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According to the Royal Society of Chemistry, Smithson Tennant discovered iridium by dissolving crude platinum in diluted aqua regia (a mixture of nitric and hydrochloric acids), then by treating the black residue left behind in turn with alkalis and acids,. After this treatment, the residue separated into two new elements. At the Royal Institution in London he announced his findings and named one element iridium and the other osmium. The name iridium comes from the Latin word iris, which means rainbow. Though the metal itself isn’t rainbow coloured, it is called this because of its multi-coloured compounds.

Iridium is a hard, brittle, lustrous, dense, transition metal of the platinum family. It is silvery-white and it is notable for being the most corrosion resistant element known. It is unaffected by air, water and acids. Iridium is the most corrosion-resistant element on the Periodic Table of Elements. Because iridium is very resistant to corrosion, the standard meter bar was made of 90 % platinum and 10 % iridium. This bar was replaced as the definition of a meter in 1960, though. The meter was redefined in terms of the orange-red spectral line of krypton. However, the international prototype kilogram, which defines a kilogram, also made of a platinum and platinum/iridium alloy, is still in use around the world.

It also has the highest density of all the elements, which makes it is hard to machine, form or work it unless it is heated to extreme temperatures.

Ore containing iridium is found in Brazil, the United States, Myanmar, South Africa, Russia and Australia. Annual world production amounts to around 3 tonnes. Reserves have not been estimated. Today, iridium is commercially recovered as a byproduct of copper or nickel mining. Pure iridium is very rare on the Earth’s crust with only about 2 parts per billion located there.

Nowadays demand for iridium comes mainly from the electronic industry, the automotive industry and from the chemical industry, where it is used to coat the electrodes in the chlor-alkali process, and in catalyst. Some applications are in pivot bearings and in scientific and other special equipment. However, it is principally used in alloys.

A compound of osmium and iridium, called osmiridium, is used in fountain pen tips and compass bearings.

Iridium is also combined with platinum to make the latter harder. This alloy is used to make devices needed for high temperatures such as crucibles and electrical contacts. It is also used on some optical lenses to reduce glare as well as durable jewellery.

Finally, radioactive isotopes of iridium are used in radiation therapy for the treatment of cancer.