sedimentary

Banded Iron Formation (BIF)

Banded Iron Formation

AI-generated identification · may be incorrect

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

Identified June 10, 2026More sedimentary →

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Physical properties

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.