Iridium

Iridium was discovered in 1803 by the English chemist Smithson Tennant, alongside osmium, in the residues of dissolved platinum ore. He named it after Iris, the Greek goddess of the rainbow, because its salts showed so many different colours. Iridium is a hard, brittle, very dense and exceptionally corrosion-resistant member of the platinum group, the most corrosion-resistant metal known, used in spark plugs, crucibles and the tips of fine pen nibs. Able to endure great heat and almost immune to attack, iridium is used in long-lasting spark plugs, in crucibles for growing crystals, in the hard-wearing electrodes and tips of specialist instruments, and once helped define the standard kilogram in a platinum-iridium alloy.

The International Prototype Kilogram, a cylinder of ninety per cent platinum and ten per cent iridium kept at the International Bureau of Weights and Measures near Paris since 1889, served as the world's literal physical definition of mass for one hundred and thirty years, until the kilogram was finally redefined in 2019 in terms of the fixed value of the Planck constant, retiring the iridium-alloy cylinder from active duty. Iridium's near-total resistance to corrosion and stable density made it the obvious choice of alloying partner when the standard was cast in the nineteenth century.

Tennant named iridium for Iris, the rainbow goddess who in Greek myth carried messages between the gods and mortals along the arc of the rainbow, choosing her for the varied colours of the metal's compounds, so that iridium joins chromium and rhodium among the elements christened for colour. The metal later acquired a startling scientific fame: iridium is rare in the Earth's crust but relatively abundant in asteroids, and in 1980 the physicist Luis Alvarez and his son Walter found a thin worldwide layer unusually rich in iridium at exactly the geological boundary marking the extinction of the dinosaurs. That iridium anomaly became the key evidence that a giant asteroid impact ended the age of the dinosaurs, giving the rainbow-named element an unexpected role in the story of life on Earth.

The Alvarez hypothesis, initially controversial, gained decisive physical support in the 1990s when geologists identified the Chicxulub crater buried beneath Mexico's Yucatan Peninsula, an impact structure of almost exactly the right size and age to match the iridium-rich boundary layer found worldwide, closing much of the debate over what had actually struck the Earth sixty-six million years ago. Iridium anomalies have since become a standard tool geologists check for whenever they suspect an impact event anywhere in the rock record.