Banded iron formation (BIF) is a chemical sedimentary rock containing >15 wt% iron (Fe). Most commonly, Fe-rich layers alternate with chert, although they may also be interbedded with layers made up of carbonate or silicate minerals, or sulphidic shale. As the major source of iron ore, BIF is of crucial economic importance. Sedimentologic and geochemical investigations of BIF continue to yield significant insights into the chemistry of the Precambrian atmosphere-hydrosphere system and the evolution of life.