sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation (predominantly Hematite, Magnetite, and Chert/Quartz)
Hardness: 5-7 (Mohs); Color: Alternating bands of silver-grey, black, and red or brown; Luster: Metallic to dull; Crystal structure: Microcrystalline; Cleavage: None/Conchoidal fracture; Specific gravity: 3.0-5.3 (depends on iron content).
- Hardness
- 5-7 (Mohs)
- Color
- Alternating bands of silver-grey, black, and red or brown
- Luster
- Metallic to dull
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Physical properties
Hardness: 5-7 (Mohs); Color: Alternating bands of silver-grey, black, and red or brown; Luster: Metallic to dull; Crystal structure: Microcrystalline; Cleavage: None/Conchoidal fracture; Specific gravity: 3.0-5.3 (depends on iron content).
Formation & geological history
Formed in Precambrian oceans between 2.4 and 1.8 billion years ago. They result from the precipitation of iron and silica from seawater due to oxygen produced by early photosynthetic cyanobacteria (the Great Oxidation Event).
Uses & applications
Primary global industrial source of iron ore; used in steel production; also used as ornamental stone (Tiger Iron) or for stratigraphic dating in geological research.
Geological facts
These rocks represent a direct record of the Earth's atmosphere changing from oxygen-poor to oxygen-rich. They are most commonly found in cratons, which are the stable, ancient cores of continents.
Field identification & locations
Identify by distinct parallel banding of dark iron oxides and lighter silica. Usually heavy for its size and may be weakly magnetic. Common in Western Australia, Minnesota (USA), and Brazil. Collectors should look for high-contrast banding.
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