Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. The unusually high abundance of iridium in the clay layer at the K–T geologic boundary gave rise to the Alvarez hypothesis, which suggests that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 65 million years ago. The claimed benefits are greater economy and improved acceleration. In the automotive industry, iridium is used in high-end after-market sparkplugs as the center electrode, replacing more commonly-used metals. Iridium compounds also find applications as catalysts for the production of acetic acid. Iridium is employed when high corrosion resistance at high temperatures is needed, as in spark plugs, crucibles for recrystallization of semiconductors at high temperatures, electrodes for the production of chlorine in the chloralkali process, and radioisotope thermoelectric generators used in unmanned spacecraft. Although it is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes, it has a number of specialized industrial and scientific applications. Iridium was discovered in 1803 by Smithson Tennant in London, England, among insoluble impurities in natural platinum from South America. 191Ir and 193Ir are the only two naturally occurring isotopes of iridium as well as the only stable isotopes the latter is the more abundant of the two. The most important iridium compounds in use are the salts and acids it forms with chlorine, though iridium also forms a number of organometallic compounds used in catalysis and in research. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can even be flammable. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second densest element (after osmium) and is the most corrosion-resistant metal, even at temperatures as high as 2000 ☌. Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir.

Iridium metal type