Iridium is a very hard and silvery-white member of the platinum group metals (PGM). It is among the densest and most inert metals known. It always occurs as a trace element with the much-more-common platinum.
Although platinum was discovered in the eighteenth century, the residue left from its extraction was ignored for fifty years. A few chemists tackled the identification of this substance in the early nineteenth century. British scientist Smithson Tenant was the one who isolated and named iridium, as well as the related metal osmium, in the period 1803-04. Tenant was a colleague of William Hyde Wollaston, discoverer of the PGM elements platinum and palladium.
Pure iridium is a greater challenge and waited almost another 40 years. Iridium has a very high melting point, and purifying it by melting required either a powerful electrical arc or a very efficient forced-air furnace.
Iridium is more common in the rocky bodies of the solar system than the earth's crust, but a high concentration of iridium at the 65-million-year depth (the K-T boundary) marks the end of the Cretaceous Period of geologic time and the extinction of the dinosaurs. It is thought to be a result of an asteroid impact that scattered iridium-rich debris world-wide.
Mining and Production
Iridium is found in platinum deposits, including alloyed with platinum in nickel and copper ores. The largest reserves are in Russia and South Africa. The base metal is refined by electrolysis, leaving behind the PGM component as dross. Several chemical cycles of dissolving and precipitating will separate the various precious metals.
Properties and Uses
Iridium has several extreme properties that make suitable for extreme environments. It is very hard, nearly inert, and maintains its strength to very high temperature (1600°C). Thus, iridium (usually alloyed with platinum or osmium) is used for jet-engine bearings, deep-ocean pipes, spark plugs, and some industrial crucibles.
These same properties make iridium very difficult to machine or shape. Consequently, it is usually produced and delivered in powder form.
The official kilogram mass and meter bar were constructed in 1889 from platinum and iridium. The meter bar was eventually retired, but the kilogram remains (right).
Iridium is the second-densest metal known, just barely behind osmium. For simple ballast, though, lead and depleted uranium are much more practical choices.
Among the PGMs, only platinum and palladium have developed futures markets. Spot prices for iridium are quoted by Johnson Matthey, while actual trades are settled in a range of prices around the quote. Buyers are normally industrial users, though it is possible for wealthy investors to buy in lots of 100 oz. or more.
If you are interested in iridium, you can invest in the mining companies that produce it. These companies, however, are primarily extracting platinum, nickel, and copper, with the other PGMs as minor byproducts. Thus, their performance is governed by the major metals they produce.