sedimentary
Banded Iron Formation
Banded Iron Formation (BIF)
Hardness: 5.5-6.5 on Mohs scale; Color: Gray, black, and reddish-brown bands; Luster: Sub-metallic to dull; Crystal structure: Microcrystalline; Specific gravity: 3.0-5.0 depending on iron content.
- Hardness
- 5
- Color
- Gray, black, and reddish-brown bands
- Luster
- Sub-metallic to dull
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Physical properties
Hardness: 5.5-6.5 on Mohs scale; Color: Gray, black, and reddish-brown bands; Luster: Sub-metallic to dull; Crystal structure: Microcrystalline; Specific gravity: 3.0-5.0 depending on iron content.
Formation & geological history
Formed in Precambrian oceans (largely between 2.4 and 1.8 billion years ago) when oxygen produced by cyanobacteria reacted with dissolved iron in seawater, causing it to precipitate as iron oxides on the seafloor.
Uses & applications
The primary global source of iron ore for steel production; also used as decorative stone and for geological study.
Geological facts
BIFs are the primary evidence for the 'Great Oxidation Event'. They are mostly found in very old Precambrian shields like those in Australia, Brazil, and the Lake Superior region.
Field identification & locations
Identify by characteristic alternating dark (magnetite/hematite) and light (chert/jasper) layers. Often magnetic or very heavy for its size. Common in the Mesabi Range of Minnesota and the Pilbara region of Western Australia.
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