sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation (predominantly Hematite Fe2O3 and magnetite Fe3O4 alternating with Chert SiO2)
Hardness: 5.5-6.5 (iron oxides) and 7 (chert); Color: Alternating bands of grey/black and reddish-brown; Luster: Metallic to dull; Crystal structure: Microcrystalline; Cleavage: None, breaks along bedding planes.
- Hardness
- 5
- Color
- Alternating bands of grey/black and reddish-brown
- Luster
- Metallic to dull
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Physical properties
Hardness: 5.5-6.5 (iron oxides) and 7 (chert); Color: Alternating bands of grey/black and reddish-brown; Luster: Metallic to dull; Crystal structure: Microcrystalline; Cleavage: None, breaks along bedding planes.
Formation & geological history
Formed in Precambrian oceans, mostly between 2.4 and 1.8 billion years ago. Created by the production of oxygen by cyanobacteria, which caused dissolved iron in the seawater to oxidize and precipitate out as layers on the ocean floor.
Uses & applications
Primary source of iron ore globally for the production of steel. Occasionally used as ornamental stone (Tiger Iron) or for lapidary work.
Geological facts
Banded Iron Formations provide key evidence for the Great Oxygenation Event, recording the transition of Earth's atmosphere from oxygen-poor to oxygen-rich.
Field identification & locations
Identify by distinct, rhythmic dark (iron-rich) and light (silica-rich) layering. Often magnetic if magnetite is present. Found in ancient 'cratons' like those in Western Australia, Brazil, and the Lake Superior region.
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