Peacock Ore (when tarnished)

Description

Chalcopyrite with Bornite refers to an assemblage of these two distinct copper-iron sulfide minerals. Chalcopyrite is a brassy yellow mineral with a metallic luster, often tarnishing to iridescent blues, purples, and reds. Bornite, when freshly broken, is a reddish-brown to copper-red, but it rapidly tarnishes upon exposure to air, developing a characteristic iridescent blue, purple, and red sheen, which gives it the common name 'Peacock Ore'. This tarnish is due to the formation of thin oxide films on the surface. The two minerals can occur intimately intergrown, as exsolution lamellae, or as distinct grains within the same ore body. Their presence together signifies a significant copper mineralization.

How to Identify

Color

Chalcopyrite: Brassy yellow, often with iridescent tarnish (blue, purple, red). Bornite: Freshly broken, reddish-brown to copper-red; rapidly tarnishes to iridescent blue, purple, and red ('Peacock Ore').

Luster

Metallic.

Texture

Typically massive, granular, or disseminated. Can also form in veins or as coatings.

Crystal Form

Chalcopyrite: Tetragonal, often forming pseudotetrahedral or disphenoidal crystals, but commonly massive. Bornite: Orthorhombic (pseudocubic), typically massive, granular, or as disseminated grains. Less commonly forms distinct crystals.

Cleavage

Chalcopyrite: Poor, indistinct {011} cleavage. Bornite: Poor, indistinct {111} cleavage.

Geological Environment

Hydrothermal veins, porphyry copper deposits, volcanogenic massive sulfide (VMS) deposits, skarn deposits, contact metamorphic deposits, and sedimentary copper deposits.

Key Facts

• Hardness: Chalcopyrite: 3.5-4 (Mohs). Bornite: 3 (Mohs).
• Specific Gravity: Chalcopyrite: 4.1-4.3. Bornite: 4.9-5.3.
• Crystal System: Chalcopyrite: Tetragonal. Bornite: Orthorhombic (pseudocubic).
• Color: Chalcopyrite: Brassy yellow, often tarnished. Bornite: Reddish-brown to copper-red, rapidly tarnishing iridescent.
• Luster: Metallic.
• Transparency: Opaque.
• Fracture: Chalcopyrite: Uneven to conchoidal. Bornite: Uneven to subconchoidal.
• Cleavage: Chalcopyrite: Poor, indistinct {011}. Bornite: Poor, indistinct {111}.
• Composition: Chalcopyrite: Copper iron sulfide (CuFeS2). Bornite: Copper iron sulfide (Cu5FeS4).

Quick Check

• Color: Chalcopyrite: Brassy yellow, often tarnished iridescent. Bornite: Reddish-brown (fresh), rapidly tarnishing to iridescent blue, purple, red.
• Luster: Metallic.
• Streak: Chalcopyrite: Greenish-black. Bornite: Grayish-black.

Physical Characteristics

• Crystal Habit: Chalcopyrite: Commonly massive, disseminated, or in pseudotetrahedral crystals. Bornite: Commonly massive, granular, disseminated, or as exsolution lamellae within chalcopyrite.
• Cleavage Type: Poor/Indistinct.
• Fracture Type: Uneven to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Metallic.

Formation

Chalcopyrite and Bornite commonly form together in a variety of ore-forming environments. They are primary copper-iron sulfide minerals. Chalcopyrite is typically the more abundant primary copper mineral in many porphyry copper deposits, volcanogenic massive sulfide (VMS) deposits, and skarn deposits. Bornite often forms in similar environments, frequently as a later-stage mineral or through exsolution from chalcopyrite at lower temperatures. They can also occur in hydrothermal veins, contact metamorphic deposits, and sedimentary copper deposits (e.g., Kupferschiefer type). The association often indicates a copper-rich, iron-sulfide-bearing environment.

Usage

Both chalcopyrite and bornite are primary ore minerals for copper, which is essential for electrical wiring, plumbing, construction, and various alloys. Bornite, due to its higher copper content (approximately 63.3% by weight) compared to chalcopyrite (approximately 34.6% by weight), is a particularly valuable copper ore. The aesthetic appeal of tarnished bornite (often sold as 'Peacock Ore') also makes it popular for ornamental purposes and mineral collecting.

Age Distribution

Found in deposits ranging from Precambrian to Cenozoic, often associated with major orogenic events and magmatic activity.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Sulfides.

Group

Chalcopyrite Group (for Chalcopyrite), Bornite Group (for Bornite).

Crystal System

Chalcopyrite: Tetragonal. Bornite: Orthorhombic (pseudocubic).

Chemical Formula

Chalcopyrite: CuFeS2. Bornite: Cu5FeS4.

Composition

Chalcopyrite: Copper (34.6%), Iron (30.4%), Sulfur (35.0%). Bornite: Copper (63.3%), Iron (11.2%), Sulfur (25.5%).

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