sedimentary
Banded Iron Formation
BIF (Banded Iron Formation)
Hardness (6-7), deep red to black and metallic gray bands, dull to metallic luster, trigonal crystal system in hematite, high specific gravity (~5.0)
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Physical properties
Hardness (6-7), deep red to black and metallic gray bands, dull to metallic luster, trigonal crystal system in hematite, high specific gravity (~5.0)
Formation & geological history
Formed in Precambrian oceans between 2.4 and 1.8 billion years ago as photosynthetic cyanobacteria produced oxygen, causing dissolved iron to precipitate out of seawater as iron oxide minerals.
Uses & applications
The primary global source of iron ore for steel production; also used as a decorative stone (Tiger Iron) and for jewelry lapidary.
Geological facts
These formations provide the primary geological evidence for the Great Oxygenation Event, a period when Earth's atmosphere first became enriched with oxygen.
Field identification & locations
Identify by distinct rhythmic alternation of red (chert/jasper) and metallic (hematite/magnetite) layers; magnetic varieties will attract a neodymium magnet. Found in Ancient Shields like the Lake Superior region.
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