Iridium (Ir)

Iridium is scarce at the Earth's surface for a specific reason, and that reason is what made it the most consequential trace element in twentieth-century geology.

Iridium is siderophile — it dissolves readily in molten iron. When the young Earth differentiated and its iron sank, iridium went down with it, leaving the crust depleted by a factor of thousands relative to the solar system's overall composition. Undifferentiated meteorites never went through that process and retain the original abundance. So iridium in a rock is a rough measure of how much extraterrestrial material is mixed into it.

A centimetre of clay in Gubbio

In the late 1970s Walter Alvarez was working on a limestone sequence in a gorge near Gubbio in Umbria, where a thin bed of clay separates Cretaceous rock below from Palaeogene rock above. He wanted to know how long that clay had taken to deposit, and his father Luis Alvarez suggested a clock: cosmic dust falls at a roughly constant rate and carries iridium, so measuring the iridium in the clay would give the time.

Frank Asaro and Helen Michel ran the neutron activation analysis at Berkeley. The clay held about thirty times the iridium of the limestone above and below it — far more than slow dust accumulation could deliver. The same enrichment turned up at Stevns Klint in Denmark and at Woodside Creek in New Zealand. In 1980 the four of them published the proposal that a large asteroid had struck the Earth at the end of the Cretaceous.

The hypothesis was ridiculed for a decade, largely by palaeontologists who saw a physicist telling them how their fossils worked. The buried Chicxulub structure on the Yucatán margin was identified as the crater in 1991, from petroleum geophysics data that had been sitting unexamined. Debate now centres not on whether the impact happened but on how much of the extinction it caused, given that the Deccan Traps in India were erupting on a colossal scale across the same interval. The iridium layer itself has never been in doubt: it is global, it is the right age, and its platinum-group ratios match a chondrite rather than anything terrestrial.

What survives when nothing else does

Iridium is the most corrosion-resistant metal known. Acids do not touch it, individually or in combination — aqua regia, which dissolves gold and platinum, leaves iridium alone. It resists molten salts and oxidising atmospheres up to extreme temperature, and only halogens and molten alkalis get anywhere with it, and only when hot.

Its name comes from that chemistry's opposite face. Smithson Tennant, who separated iridium from the same black platinum residue that yielded osmium, found that its salts came in an implausible range of colours — reds, violets, blues, greens — and named it after Iris, the rainbow.

Chemical indifference plus a very high melting point defines what iridium is for. The largest industrial use is crucibles: growing single crystals of sapphire, yttrium aluminum garnet, lithium tantalate and similar oxides requires holding a melt above 2000 °C in an oxidising atmosphere for days, and iridium is the only practical container. Every synthetic sapphire watch face and LED substrate has passed through one.

Beyond that, iridium clusters into a handful of jobs that are individually small and collectively indispensable:

• Fine-wire electrode tips in long-life spark plugs, where the erosion rate sets the service interval.
• Mixed iridium-tantalum oxide coatings on the dimensionally stable anodes used in chlor-alkali cells and industrial electroplating.
• Contacts, pivots and crucible liners wherever wear and chemical attack coincide.

The capsule around a plutonium heat source

Every radioisotope thermoelectric generator flown by NASA since the 1970s carries its plutonium-238 fuel inside an iridium shell.

The alloy is specific — iridium with a fraction of a per cent of tungsten and traces of thorium and aluminum, designated DOP-26 — and it was developed because pure iridium is brittle and would shatter. The requirement is not merely to contain the fuel in normal operation but to survive a launch failure: the capsule must stay intact through an explosion on the pad, through re-entry, and through impact on rock or water, so that plutonium is not dispersed. Iridium's melting point is above plutonium dioxide's, it does not react with the fuel, and the DOP-26 alloy stays ductile at the temperatures reached during atmospheric re-entry.

Those capsules are on Voyager 1 and 2, Galileo, Ulysses, Cassini, New Horizons, Curiosity and Perseverance. It is the only element in that role and there is no substitute for it.

Why green hydrogen has an iridium problem

Proton-exchange-membrane electrolysers split water using two catalysts, and the oxygen side is the hard one. The oxygen evolution reaction runs in a hot, strongly acidic, strongly oxidising environment that destroys almost every catalyst material within hours. Iridium oxide survives it. Nothing else that works has yet been shown to last.

The arithmetic is uncomfortable. World iridium production is on the order of seven to eight tonnes a year, entirely as a byproduct of platinum and nickel mining and therefore not expandable on demand. Current electrolysers use something in the region of a few hundred milligrams of iridium per kilowatt. Scaling PEM electrolysis to the capacity implied by national hydrogen strategies would require either loadings roughly an order of magnitude lower or a different catalyst, and reducing iridium loading is now one of the most heavily funded problems in electrocatalysis.

Two stable isotopes and a seventy-four-day workhorse

Iridium has just two stable isotopes, and the one that matters commercially sits between them. Iridium-192 has a half-life of 73.8 days and emits gamma rays energetic enough to penetrate steel but not so energetic as to require impossible shielding.

That makes it the standard source for industrial radiography — checking welds in pipelines, pressure vessels and aircraft structures — and a mainstay of high-dose-rate brachytherapy, where a source is driven briefly into a catheter placed in a tumour. Its half-life is short enough that sources must be replaced a few times a year, which means a continuous global traffic in small, intensely radioactive capsules.

That traffic has a casualty list. Iridium-192 sources are physically small, valuable-looking and occasionally lost, and orphan sources picked up by people who do not know what they are have caused severe radiation injuries and deaths, notably at Yanango in Peru in 1999.

The kilogram, and a satellite constellation

Iridium's most famous single object no longer has a job. The International Prototype Kilogram — a cylinder of 90% platinum and 10% iridium, iridium added for hardness and stability — defined the unit of mass from 1889 until the SI redefinition took effect in 2019. It sits in a vault at Sèvres, now an artefact rather than a standard.

The satellite constellation carries the name for a reason that stopped being true before launch: the original design called for 77 satellites, matching iridium's atomic number. The system as built used 66. Nobody renamed it dysprosium.