sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation (Magnetite/Hematite-Chert)
Hardness: 6-7 (Mohs scale), Color: Alternating bands of silver-grey (magnetite/hematite) and red, brown, or yellow (chert/jasper), Luster: Metallic to dull, Crystal structure: Microcrystalline, Specific gravity: 3.0-5.3 depending on iron content
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Alternating bands of silver-grey (magnetite/hematite) and red, brown, or yellow (chert/jasper), Luster: Metallic to dull, Crystal structure: Microcrystalline, Specific gravity: 3.0-5.3 depending on iron content
Formation & geological history
Formed in Precambrian oceans (2.4 to 1.8 billion years ago) during the Great Oxygenation Event. Cyanobacteria produced oxygen which reacted with dissolved iron, causing it to precipitate as iron oxide layers alternating with silica layers.
Uses & applications
Primary global source of iron ore for steel production; also used as a decorative stone for lapidary work and educational geological specimens.
Geological facts
These rocks provide the primary evidence for the atmospheric evolution of Earth; they represent a time when the world's oceans turned from green to blue as iron was successfully removed from the water.
Field identification & locations
Can be identified by distinctive stripes and high density. Many specimens are magnetic. Commonly found in ancient cratons like the Lake Superior region (USA), Western Australia, and Brazil.
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