Iron

Iron is the most abundant element on Earth if the whole planet is counted: the core is mostly iron and nickel. In the crust it ranks fourth, and in importance to human technology it has no rival — around 95 % of all metal produced in the world is iron and its alloys.

It is also responsible for one of the most ordinary chemical phenomena there is: rust, which destroys a noticeable fraction of all the metal ever made, every year.

The principal ores are oxides: magnetite Fe₃O₄, haematite Fe₂O₃, and limonite. Siderite FeCO₃ and pyrite FeS₂ matter too, though the sulfur in the latter makes it awkward for metallurgy.

The largest deposits are banded iron formations laid down some two billion years ago. Their origin is directly tied to the appearance of oxygen: iron(II) dissolved in the ancient ocean was oxidised to insoluble iron(III) and settled to the seabed. The ores that steel is made from are, in effect, a trace of the Great Oxidation Event.

No date can be given: iron has been known since prehistory. The first objects were made from meteoritic iron — the ancient Egyptian name meant something close to "metal from the sky", and such things were valued above gold.

Smelting from ore needs a higher temperature than smelting copper, and spread from roughly the twelfth century BC. The shift to iron changed both economies and warfare: copper and tin rarely occur together, while iron ore is almost everywhere.

Iron is a transition metal, and like most transition metals it has more than one stable oxidation state. The main ones are +2 and +3, and in the presence of oxygen the +2 gradually becomes +3.

Corrosion is an electrochemical process rather than simple oxidation: it needs oxygen and water together, and it speeds up markedly in the presence of salt. The rust that forms is loose and does not protect the metal underneath — unlike the oxide film on aluminium — so the process runs to completion.

There are several defences: paint, galvanising, alloying with chromium. In stainless steel the chromium forms a thin dense oxide film, and that is what stops the corrosion.

Cast iron and steel are both alloys of iron with carbon, and the difference between them is how much: above about 2 % carbon the metal is cast iron and brittle; below it, steel and workable. Further properties come from additions — chromium for corrosion resistance, manganese for wear resistance, tungsten for keeping hardness when hot.

Iron compounds have their own uses: pigments such as ochre and red oxide, catalysts including the one in ammonia synthesis, and medicines for anaemia.

Iron sits at the centre of haem, the fragment of the haemoglobin molecule that binds oxygen reversibly and carries it around the body. It is the iron in haem that makes blood red.

It also sits in the cytochromes of the respiratory chain, where it cycles between +2 and +3 as it passes electrons along. A person needs only tens of milligrams a day, but iron deficiency is the most widespread nutritional deficiency in the world.