sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation
Hardness: 5-6 (Mohs); Color: Alternating layers of gray/black (magnetite/hematite) and red/brown (chert/jasper); Luster: Dull to metallic; Crystal structure: Microcrystalline; Cleavage: None/Conchoidal.
- Hardness
- 5-6 (Mohs)
- Luster
- Dull to metallic
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Physical properties
Hardness: 5-6 (Mohs); Color: Alternating layers of gray/black (magnetite/hematite) and red/brown (chert/jasper); Luster: Dull to metallic; Crystal structure: Microcrystalline; Cleavage: None/Conchoidal.
Formation & geological history
Formed in marine environments billions of years ago (mostly 2.4 to 1.8 billion years ago) during the Great Oxidation Event when dissolved iron in sea water combined with oxygen produced by cyanobacteria, precipitating as mineral layers.
Uses & applications
Primary source of iron ore globally for the steel industry; polished specimens are used in jewelry (Tiger's Eye is a variant) and for mineral collecting.
Geological facts
These formations provide the best evidence for when Earth's atmosphere first became oxygenated. They are almost exclusively found in Precambrian rocks.
Field identification & locations
Identified by distinct rhythmic layering of iron-rich and silica-rich bands. Often magnetic if magnetite is present. Found in ancient shield areas like Western Australia, Brazil, and the Lake Superior region.
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