A typical BIF consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shales and cherts, often red in color, of similar thickness, and containing microbands (sub-millimeter) of iron oxides.
|Banded iron formation, Karijini National Park, Western Australia|
Some of the oldest known rock formations, formed over 3,700 million years ago, include banded iron layers. Banded layers rich in iron were mostly deposited between 2,400 and 1,800 mya. Phanerozoic ironstones generally have a different genesis.
Banded iron beds are an important commercial source of iron ore, such as the Pilbara region of Western Australia and the Animikie Group in Minnesota.