It has the 10th highest boiling point among all elements and becomes a superconductor at temperatures below 0.14 K (−273.010 ☌ −459.418 ☏). It is the only metal to maintain good mechanical properties in air at temperatures above 1,600 ☌ (2,910 ☏). Because of its hardness, brittleness, and very high melting point, solid iridium is difficult to machine, form, or work thus powder metallurgy is commonly employed instead.
One troy ounce (31.1035 grams) of arc-melted iridiumĪ member of the platinum group metals, iridium is white, resembling platinum, but with a slight yellowish cast. It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum-group metals, the high density and tendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten. Similarly, an iridium anomaly in core samples from the Pacific Ocean suggested the Eltanin impact of about 2.5 million years ago. For this reason, the unusually high abundance of iridium in the clay layer at the Cretaceous–Paleogene boundary gave rise to the Alvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 66 million years ago, now known to be produced by the impact that formed the Chicxulub crater. Iridium is found in meteorites in much higher abundance than in the Earth's crust. Iridium radioisotopes are used in some radioisotope thermoelectric generators. Iridium metal is employed when high corrosion resistance at high temperatures is needed, as in high-performance spark plugs, crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the chloralkali process. 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 industrial catalysis, and in research. 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. Iridium is one of the rarest elements in Earth's crust, with annual production and consumption of only 3 tonnes (6.6 thousand pounds).
Smithson Tennant, the primary discoverer, named it after the Greek goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium was discovered in 1803 among insoluble impurities in natural platinum. However, corrosion-resistance is not quantifiable in absolute terms although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable, whereas gold dust is not flammable but can be attacked by substances that iridium resists, such as aqua regia. It is one of the most corrosion-resistant metals, even at temperatures as high as 2,000 ☌ (3,630 ☏). A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of 22.56 g/cm 3 (0.815 lb/cu in) as defined by experimental X-ray crystallography. Iridium is a chemical element with the symbol Ir and atomic number 77.
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