

These associations are typical of copper ore systems.

Common Locations

Major occurrences of Bornite include:

• United States: Arizona, Montana
• Peru and Chile: Major copper-producing regions
• Australia: Sulfide deposits
• Kazakhstan: Large ore bodies

It is widely distributed in copper mining regions worldwide.

Uses and Practical Significance

Bornite is economically important:

Copper ore

• Important source of copper
• Often processed along with:
  • Chalcopyrite and other sulfides

Collecting

• Highly valued for:
  • Iridescent tarnish (“peacock ore”)
  • Metallic appearance

Similar and Related Minerals

Bornite may be confused with:

• Chalcopyrite: Brassy yellow and less vividly iridescent
• Chalcocite: Darker and less colorful
• Covellite: Deep blue but typically softer and more platy

Distinguishing features include:

• Iridescent tarnish colors
• Copper-red fresh surface
• Softness (Mohs 3)

Identification Notes for Collectors

• Look for iridescent purple and blue tarnish on metallic surfaces
• Check for copper-red color on freshly broken surfaces
• Note association with copper sulfide minerals
• Test hardness (~3)
• Be aware that similar colors can occur on tarnished chalcopyrite

Bornite is a copper iron sulfide mineral, recognized for its distinctive iridescent tarnish and importance as a copper ore, making it both economically significant and visually striking for collectors.