CoachA banded iron formation is a sedimentary rock of thin alternating layers of iron oxides, such as hematite, and of chert, a fine-grained silica poor in iron. Almost all are older than 1.8 billion years.
Earth and Space Sciences · Glossary
What is Banded iron formation?
A banded iron formation (BIF) is a chemical sediment of thin alternating layers of iron oxides — hematite, magnetite — and of chert. It precipitates from sea water that carries dissolved iron(II), when that iron is oxidised to insoluble iron(III); it therefore needs a deep ocean without oxygen, a supply of iron, and an oxidant where the iron is deposited.