Banded iron formation

In short

A banded iron formation (BIF) is a sedimentary rock built of iron-oxide and chert (silica) bands, each a few millimetres to centimetres thick. Most formed around 2.5 billion years ago, when the oceans were rich in dissolved iron and the air held no free oxygen. The bands are read as a record of iron in the sea combining with oxygen and settling to the bottom, the rusting of the planet turned to stone. More than 60% of the world's iron reserves lie in these rocks.

Photos

How to recognise it

1. 1

   It shows regular stripes of metallic grey, black or red iron bands alternating with pale grey, red or yellowish chert bands.

2. 2

   It is very hard and heavy; the chert bands cannot be scratched with a steel knife.

3. 3

   Specimens rich in magnetite bands attract a magnet.

4. 4

   The bands are millimetres to centimetres thick and often keep the same pattern over great distances along the outcrop; they may be bent or folded.

Myths and misconceptions

• MythTrees and forests created the oxygen in the air.

  FactThe banded iron record shows that oxygen began to accumulate in the atmosphere around 2.4 billion years ago, long before land plants, most probably through photosynthesis by microscopic cyanobacteria. Exactly how the bands formed is still debated.

• MythBanded iron stone is pure iron.

  FactBanded iron formations mostly contain about 30% iron, and most of the rest is silica (chert). To be used as ore, the rock has to be crushed and enriched.

Did you know?

Where is it found?

• Hamersley Range and Karijini National Park, Pilbara, Western Australia · Major depositBanded iron formations about 2.5 billion years old, the thickest and most extensive in the world, over 900 m thick. Rio Tinto mines such as Mount Tom Price remove more than 100 million tonnes of iron ore a year.
• Mesabi Range, Minnesota, USA · Major depositIron ore in the Biwabik Iron Formation was found by the Merritt brothers in 1890. Today the only product is taconite pellets, made from the harder, leaner (30–35% iron) banded iron stone. The Hull–Rust–Mahoning pit at Hibbing is the world's largest open-pit iron mine.
• Isua Greenstone Belt, south-west Greenland · Known localityAbout 150 km north-east of Nuuk; at about 3.8 billion years old it is probably the oldest banded iron formation in the world. Found by a Danish company in 1962, with a resource of about 2 billion tonnes of ore (averaging 32.9% iron) estimated.
• Quadrilátero Ferrífero (Iron Quadrangle), Minas Gerais, Brazil · Major depositAn area of about 7,000 km² with itabirite, a metamorphosed banded iron formation. The region's Paleoproterozoic banded iron formations were named an IUGS Geological Heritage Site in 2022.
• Kryvyi Rih, Ukraine · Known localityThe Kryvbas iron ore basin; mining began in the 1880s with open pits, and with the first underground mine in 1886. By the end of the Soviet era it produced 42% of the USSR's iron ore.
• Kuruman, Northern Cape, South Africa · Known localityThe Kuruman Iron Formation is one of the main banded iron formations of South Africa's Transvaal Basin.
• Anshan, Liaoning, China · Known localityOne of China's major banded iron formation deposits; mining began in 1918.
• Temagami, Ontario, Canada · Known localityThe iron formation in the Temagami Greenstone Belt is 2.74–2.69 billion years old and about 60 m thick.

Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →