Iron (Fe)

Almost none of the iron in the universe was made as iron. It is made as nickel-56, in the last seconds of a massive star or in the runaway detonation of a white dwarf, because at the temperatures involved a nucleus with equal numbers of protons and neutrons is what forms most readily. Nickel-56 then decays to cobalt-56 with a half-life of about six days, and cobalt-56 decays to iron-56 over roughly seventy-seven days.

That second step is why a type Ia supernova stays bright. The explosion itself dims within days; the glow that astronomers track for months afterwards is powered by gamma rays from cobalt turning into iron, deposited in the expanding debris. The characteristic shape of the fading light curve — a decline that settles onto a straight line in magnitudes, at a rate matching cobalt-56's half-life — was the evidence that this was happening, and it is the reason type Ia supernovae are standard candles reliable enough to have been used to detect the accelerating expansion of the universe.

The end of the line for fusion

Fusing light nuclei releases energy. Fusing heavy ones costs it. The crossover sits at the iron group, where binding energy per nucleon reaches its maximum, and that single fact structures the death of every large star.

A star heavy enough to burn silicon builds an iron core it cannot do anything with. There is no further exothermic reaction available; the core is held up only by electron degeneracy pressure, and once it exceeds roughly 1.4 solar masses even that fails. What follows takes under a second. Gamma rays tear the iron nuclei back apart into helium in a process called photodisintegration, which absorbs energy rather than releasing it and accelerates the collapse; electrons and protons combine into neutrons, releasing a flood of neutrinos; and the core falls inward at a substantial fraction of the speed of light before rebounding off nuclear-density material.

Iron is therefore not merely common. It is where stellar nucleosynthesis runs out of profit, and the cause of the explosions that distribute everything heavier.

Iron of the sky

The Iron Age is conventionally dated from the point at which smelting became routine, but people were working iron for well over a thousand years before that, from a source that required no metallurgy at all.

Iron meteorites are already metal. They are also alloyed with nickel at levels no terrestrial ore produces, typically between five and twenty per cent, which makes them chemically identifiable. The Egyptian term for the material, bia-n-pet, translates roughly as iron of the sky. In 2016 a team using portable X-ray fluorescence examined the dagger buried with Tutankhamun and found nickel around eleven per cent along with cobalt in the right proportion: the blade is meteoritic, made several centuries before Egypt had any iron industry.

The largest documented case is the Cape York meteorite in Greenland, from which Inuit communities cold-worked harpoon tips and knife blades for centuries, hammering flakes from masses of iron weighing tens of tonnes.

Why the symbol and the name disagree

English took iron from Germanic roots; the symbol Fe comes from Latin ferrum, and so do the words used in French, Spanish, Italian and Portuguese. The split runs right through the vocabulary: an English speaker says iron but also ferrous, ferric, ferrite and ferromagnetic, switching to the Latin stem the moment the context becomes technical. There is no chemical significance to the divide at all, only the accident of which language wrote the textbooks and which wrote the everyday speech.

The ore is a fossil of an atmospheric change

The great iron ore bodies — the Hamersley Basin in Western Australia, the Transvaal, the Lake Superior ranges, Carajás in Brazil — are almost all banded iron formations, and almost all of them were laid down in a geologically narrow window between roughly 2.6 and 1.8 billion years ago.

They are the sediment of a chemical transition that happened once and cannot happen again. Early oceans held enormous quantities of iron in solution in its reduced ferrous form, which is soluble. Oxygen from the first photosynthesisers converted it to the ferric form, which is not, and it precipitated out across the shallow seas in alternating iron-rich and silica-rich layers. When the oceans finally ran out of dissolved iron, the process stopped permanently.

Every blast furnace on Earth is therefore consuming a limited deposit that formed under atmospheric conditions no longer available. The world mines something like two and a half billion tonnes of it a year, more than every other metal ore combined.

Rust is measured in percentage points of world output

Iron's oxide is the reason the metal is cheap and the reason it is a liability. Aluminum and chromium both grow oxide films that are dense, adherent and self-limiting. Iron's oxide occupies substantially more volume than the metal it replaced, so it cracks, flakes away, and exposes fresh surface to be attacked in turn. Corrosion of iron does not slow down; it proceeds until there is nothing left.

The cost is large enough to be a macroeconomic figure. A widely cited study published by NACE International in 2016 put the global direct cost of corrosion at around 2.5 trillion US dollars a year, roughly 3.4% of world GDP, the majority of it iron and steel. Galvanising, painting, alloying with chromium and cathodic protection all exist to fight one badly-behaved oxide.

Steel, and the carbon problem it cannot design away

Crude steel production is close to two billion tonnes a year. The dominant route remains the blast furnace, where coke serves as both the fuel and the chemical reducing agent — the carbon takes the oxygen away from the iron and leaves as carbon dioxide. That chemistry is not an inefficiency to be engineered out; it is the reaction.

Steelmaking is responsible for somewhere between seven and nine per cent of world carbon dioxide emissions, and the only routes around it change the reductant rather than the furnace. Direct reduction with hydrogen produces water instead of carbon dioxide, and the Swedish HYBRIT venture delivered the first commercial batch of steel made this way to Volvo in 2021. Electrolytic reduction of iron ore is further off. Both compete against a process that is, on cost alone, extraordinarily good at its job.

An isotope that records a nearby supernova

Iron-60 is radioactive with a half-life of about 2.6 million years — long enough to survive a trip across interstellar space, far too short for any of it to remain from the formation of the solar system.

It is nevertheless present, in measurable quantities, in slowly accumulating ferromanganese crusts on the deep ocean floor and in lunar regolith samples. The depth at which it appears dates its arrival to somewhere around two to three million years ago. Since the only plausible source is a supernova, the interpretation is direct: a massive star exploded close enough to Earth, in the Pliocene, to dust the planet with its debris. Estimates of the distance cluster around a few hundred light years.

The metal life could not do without

Haemoglobin carries four iron atoms, each held in a porphyrin ring, and each binds one oxygen molecule reversibly — a reaction that works precisely because iron's charge state can shift without the bond becoming permanent. Beyond blood, iron-sulfur clusters run a large fraction of cellular electron transport, and they are thought to be among the most ancient cofactors in biochemistry, plausibly older than photosynthesis.

The awkward corollary is that iron is simultaneously the most abundant element on Earth by mass and one of the hardest for organisms to obtain, because oxidised iron is nearly insoluble at neutral pH. Bacteria secrete dedicated scavenging molecules called siderophores to prise it out of their surroundings, vertebrates hoard it in ferritin, and iron deficiency remains the most widespread nutritional disorder in the human population.