Iridium
Also: Ir, Iridium metal
Iridium (Ir) is a silvery-white platinum-group precious metal with one of the highest densities of any element (22.56 g/cm3, just behind osmium) and exceptional resistance to corrosion.
Iridium belongs to the platinum group metals and ranks among the rarest and most difficult-to-work metals on Earth. With a density of 22.56 g/cm3, it is the second-densest naturally occurring element, just behind its relative osmium (22.59 g/cm3). It was discovered in 1803 by Smithson Tennant, who named it after the Greek goddess Iris (the rainbow) because its salts appear in strikingly varied colours.
Occurrence and extraction
Iridium is present in the Earth's crust at concentrations of roughly 0.001 ppb, about ten times rarer than gold. It is won almost exclusively as a by-product of nickel and copper extraction from platinum ores, chiefly in South Africa (the Bushveld Complex), Russia and Zimbabwe. World annual output fluctuates between seven and nine tonnes, far below that of platinum or palladium.
Properties
Iridium's technical data are extraordinary:
- The highest corrosion resistance of all metals: iridium dissolves in neither hydrochloric nor sulphuric acid. Only aqua regia attacks it, and then only under certain conditions.
- An extremely high melting point of 2,446 C, higher than nearly all technically usable metals.
- High hardness (Mohs 6.5) combined with marked brittleness, which makes mechanical working considerably harder.
Industrial applications
Because of its resilience, iridium finds use in high-temperature and high-precision applications:
| Field | Application |
|---|---|
| Automotive | Spark-plug electrodes (Ir alloy) |
| Chemistry | Crucible material for single-crystal growth |
| Medicine | Radiotherapy (Ir-192 implants) |
| Aerospace | Rocket engines, heat shielding |
| Metrology | Historically the prototype kilogram alloy (Pt-10%Ir) |
Iridium as an investment and trading object
Unlike gold, silver, platinum or palladium, iridium has no established futures market (such as COMEX) and no daily fixing. Prices are published as indicative levels by large industrial dealers such as Johnson Matthey or Heraeus. The extreme thinness of the market, with a few tonnes of annual output and few large buyers, can produce sharp price swings on small trading volumes. Iridium is primarily an industrial metal; liquid investment products (coins, ETCs) are barely available. Comparable historical price movements of the platinum-group metals can be followed on the platform.
Tax and investment-related considerations regarding iridium as an investment object should be checked individually; this entry is not tax or investment advice.
Key takeaway
Iridium is the densest and most corrosion-resistant metal on Earth, arising almost entirely as a by-product of platinum mining. Because of its extreme rarity and the absence of liquid trading markets, it counts primarily as an industrial metal. Current prices of the related platinum group metals offer a guide to the price level of this market segment.