sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation (principally Hematite/Magnetite and Chert)
Hardness: 5-7 (Mohs), Color: Rusty brown, dark grey, to red, Luster: Metallic to dull/earthy, Crystal Structure: Microcrystalline, Cleavage: None/Conchoidal, Specific Gravity: 3.0-5.3 (relatively heavy)
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Physical properties
Hardness: 5-7 (Mohs), Color: Rusty brown, dark grey, to red, Luster: Metallic to dull/earthy, Crystal Structure: Microcrystalline, Cleavage: None/Conchoidal, Specific Gravity: 3.0-5.3 (relatively heavy)
Formation & geological history
Formed in Precambrian seawater (mostly 1.8 to 2.5 billion years ago) through the precipitation of iron and silica as the Earth's atmosphere and oceans became oxygenated by early photosynthetic bacteria.
Uses & applications
Major source of iron ore for steel production; occasionally used for lapidary work, jewelry (Tiger Iron), or educational specimens.
Geological facts
Banded Iron Formations represent the 'Great Oxidation Event.' They are evidence of when algae first began producing oxygen, which reacted with dissolved iron to create these striped layers on the ocean floor.
Field identification & locations
Identify by alternating layers of metallic iron minerals and silica; often magnetic and noticeably heavy for its size. Commonly found in ancient cratons like those in Minnesota (Mesabi Range), Australia, and Brazil.
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