Almost all the iron in every car, bridge and building on Earth comes from a rock that stopped forming roughly 1.8 billion years ago and has essentially never formed since. Banded iron formations are not just an ore. They are a record of the single largest chemical change in the planet’s history.
What they look like
Alternating layers, millimetres to centimetres thick, of iron oxide minerals, mostly magnetite and hematite, interbedded with silica-rich chert. The banding runs for hundreds of kilometres with individual layers traceable across entire basins. Polished slabs are genuinely beautiful, in deep red, silver-grey and black.
They are also enormous. The Hamersley Basin in Western Australia, the Transvaal in South Africa, the Animikie in Minnesota and Michigan, the Quadrilátero Ferrífero in Brazil. These are the source of the world’s iron.
The chemistry problem they solve
For the first half of Earth’s existence the atmosphere had essentially no free oxygen. In an oxygen-free ocean, iron dissolves readily as ferrous iron, so the early oceans carried a large dissolved iron load delivered by hydrothermal vents.
Then cyanobacteria evolved oxygenic photosynthesis. The oxygen they released did not accumulate in the air, because the oceans were full of dissolved iron and iron reacts with oxygen immediately. Ferrous iron was oxidised to ferric iron, which is nearly insoluble in water, so it precipitated and sank.
The oceans rusted. That precipitate, accumulating across hundreds of millions of years, is what banded iron formations are.
Why banded
Still argued about, which is part of the appeal. The leading explanations involve cycles: seasonal or longer-period fluctuations in cyanobacterial productivity, periodic upwelling of iron-rich deep water, or oscillation between oxygen production and iron supply, with silica precipitating during the iron-poor intervals. Whatever the driver, the layers record a rhythm, which is the general case made in banding in mineral growth.
Why they stopped
Around 2.4 billion years ago free oxygen finally began accumulating in the atmosphere, the Great Oxidation Event. Once the ocean’s dissolved iron was used up and conditions turned oxidising, iron could no longer travel in solution. It oxidised where it sat instead of being transported and deposited in vast layered beds.
Major banded iron formation deposition ends around 1.8 billion years ago. A brief return in the Neoproterozoic is tied to the Snowball Earth glaciations, when a globally ice-covered ocean went anoxic again. Otherwise the process is finished. These deposits are non-renewable in the strongest possible sense.
The uncomfortable part
The Great Oxidation Event was, for most life then existing, a catastrophe. Oxygen is highly reactive and was toxic to the anaerobic organisms that dominated the planet. Cyanobacteria caused what may be the largest extinction in Earth’s history by excreting a waste product.
Every iron oxide mineral at the surface today, including the material behind hematite and goethite and our iridescent rainbow hematite from Graves Mountain, exists because of an atmosphere that biology built.
The connection to everything else
Iron oxides are also why so many rocks are red, why carnelian and jasper are the colours they are, and why the streak test is so useful in the first place. And on a planetary scale the same reaction happened without the biology on Mars, which is red for the same chemical reason and never developed the atmosphere that followed here.
