Iridium

Iridium is a chemical element that has the symbol Ir and atomic number 77. A dense, very hard, brittle, silvery-white transition metal of the platinum family, iridium is used in high-strength alloys that can withstand high temperatures and occurs in natural alloys with platinum or osmium. Iridium is notable for being the most corrosion-resistant element known and for its significance in the determination of the probable cause of the extinction, by a meteorite strike, of the dinosaurs. It is used in high-temperature apparatus, electrical contacts, and as a hardening agent for platinum.

Characteristics
A platinum group metal, iridium is white, resembling platinum, but with a slight yellowish cast. Due to its extreme hardness and brittleness, iridium is difficult to machine, form, or work. It is the most corrosion-resistant metal known: iridium cannot be attacked by any acids or by aqua regia, but it can be attacked by molten salts, such as NaCl and NaCN.

The measured density of iridium is only slightly lower than that of osmium, which is often listed as the densest element known. However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22,650 kg/m³ for iridium versus 22,610 kg/m³ for osmium. Definitive selection between the two is therefore not possible at this time.

Applications
The principal use of iridium is as a hardening agent in platinum alloys. Other uses: At one time iridium, as an alloy with platinum, was used in bushing the vents of heavy ordnance, and in a finely powdered condition (iridium black), for painting porcelain black.
 * Crucibles and devices that require high temperatures.
 * Electrical contacts (notable example: Pt-Ir spark plugs).
 * Osmium-iridium alloys are used for compass bearings.
 * Iridium is commonly used in complexes like Ir(mppy)3 and other complexes in polymer LED technology to increase the efficiency from 25% to almost 100% due to triplet harvesting.
 * Used as a source of radioactivity (Ir-192) for the treatment of cancer (in particular High Dose Rate prostate brachytherapy and intracavitary cervix brachytherapy)
 * Iridium is used as a catalyst for carbonylation of methanol to produce acetic acid
 * Iridium is used in supercolliders in the production of antimatter, specifically antiprotons

Iridium was used to tip some early-twentieth-century fountain pen nibs. The tip material in modern fountain pens is still conventionally called "iridium," although there is seldom any iridium in it. An exception to this are the JML Fountain pens, sold in the UK.

History
Iridium was discovered in 1803 by British scientist Smithson Tennant in London, England along with osmium in the dark-coloured residue of dissolving crude platinum in aqua regia (a mixture of hydrochloric and nitric acid). The element was named after the Latin word for rainbow (iris; iridium means "of rainbows") because many of its salts are strongly coloured.

An alloy of 90% platinum and 10% iridium was used in 1889 to construct the standard metre bar and kilogramme mass, kept by the International Bureau of Weights and Measures near Paris. The metre bar was replaced as the definition of the fundamental unit of length in 1960 (see krypton), but the kilogram prototype is still the international standard of mass.

K-T boundary
The K–T boundary of 65 million years ago, marking the temporal border between the Cretaceous and Tertiary eras of geological time, was identified by a thin stratum of iridium-rich clay. A team led by Luis Alvarez (1980) proposed an extraterrestrial origin for this iridium, attributing it to an asteroid or comet impact. Their theory is widely accepted to explain the demise of the dinosaurs. A large buried impact crater structure with an estimated age of about 65 million years was later identified near what is now Yucatán Peninsula. Dewey M. McLean and others argue that the iridium may have been of volcanic origin instead. The Earth's core is rich in iridium, and Piton de la Fournaise on Réunion, for example, is still releasing iridium today.

Occurrence
Iridium is found uncombined in nature with platinum and other platinum group metals in alluvial deposits. Naturally occurring iridium alloys include osmiridium and iridiosmium, both of which are mixtures of iridium and osmium. It is recovered commercially as a by-product from nickel mining and processing.

Iridium is the rarest non-radioactive, non-noble gas element in the Earth's crust, but it is relatively common in meteorites. Iridium and osmium are the densest elements, and both are believed to have dropped below the Earth's crust toward the core when the Earth was young and molten. The concentration of iridium in meteorites matches the concentration of iridium in the Earth as a whole.

Isotopes
There are two natural isotopes of iridium, and many radioisotopes, the most stable radioisotope being Ir-192 with a half-life of 73.83 days. Ir-192 beta decays into platinum-192, while most of the other radioisotopes decay into osmium.

Precautions
Iridium metal is mostly non-toxic due to its relative unreactivity.