Banded iron formations (also known as banded ironstone formations or BIFs) are a distinctive type of rock often found in primordial (Precambrian) sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shale and chert. Some of the oldest known rock formations, formed over 3,700 million years ago, include banded iron layers,[1] and the banded layers are a common feature in sediments for much of the Earth's early history. The formations are abundant around the time of the Great oxygenation event,[2], and become less common after 1.8 Ga.[citation needed] The reappearance of BIF conditions at 1,900 million years ago,[3] and in association with Snowball Earth 750 million years ago,[4] is problematic to explain.

The total amount of oxygen locked up in the banded iron beds is estimated to be perhaps twenty times the volume of oxygen present in the modern atmosphere. Banded iron beds are an important commercial source of iron ore, such as the Pilbara region of Western Australia and the Mesabi Range in Minnesota.