Resource Dictionary

Banded iron formation

Sedimentary deposits consisting of thin layers of iron minerals alternated with beds of silica. The most important worldwide sources of mined iron. Most banded iron formations (or BIFs) formed during the Proterozoic era, from 2.5 to 1.8 billion years ago, the layering being controlled by fluctuations in atmospheric oxygen. Various classes are recognized, distinguished mainly by the mineralogical form of iron – sulfide, oxide, carbonate, or silicate.