![]() There had been some ambiguity regarding which of the two elements was denser, due to the small size of the difference in density and difficulties in measuring it accurately, but, with increased accuracy in factors used for calculating density X-ray crystallographic data yielded densities of 22.56 g/cm 3 for iridium and 22.59 g/cm 3 for osmium. The measured density of iridium is only slightly lower (by about 0.12%) than that of osmium, the densest element known. Despite these limitations and iridium's high cost, a number of applications have developed where mechanical strength is an essential factor in some of the extremely severe conditions encountered in modern technology. This, together with a high shear modulus and a very low figure for Poisson's ratio (the relationship of longitudinal to lateral strain), indicate the high degree of stiffness and resistance to deformation that have rendered its fabrication into useful components a matter of great difficulty. Iridium's modulus of elasticity is the second-highest among the metals, only being surpassed by osmium. Iridium has a very high boiling point ( 10th among all elements) and becomes a superconductor at temperatures below 0.14 K. It is the only metal to maintain good mechanical properties in air at temperatures above 1600 ☌. ![]() Because of its hardness, brittleness, and very high melting point, solid iridium is difficult to machine, form, or work, and thus powder metallurgy is commonly employed instead. 1 troy ounce (31 g) of arc-melted iridiumĪ member of the platinum group metals, iridium is white, resembling platinum, but with a slight yellowish cast. 3.1 Cretaceous–Paleogene boundary presence Ĭharacteristics Physical properties.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. 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 65 million years ago. Iridium is found in meteorites with an abundance much higher than its average abundance 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-end 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 of the two. Iridium is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes. Smithson Tennant, the primary discoverer, named the iridium for 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. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second- densestelement (after osmium) and is the most corrosion-resistant metal, even at temperatures as high as 2000 ☌. ![]() Iridium is the chemical element with symbol Ir and atomic number 77.
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