Iridium

Iridium is a chemical element in the periodic table that has the symbol Ir and atomic number 77. A heavy, 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 association with the demise of the dinosaurs. It is used in high temperature apparatus, electrical contacts, and as a hardening agent for platinum.

Notable Characteristics

A platinum family metal, iridium, is white, resembling platinum, but with a slight yellowish cast. Due to its extreme hardness and brittle properties, iridium is difficult to machine, form, or work. Iridium is the most corrosion-resistant metal known

Iridiuim can not be attacked by any acids or by aqua regia, but it can be attacked by molten salts, such as Na Cl and NaCN. The specific gravity of this element is only slightly lower than osmium, which is generally considered to be the heaviest 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 22650 for iridium versus 22661 for osmium. Definitive selection between the two is therefore not possible at this time. It's just too close to call.

Applications

The principal use of iridium is as a hardening agent in platinum alloys. Other uses;

For making crucibles and devices that require high temperatures.
Electrical contacts (notable example; Pt/Ir sparkplugs).
Osmium/iridium alloys are used for ball point pen tips and for compass bearings.

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.

History

Iridium (Latin iris meaning "rainbow") was discovered in 1803 by Smithson Tennant in London, England along with osmium in the dark colored residue of dissolving crude platinum in aqua regia (a mixture of hydrochloric and nitric acid). This element was named after the Latin word for rainbow because its salts are highly colored.

This metal was to make the standard meter bar in Paris, which is an alloy of 90 % platinum and 10 % iridium. In 1960 the Paris meter bar was replaced as definition of the fundamental unit of length (see krypton).

The KT event, marking the temporal border between the Cretaceous and Tertiary eras of geological time, was identified by a thin stratum of iridium. According to many scientists, such as Luis Alvarez of Berkeley, California, this iridium was of extraterrestrial origin, attributed to a comet thought to have struck near what is now the Yucatan Peninsula.

However, there are others such as Dewey M. McLean of Virginia Polytechnic Institute who argue that the iridium was of volcanic origin. 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.

Isotopes

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

Precautions

Iridium metal is generally non-toxic due to its relative unreactivity, but iridium compounds should be considered highly toxic.