sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation (predominantly Hematite and Chert)
Hardness: 5.5-7 (Mohs scale); Color: Metallic grey/black bands alternating with red, brown, or jasper; Luster: Sub-metallic to dull; Crystal structure: Microcrystalline; Specific gravity: 3.0-5.3
- Hardness
- 5
- Luster
- Sub-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 jasper; Luster: Sub-metallic to dull; Crystal structure: Microcrystalline; Specific gravity: 3.0-5.3
Formation & geological history
Formed in marine environments billions of years ago (mostly Archean and Proterozoic eons) when oxygen produced by cyanobacteria reacted with dissolved iron in the oceans, causing it to precipitate on the sea floor.
Uses & applications
Primary global source of iron ore for steel production; occasionally used for jewelry (as Tiger Iron or decorative stones) and collecting.
Geological facts
These formations are responsible for the 'Great Oxygenation Event' and represent a time when Earth's atmosphere was transitioning to contain free oxygen.
Field identification & locations
Identified by distinct alternating layers of iron oxides and silica. Often magnetic if magnetite is present. Common in ancient shield areas like Western Australia, Brazil, and the Lake Superior region.
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