sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation (Fe2O3 + SiO2)
Hardness: 5.5-7 (Mohs scale); Color: Metallic grey/black bands alternating with red, brown, or yellow; Luster: Metallic to dull; Structure: Fine-grained, laminated; Cleavage: None
- Hardness
- 5
- Luster
- Metallic to dull
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Physical properties
Hardness: 5.5-7 (Mohs scale); Color: Metallic grey/black bands alternating with red, brown, or yellow; Luster: Metallic to dull; Structure: Fine-grained, laminated; Cleavage: None
Formation & geological history
Formed in Precambrian oceans (largely 2.4 to 1.8 billion years ago) when oxygen produced by cyanobacteria reacted with dissolved iron, precipitating it onto the ocean floor.
Uses & applications
Primary global source of iron ore used in steel production; also used as decorative garden stone or lapidary material (Tiger Iron).
Geological facts
BIFs are the primary evidence for the 'Great Oxidation Event'; they represent a time when the Earth's atmosphere was transitioning to an oxygen-rich environment.
Field identification & locations
Identify by distinct alternating dark (hematite/magnetite) and light/red (chert/jasper) layers; often magnetic if magnetite is present. Found in ancient cratons like the Mesabi Range (USA) or Pilbara (Australia).
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