Iridium is a transition metal belonging to the platinum group. Its main characteristics are that of being hard and brittle and of being silvery-white in appearance. It ranks next to osmium in being the densest element. As to its main property, this is best expressed in its being considered the most corrosive resistant of all the precious metals. In fact, this is shown even in temperatures of as high as 2000°C (3632°F or 2273.15°K).

Iridium was discovered in 1803 by the English chemist Smithson Tennant. It was identified from the residue of platinum ore which was dissolved in nitro-hydrochloric acid (also known as aqua regia). Platinum ores are still the main sources today of iridium. The precious metal is likewise obtained as a by-product of mining nickel.

Below are some of the properties of iridium.

General:

• Chemical Symbol: Ir

• Atomic Number: 77

• Category (as an element): Transition Metal

• Group/ Period/ Block (in the Periodic Table): 9/ 6/ d

• Atomic Weight: 192.217 g.mol-1

• Electron Configuration: [Xe] 4f14 5d7 6s2

Physical:

• Density (near room temperature): 22.56 g.cm-3

• Liquid Density (at melting point): 19 g.cm-3

• Melting Point: 2466°C, 4471°F, 2739°K

• Boiling Point: 4428°C, 8002°F, 4701°K

• Heat of Fusion: 41.12 kJ.mol-1

• Heat of Vaporization: 563 kJ.mol-1

Atomic:

• Oxidation States: -3, -1, 0, 1, 2, 3, 4, 5, 6

• Electronegativity: 2.20 (Pauling scale)

• Atomic Radius: 136 picometre

• Covalent Radius: 141±6 picometre

Because of its characteristic of being very brittle, pure iridium is quite difficult - almost impossible, in fact - to machine. Its primary use is as a hardening agent for platinum. High-temperature equipment, such as crucibles, are made from platinum-iridium alloys. Compass bearings, balances and fountain pen tips, on the other hand, are made from osmium-iridium alloys.

Again, iridium is the most corrosive resistant precious metal known. Coupled with its resistance to extremely high temperatures, this special characteristic makes iridium ideal for use in certain parts of aircraft engines. It is also alloyed with titanium to make deep-water pipes.

Other uses of iridium include the following:

1. Electrical contacts for spark plugs (due to its resistance to arc erosion);

2. Computer memory devices;

3. Direct-ignition engine (as a catalyst);

4. Radiotherapy (as a source of radiation);

5. X-ray telescopes.

In 2007, worldwide demand for iridium reached 3,701 kilograms (119,000 troy ounces). Distribution of these were as follows: electrochemical uses (1,100 kilograms); electrical uses (780 kilograms); for catalysis (750 kilograms); and other applications (1,100 kilograms).

Iridium is found at highest concentrations within the Earth's crust in three specific types of geologic structures: in impact craters, in igneous deposits, and in deposits reworked from either of the first two. The Bushveld igneous complex in South Africa is the largest known primary reserves for iridium in the world. Other important sources of this precious metal are the Sudbury Basin in Canada and the nickel-copper-palladium deposits near Norilsk in Russia. Several smaller iridium reserves are also found in the United States.

Beginning the year 2000, the annual production of iridium is about 3 tonnes (96,500 troy ounces). Its price as of 2007 is 14,667 U.S. dollars per kilogram (440 U.S. dollars per troy ounce).