Iridium

• 1. Introduction

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  Iridium carries the symbol Ir and the atomic number 77, and it answers to a name borrowed from a goddess of the rainbow. Smithson Tennant pulled it from a dark, stubborn residue that other chemists had dismissed as graphite. The salts he coaxed out of that black powder glowed in many colors, so he reached for Iris, the Greek winged messenger of the gods. That residue had been thrown away for years as nothing more than dirt clinging to platinum. How did a discard become one of the rarest metals in Earth's crust, with only about 15,000 pounds produced in a single recent year? Why does it turn up in meteorites far more readily than in the ground beneath our feet? And how did a thin band of iridium-rich clay come to explain the death of the dinosaurs? The answers run from the inside of a fountain pen nib to the core of the planet itself.

• 2. A Metal That Resists Everything

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  Aqua regia, the acid mixture that dissolves gold, cannot touch iridium. It is the most corrosion-resistant metal known, holding firm even at temperatures as high as 2000 degrees Celsius. Concentrated hydrochloric acid will only dissolve it with sodium perchlorate present to force the reaction. With oxygen around, it reacts with cyanide salts, and the halogens and sulfur can attack it at higher temperatures, yielding iridium disulfide.

  This silvery-white transition metal of the platinum group resembles platinum but wears a slight yellowish cast. Its modulus of elasticity is the second-highest among the metals, beaten only by osmium, and its very low Poisson's ratio marks a stiffness that fights any deformation. Those same virtues make it a nightmare to handle. Solid iridium is so hard, brittle, and high-melting that workers turn to powder metallurgy rather than try to machine or form it.

  The brittleness goes so far that iridium is hard to weld, because the heat-affected zone cracks. A small fix exists. Adding tiny amounts of titanium and zirconium, about 0.2 percent of each, makes the metal noticeably more ductile. Alloying changes its hardness dramatically too. Pure platinum measures 56 on the Vickers scale, while a fifty-fifty platinum-iridium alloy can climb past 500 HV.

  Its density sits at 22.56 grams per cubic centimeter, only about 0.12 percent below osmium at 22.59. For a time nobody could say which of the two was denser, since the gap was so slim and so hard to measure. X-ray crystallographic data finally settled the contest in osmium's favor, leaving iridium as the second-densest naturally occurring metal.

• 3. Two Stable Twins, One Famous Isotope

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  191Ir and 193Ir are the only two stable isotopes iridium possesses, with natural abundances of 37.3 percent and 62.7 percent. Around them sit 40 known radioisotopes spanning mass numbers from 164 to 205, plus at least 32 metastable isomers. The most stable isomer, 192m2Ir, decays by isomeric transition with a half-life of 241 years, outlasting every ground-state radioisotope of the element.

  192Ir falls squarely between the two stable twins and is the most stable radioisotope, with a half-life of 73.82 days. It splits its decay two ways, with beta-plus decay accounting for 95.24 percent and electron capture for 4.76 percent. Doctors use it in brachytherapy, and engineers use it in industrial radiography to test welds in steel for the oil and gas industries. That same usefulness carries danger, and iridium-192 sources have been involved in a number of radiological accidents.

  191Ir earned a place in physics history as the first isotope of any element shown to display the Mössbauer effect. In Munich in 1957, Rudolf Mössbauer observed the recoil-free emission and absorption of gamma rays in a solid sample containing only 191Ir. The work has been called one of the landmark experiments in twentieth-century physics. It brought him the Nobel Prize in Physics in 1961, when he was 32, just three years after publishing the discovery.

• 4. From Discarded Residue To New Element

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  Joseph Louis Proust looked at the dark, insoluble residue left when platinum dissolved in aqua regia and guessed it was graphite. In 1803 a trio of French chemists, Victor Collet-Descotils, Antoine François de Fourcroy, and Louis Nicolas Vauquelin, also saw the black residue but could not gather enough to study it. Vauquelin treated his powder with alternating alkali and acid, drew off a volatile oxide, and named the supposed metal ptene, from the Greek for winged.

  Smithson Tennant, who lived from 1761 to 1815, held the advantage of a far larger amount of residue. Working through reactions with sodium hydroxide and hydrochloric acid, he produced dark red crystals and concluded the black powder hid not one but two new elements: iridium and osmium. He announced the find in a letter to the Royal Society on the 21st of June 1804.

  The discovery sat inside a much older story about platinum. The first European mention of that metal came in 1557, when the Italian humanist Julius Caesar Scaliger described an unknown noble metal found between Darién and Mexico, one that no fire nor any Spanish artifice had been able to liquefy. The Spanish treated it as an impurity in gold and even passed a decree against adulterating gold with it. In 1735 Antonio de Ulloa and Jorge Juan y Santacilia watched Native Americans mining platinum during eight years travelling through Colombia and Peru, and carried nuggets home to Spain.

