February 18, 2019History of Medicine
Iridium is found in meteorites in much higher abundance than in the Earth's crust. 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. Similarly, an iridium anomaly in core samples from the Pacific Ocean suggested the Eltanin impact of about 2.5 million years ago. It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum-group metals, the high density and tendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.
The discovery of iridium is intertwined with that of platinum and the other metals of the platinum group. Native platinum used by ancient Ethiopians and by South American cultures always contained a small amount of the other platinum group metals, including iridium. Platinum reached Europe as platina (silverette), found in the 17th century by the Spanish conquerors in a region today known as the department of Choco in Colombia. The discovery that this metal was not an alloy of known elements, but instead a distinct new element, did not occur until 1748.
Chemists who studied platinum dissolve it in aqua regia (a mixture of hydrochloric and nitric acids) to create soluble salts. When they did this, they always observed a small amount of a dark, insoluble residue. Joseph Louis Proust thought that the residue was graphite. The French chemists Victor Collet-Descotils, Antoine Francois, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough for further experiments. In 1803, British scientist Smithson Tennant (1761-1815) analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternately with alkali and acids and obtained a volatile new oxide, which he believed to be of this new metal which he named ptene, from the Greek word winged'. Tennant, who had the advantage of a much greater amount of residue, continued his research and identified the two previously undiscovered elements in the black residue, iridium and osmium. He obtained dark red crystals (probably of Na2[IrCl6]?nH2O) by a sequence of reactions with sodium hydroxide and hydrochloric acid. He named iridium after Iris, the Greek winged goddess of the rainbow and the messenger of the Olympian gods, because many of the salts he obtained were strongly colored. Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.
British scientist John George Children was the first to melt a sample of iridium in 1813 with the aid of the greatest galvanic battery that has ever been constructed (at that time). The first to obtain high-purity iridium was Robert Hare in 1842. He found it had a density of around 21.8 g/cm3 and noted the metal is nearly immalleable and very hard. The first melting in appreciable quantity was done by Henri Sainte-Claire Deville and Jules Henri Debray in 1860. They required burning more than 300 liters of pure O2 and H2 gas for each kilogram of iridium. These extreme difficulties in melting the metal limited the possibilities for handling iridium. John Isaac Hawkins was looking to obtain a fine and hard point for fountain pen nibs, and in 1834 managed to create an iridium-pointed gold pen. In 1880, John Holland and William Lofland Dudley were able to melt iridium by adding phosphorus and patented the process in the United States; British company Johnson Matthey later stated they had been using a similar process since 1837 and had already presented fused iridium at a number of World Fairs. The first use of an alloy of iridium with ruthenium in thermocouples was made by Otto Feussner in 1933. These allowed for the measurement of high temperatures in air up to 2000 oC. In Munich, Germany in 1957 Rudolf Mossbauer, in what has been called one of the landmark experiments in twentieth-century physics, discovered the resonant and recoil-free emission and absorption of gamma rays by atoms in a solid metal sample containing only 191Ir. This phenomenon, known as the Mossbauer effect (which has since been observed for other nuclei, such as 57Fe), and developed as Mossbauer spectroscopy, has made important contributions to research in physics, chemistry, biochemistry, metallurgy, and mineralogy. Mossbauer received the Nobel Prize in Physics in 1961, at the age 32, just three years after he published his discovery. In 1986 Rudolf Mossbauer was honored for his achievements with the Albert Einstein Medal and the Elliot Cresson Medal.