![]() Références Portail de la chimie La dernière modification de cette page a été faite le 21 juin 2022 à 13:38. The dust also would have blocked the sun for a time, resulting in the extinction of many plant and animal species, including the dinosaurs. Les quatre températures clefs dans ces plages sont respectivement le point de fusion de liridium, le point débullition du béryllium, le point de fusion du niobium, et le point débullition de laluminium. Dust from the impact would have spread around the globe, depositing the iridium. Since meteors and asteroids contain a higher percentage of iridium than the earth's crust, this iridium enriched layer is seen as evidence that the earth was struck by a large meteor or asteroid at that time. This bar was replaced as the definition of the meter in 1960 when the meter was redefined in terms of the orange-red spectral line of krypton-86.Ī thin, worldwide layer of iridium exists in a layer of sediment that was put down at the end of the Cretaceous period. For this reason, the standard meter bar was created from an alloy of 90% platinum and 10% iridium. Iridium is the most corrosive resistant metal known. Iridium is also alloyed with osmium to make the tips of fountain pens and compass bearings. It has an abundance in the earths crust of almost 3x10 6 ppm. Platinum-iridium alloys are used to make crucibles and other high temperature equipment. In appearance, iridium is a lustrous, silvery metal. It is primarily used as a hardening agent for platinum. Pure iridium is very brittle and is nearly impossible to machine. ![]() Today, iridium is still obtained from platinum ores and as a by-product of mining nickel. Iridium and osmium were identified in the black residue remaining after dissolving platinum ore with aqua regia, a mixture of 25% nitric acid (HNO 3) and 75% hydrochloric acid (HCl). Iridium and osmium were discovered at the same time by the British chemist Smithson Tennant in 1803.
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