• 5. The Battle To Melt It

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  John George Children melted a sample of iridium in 1813 using what was called the greatest galvanic battery ever constructed at the time. The metal fought every attempt to work it. Robert Hare became the first to obtain high-purity iridium in 1842, measuring a density near 21.8 grams per cubic centimeter and noting it was nearly immalleable and very hard. When Henri Sainte-Claire Deville and Jules Henri Debray achieved the first melting in appreciable quantity in 1860, they had to burn more than 300 liters of gas for every kilogram of iridium.

  John Isaac Hawkins wanted a fine, hard point for fountain pen nibs, and in 1834 he managed an iridium-pointed gold pen. A workable melting process arrived in 1880, when John Holland and William Lofland Dudley melted iridium by adding phosphorus and patented the method in the United States. The British firm Johnson Matthey later claimed it had used a similar process since 1837 and had already shown fused iridium at several World Fairs.

  High-temperature measurement opened another use. Otto Feussner built the first thermocouples from an iridium-ruthenium alloy in 1933, letting engineers read temperatures in air up to 2000 degrees Celsius. The metal that resisted the furnace had become a tool for measuring the furnace's heat.

• 6. Born In Stars, Hidden In The Core

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  Neutron star mergers forge iridium through the r-process, the rapid capture of neutrons, with rare supernovae possibly contributing too. The element ranks among the nine least abundant stable elements in Earth's crust, averaging just 0.001 parts per million in crustal rock. Gold is four times more common there, platinum ten times, and silver and mercury eighty times.

  The scarcity at the surface hides a deeper truth. Earth's overall iridium content is thought to be far higher than crustal rock suggests, but the metal is dense and siderophilic, an iron-lover, so it sank into the core while the planet was still molten. In meteorites, untouched by that sorting, iridium reaches concentrations of 0.5 parts per million or more.

  Where iridium does surface, it favors three kinds of geology: igneous deposits, impact craters, and deposits reworked from them. The largest known primary reserves lie in the Bushveld igneous complex in South Africa, near the Vredefort impact structure. The copper-nickel deposits near Norilsk in Russia and the Sudbury Basin in Canada, itself an impact crater, are also significant. In nature the metal appears uncombined or in alloys such as osmiridium and iridosmium, and pre-Columbian people in the Chocó Department of Colombia worked alluvial deposits that still yield platinum-group metals today.

• 7. The Clay That Killed The Dinosaurs

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  A thin stratum of iridium-rich clay marks the Cretaceous-Paleogene boundary, the line in geological time drawn 66 million years ago. In 1980 a team led by Luis Alvarez argued that this iridium came from beyond Earth, delivered by an asteroid or comet impact. The Alvarez hypothesis is now widely accepted to explain the extinction of the non-avian dinosaurs. A large buried crater of about the same age was later found beneath the Yucatán Peninsula, the Chicxulub crater.

  Not everyone reads the clay the same way. Dewey M. McLean and others argue the iridium could be volcanic, since Earth's core is rich in it and active volcanoes such as Piton de la Fournaise on Réunion still release the metal. The element's extraterrestrial signature shows up elsewhere too. Core samples from the Pacific Ocean carrying elevated iridium pointed to the Eltanin impact of about 2.5 million years ago.

  Iridium drifts through the oceans as well, found in marine organisms, sediments, and the water column. In organisms it sits near 20 parts per trillion, roughly five orders of magnitude below its level in K-T boundary sediment, because it does not readily form chloride complexes in seawater. Paired with osmium, it serves as a tracer for meteoritic material in sediment, a chemical fingerprint left by visitors from space.

• 8. Crucibles, Spark Plugs, And A Standard Kilogram

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  An alloy of 90 percent platinum and 10 percent iridium built the International Prototype Meter and kilogram in 1889, kept near Paris by the International Bureau of Weights and Measures. The meter bar lost its defining role in 1960 to a line in the spectrum of krypton, but the kilogram prototype reigned as the world standard of mass until the 20th of May 2019, when the kilogram was redefined through the Planck constant.

  Its resistance to heat and corrosion drives most modern uses. Iridium crucibles run the Czochralski process to grow oxide single-crystals like sapphires, melting mixed oxides under oxidizing conditions at temperatures up to 2100 degrees Celsius. Spark plug makers prize iridium alloys for the center electrodes because they resist arc erosion, and such plugs see particular use in aviation. The metal also serves in electrodes for producing chlorine, in long-life aircraft engine parts, and in spinnerets that extrude polymer melt into fibers such as rayon.

  Chemistry leans on iridium too, with an oxidation range running from minus 3 to plus 9, the highest oxidation state recorded for any element. Iridium compounds catalyze the Cativa process that carbonylates methanol into acetic acid. Iridium complexes drive asymmetric hydrogenation, the basis of the industrial route to the herbicide (S)-metolachlor, which Syngenta makes on the scale of 10,000 tons a year. The name once meant rainbow-colored salts, and today the same element quietly anchors the white glow of OLEDs and the catalysts of modern industry.