Banded iron formations

Banded iron formation

Sedimentary rock
Banded iron formation, Karijini National Park, Western Australia
Composition
Primary	iron oxides, shales and cherts
Secondary	Other

Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock that are almost always of Precambrian age.

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.

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 1,900 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.

The formations are abundant around the time of the great oxygenation event, 2,400 million years ago (mya or Ma), and become less common after 1,800 mya. Conditions of a reappearance of a sea with dissolved iron at 1,900 million years ago, and later in association with Snowball Earth BIF reappeared 750 million years ago, and that is problematic to explain (see below).