Iridium

Iridium (Ir) is a transition metal belonging to the platinum group. Its atomic number is 77. It is a white, lustrous metal that is extremely hard and is known to be one of the most corrosion-resistant substances known—it withstands high temperatures and strong chemicals without readily reacting.

Iridium is extremely rare in the Earth's crust. The usual explanation is that most of the heavy metals of this type sank toward the Earth's core during meteorite accretion and planetary differentiation in the early period. For this reason, unusually high concentrations of iridium in rock layers are often interpreted as evidence of material from space (a meteorite or cometary impact).

Physical and Chemical Properties

Iridium has an exceptionally high density—it is one of the two densest metals (alongside osmium), with a density around 22.5 grams per cubic centimeter. Its melting point is very high, above 2400 degrees Celsius. Chemically, iridium reacts with difficulty; ordinary acids, even aqua regia, do not readily attack solid metal.

Recovery and Production

Iridium is typically obtained as a by-product associated with platinum and other metals in the same group, particularly from platinum mines and also from nickel and copper mines where it is recovered during refining. Annual production is very small compared to common metals, which affects its price and applications.

Applications

Because of its hardness and resistance to corrosion, iridium is used in equipment requiring resistance to high temperatures and harsh chemicals: such as spark plugs and heat sensors, the wear-resistant tips of ballpoint pens, and platinum-iridium alloys used in precision electrical contacts. It is also used as a catalyst support in chemical manufacturing.

Significance in Earth Sciences

Iridium figures prominently in discussion of the extinction event at the end of the Cretaceous period. A thin layer unusually rich in iridium has been found at multiple locations around the world, and is taken as evidence supporting the hypothesis that a large object from space struck Earth at that time. The detailed interpretation of this evidence remains disputed, but the iridium layer is one important marker.