Rock Identifier
Banded Iron Formation (BIF) (Banded Iron Formation (composed primarily of Hematite [Fe2O3], Magnetite [Fe3O4], and Chert [SiO2])) — sedimentary
sedimentary

Banded Iron Formation (BIF)

Banded Iron Formation (composed primarily of Hematite [Fe2O3], Magnetite [Fe3O4], and Chert [SiO2])

Hardness: 5.5-7 (Mohs scale), Color: Alternating red (Jasper/Chert) and silver-black (Hematite/Magnetite) bands, Luster: Metallic to dull, Crystal structure: Trigonal/Hexagonal systems in constituent minerals, Specific gravity: 3.5-5.0 depending on iron content.

Hardness
5
Identified More sedimentary →
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Physical properties

Hardness: 5.5-7 (Mohs scale), Color: Alternating red (Jasper/Chert) and silver-black (Hematite/Magnetite) bands, Luster: Metallic to dull, Crystal structure: Trigonal/Hexagonal systems in constituent minerals, Specific gravity: 3.5-5.0 depending on iron content.

Formation & geological history

Formed in marine environments during the Precambrian Eon (mostly 2.4 to 1.8 billion years ago). These deposits resulted from the 'Great Oxygenation Event' where oxygen produced by early cyanobacteria reacted with dissolved iron in the oceans, precipitating out as iron oxides on the sea floor.

Uses & applications

Primary global source of iron ore for steel production. High-quality specimens with vivid red jasper are used as semi-precious gemstones or 'Genesis Stone' in jewelry and lapidary art.

Geological facts

BIFs are geological 'clocks' that record the moment Earth's atmosphere became oxygen-rich. They are no longer forming in modern oceans because the water is now too oxygenated to allow iron to dissolve in large quantities.

Field identification & locations

Identified by distinct alternating color bands (red and dark grey/black) and its unusually high weight (density) for its size. Often magnetic if magnetite is present. Common in ancient shield areas like Western Australia, Minnesota (USA), and South Africa.

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