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77. IridiumName: Iridium
The name iridium is appropriate, for its salts are highly coloured. Iridium is white, similar to platinum, but with a slight yellowish cast. It is very hard and brittle, making it very hard to machine, form, or work. Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN. Iridium (Latin iris meaning "rainbow", "iridium" means "of rainbows") was discovered in 1803 by Smithson Tennant in London, England along with osmium in the dark colored residue of dissolving crude platinum in aqua regia (a mixture of hydrochloric and nitric acid). This element was named after the Latin word for rainbow because its salts are highly colored. This metal was used in the standard metre bar and kilogram mass, kept by the International Bureau of Weights and Measures near Paris. These are made of an alloy of 90% platinum and 10% iridium. In 1960 the Paris metre bar was replaced as the definition of the fundamental unit of length (see krypton), but the kilogram mass is still the international standard of mass. The KT event, marking the temporal border between the Cretaceous and Tertiary eras of geological time, was identified by a thin stratum of iridium-rich clay. According to many scientists, such as Luis Alvarez, this iridium was of extraterrestrial origin, attributed to an asteroid or a comet thought to have struck near what is now the Yucatan Peninsula. However, there are others such as Dewey M. McLean of Virginia Polytechnic Institute who argue that the iridium was of volcanic origin. The Earth's core is rich in iridium, and Piton de la Fournaise on Reunion, for example, is still releasing iridium today. Iridium metal is generally non-toxic due to its relative unreactivity, but iridium compounds should be considered highly toxic. Quick links
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