Banded Iron Formations (BIF) are a distinct rock type found dominantly in Precambrian rocks between 2500 and 1800 million year old. They consist of repeated thin layers of iron oxide, magnetite (Fe₃O₄, grey) or hematite (Fe₂O₃, red), alternating with bands of iron-poor chert.

During most of Archean time the Earth's surface water and atmosphere were oxygen-free so that iron was stable in a reduced (ferrous or Fe²⁺) form which is water-soluble. Vast amounts of ferrous iron entered the oceans through volcanic activity, erosion and run-off.

Eventually, as the first photosynthetic organisms generated free oxygen in the oceans, the available dissolved ferrous iron was oxidized and precipitated as less soluble red ferric iron (Fe³⁺) oxides forming the BIF’s red Fe^- rich bands. Biological cycles, probably seasonal, resulted in intervening periods when iron or oxygen may have not been as available, resulting in the interlayered chert.