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. Important compounds of iridium are chlorides and iodides in industrial catalysis. The dominant uses of iridium are the metal itself and its alloys, 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. Iridium is one of the rarest elements in Earth's crust, with estimated annual production and consumption of only 7.3 tonnes (16 thousand pounds) in 2018. 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. It is one of the most corrosion-resistant metals, even at temperatures as high as 2,000 ☌ (3,630 ☏). 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. 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 it has symbol Ir and atomic number 77.
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