<Tr> <Th> Secondary </Th> <Td> Other </Td> </Tr> <P> Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock that are almost always of Precambrian age . </P> <P> A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe O) or hematite (Fe O), alternating with bands of iron - poor shales and cherts, often red in color, of similar thickness, and containing microbands (sub-millimeter) of iron oxides . </P> <P> Some of the oldest known rock formations, formed over 3,700 million years ago, include banded iron layers . 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 . </P>

Banded iron formations consist of alternating thin layers of chert and iron minerals