Iridium (Ir)
Overview of Iridium
Iridium is a hard, silvery-white transition metal with atomic number 77. It is one of the densest elements and has an exceptionally high melting point. Known for being almost as chemically unreactive as gold, iridium is also the most corrosion-resistant metal. Its name comes from Iris, the Greek goddess of the rainbow, because its compounds display a wide variety of vivid colors.
Why Is Iridium So Useful?
Iridium’s combination of durability, resistance, and high melting point makes it ideal for extreme conditions:
Durable alloys: Iridium is alloyed with osmium to produce extremely hard materials, such as the tips of fountain pens and compass bearings.
High-performance parts: Its stability at high temperatures allows it to be used in spark plug contacts and crucibles for growing crystals under extreme heat.
Historical standards: A platinum–iridium alloy (90% platinum, 10% iridium) was used to create the international prototype metre bar, which defined the unit of length for more than 70 years.
Iridium and the Dinosaurs
Although very rare in Earth’s crust, iridium is more common in meteors and asteroids. A thin global layer of iridium-rich clay is found at the Cretaceous–Paleogene (K–Pg) boundary, dating to 66 million years ago. This discovery provided crucial evidence for the theory that a massive asteroid impact caused the extinction of the dinosaurs.
Natural Abundance and Production of Iridium
Sources: Iridium occurs in uncombined form in river sediments and placer deposits, often alongside other platinum group metals (PGMs).
Commercial recovery: Most iridium is obtained as a by-product of nickel refining.
History of Iridium
1803 – Discovery: Iridium was discovered in London by Smithson Tennant, who was studying the black residue left after dissolving crude platinum in aqua regia.
Twin discovery: In that same residue, Tennant identified two new metals—iridium and osmium—adding to the platinum group family of elements.