Iridium

Iridium is a silvery-white metal of extraordinary hardness and corrosion resistance, the second-densest naturally occurring element after osmium. Its electron configuration, [Xe] 6s2 4f14 5d7, endows it with seven d-electrons and a tightly bound lattice that resists virtually every acid and oxidizing agent at room temperature. Even aqua regia — the mixture that dissolves gold and platinum — barely touches solid iridium. In terms of sheer chemical stubbornness, nothing in the periodic table surpasses it. But iridium's most dramatic moment in science came not from chemistry but from geology: a thin layer of iridium-enriched clay deposited worldwide at the Cretaceous-Paleogene boundary became the key evidence for a catastrophic asteroid impact 66 million years ago. Iridium is vanishingly rare in Earth's crust, yet that global clay layer contains concentrations hundreds of times higher than background — a chemical fingerprint of the impactor itself.

Iridium's principal application is in spark plugs for high-performance engines. Fine iridium wire tips, typically 0.4 mm in diameter, withstand the erosion that destroys conventional nickel or platinum electrodes, extending spark plug service life to 100,000 miles or more while maintaining consistent ignition. Crucibles made from iridium are the preferred containers for growing single crystals of high-temperature oxides — including the yttrium aluminum garnet (YAG) used in lasers and the sapphire substrates on which LED chips are grown — because iridium resists reaction with oxide melts at temperatures above 2000 °C. The international prototype kilogram, used as the physical standard for mass from 1889 to 2019, was made from a 90 percent platinum, 10 percent iridium alloy chosen for its hardness and stability. Iridium electrodes appear in electrochemical cells and dimensionally stable anodes for chlor-alkali electrolysis. Radioactive iridium-192 is widely used in brachytherapy for cancer treatment and in industrial radiographic testing of welds.

Smithson Tennant discovered iridium in 1803 while investigating the black residue that remained after dissolving crude platinum ore in aqua regia. The same dissolution procedure had puzzled chemists who noted the residue for years. Working at Cambridge, Tennant recognized that the residue contained two distinct new metals and named one iridium — from the Greek iris, meaning 'rainbow' — because its salts display a striking variety of colors. The other was osmium. Both elements were announced to the Royal Society in a 1804 paper. Throughout the nineteenth century iridium was prized mainly as a hardener for platinum alloys. Its fame took a quantum leap in 1980 when Luis and Walter Alvarez, together with Frank Asaro and Helen Michel, published data showing that a thin clay layer at the Cretaceous-Paleogene boundary worldwide contained iridium concentrations 30 times above background, proposing an asteroid impact as the cause of the mass extinction. Subsequent discovery of the Chicxulub crater in Mexico confirmed the hypothesis.

Iridium is among the rarest elements in Earth's crust, with an average concentration of roughly 0.001 parts per million. Like other platinum-group metals, it siderophile chemistry caused most of Earth's iridium to sink into the iron-nickel core during planetary differentiation, leaving the crust impoverished. The element occurs as a native metal alloyed with osmium (osmiridium) in alluvial placer deposits and in sulfide-rich nickel-copper ores, particularly in the Bushveld Igneous Complex of South Africa and the Sudbury Basin in Canada. Commercial recovery comes almost entirely from the anode slimes produced during electrolytic refining of nickel and copper. Meteorites, especially iron meteorites, contain iridium at concentrations thousands of times higher than Earth's crust, reflecting the cosmic abundance of the element before planetary differentiation — which is precisely why the K-Pg iridium layer was so diagnostic of an extraterrestrial impact.

Iridium chemistry is characterized by oxidation states of +3 and +4 being most stable, though states from -1 to +6 are documented. Iridium(III) chloride (IrCl3) is a common starting material for preparing iridium coordination compounds and catalysts. Iridium(IV) oxide (IrO2) is a black conducting solid with remarkable electrochemical stability, used as an electrocatalyst for the oxygen evolution reaction in proton-exchange membrane water electrolyzers — a role increasingly important in hydrogen fuel production. Iridium hexafluoride (IrF6) is a volatile compound in which iridium reaches the +6 state. Organometallic iridium complexes, pioneered by researchers including Robert Crabtree, are among the most effective homogeneous catalysts known: Crabtree's catalyst ([Ir(cod)(PCy3)(py)]PF6) hydrogenates highly substituted double bonds that resist palladium or platinum catalysts. Cyclometalated iridium(III) complexes are the preferred phosphorescent emitters in organic light-emitting diode (OLED) displays, responsible for the vivid colors in modern smartphone screens.

• The discovery of a worldwide iridium-rich clay layer at the Cretaceous-Paleogene boundary was the critical evidence that convinced the scientific community an asteroid impact, not volcanism alone, ended the age of non-avian dinosaurs.
• The international prototype kilogram — the physical object that defined the kilogram for all science and commerce worldwide from 1889 to 2019 — was a platinum-iridium cylinder stored under three nested glass bell jars in a vault outside Paris.
• Solid iridium is so resistant to chemical attack that it cannot be dissolved in any single acid; even aqua regia, which dissolves gold and platinum, barely reacts with it at room temperature.
• Iridium gets its name from iris, the Greek goddess of the rainbow, because the element's compounds appear in an unusually wide range of colors — reds, yellows, blues, and greens depending on ligand and oxidation state.
• Iridium-192, with a half-life of about 74 days, emits gamma rays energetic enough to penetrate steel, making it the standard source used by industrial inspectors to X-ray welds in pipelines and pressure vessels without disassembling the structure.