Banded iron formation
A chemical sedimentary rock made of alternating iron-rich and silica-rich layers, deposited mainly in Precambrian marine basins.
Dissolved iron: Iron present as ions or complexes in water rather than as solid minerals. Ancient seawater could carry abundant dissolved iron before oxygen transformed it into insoluble minerals.
Precambrian: The span of geologic time before the Cambrian Period, from Earth's formation to about 539 million years ago. Most major deposits accumulated during this vast interval of early Earth history.
Iron ore: Rock or mineral material mined for its iron content and processed to produce iron. Many banded iron formations are the world's largest sources of iron ore.
Superior-type banded iron formation: A large banded iron formation deposited in a broad marine basin, commonly far from volcanic centers. It contrasts with the smaller, volcanically associated Algoma-type deposits.
Oxygenation of Earth's atmosphere: The long-term rise of molecular oxygen in Earth's atmosphere, beginning during the Archean and Proterozoic eons. Oxygen produced by early life helped convert dissolved iron into iron oxides that settled on the seafloor.
Archean: The geologic eon from about 4.0 to 2.5 billion years ago. Some of the oldest known banded iron formations formed in Archean marine environments.
Taconite: A low-grade iron ore, commonly a fine-grained banded iron formation, processed to concentrate iron minerals. Taconite processing makes otherwise uneconomic iron-rich formations usable.
Ironstone: A sedimentary rock containing substantial iron minerals, commonly formed in younger marine or nonmarine settings. Ironstone is iron-rich but differs in age, composition, and depositional context from classic banded iron formations.
Great Oxidation Event: A major rise in atmospheric oxygen around 2.4 billion years ago. Its timing overlaps the decline of many major banded iron formation deposits.
Paleoproterozoic: The early part of the Proterozoic Eon, from about 2.5 to 1.6 billion years ago. Many extensive deposits formed during this interval, including around the time atmospheric oxygen rose.