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Iridium is a metallic chemical element with the symbol Ir, atomic number 77 and atomic weight 192.217. It is a very hard, usually brittle, extremely corrosion-resistant silver-white metal with a face-centred cubic crystalline structure. Iridium is used principally in alloys; with osmium to make fountain-pen nibs and with platinum to make heavy-duty electrical contacts[1].


The English chemist Smithson Tennant (1761-1815) discovered[2] iridium in 1804 after experimenting on the residue that remains when aqua regia is used to dissolve platinum ore. He dissolved this residue in multiple acids which resulted in colourful solutions. He noted that it could be partially precipitated when reacted with alkalis. Here is a photograph[3] of a high-purity iridium cylinder: