We continue to review some of the most important materials in heat treatment and metallurgy.  

Iridium (chemical symbol: Ir)

Iridium is a hard, silvery metal (Fig. 1) that is the most corrosion-resistant metal known to man. It is almost as unreactive as gold and extremely rare in nature. Iridium resists chemical attack by acids but is susceptible to attack by molten salts such as sodium chloride (NaCl) and sodium cyanide (NaCN). Due to iridium’s high corrosion resistance, an alloy of 90% platinum and 10% iridium was used to create the standard meter bar (Fig. 2), which defined the standard for the metric system until 1960. Iridium in its powder form is a known irritant and fire hazard with low toxicity.

Iridium was first discovered in 1803 by English chemist Smithson Tennant when he examined the residue left over when crude platinum was dissolved in Aqua Regia (a mixture of nitric and hydrochloric acids). The black residue left behind was commonly thought to be graphite before Tennant investigated further. Treating the residue with alkalis and acids, he was able to extract iridium while simultaneously discovering osmium. Tennant named the element iridium after the Latin word “iris,” meaning rainbow, since the salts he examined were highly colorful.

Iridium is one of the rarest elements (0.4 parts per billion by weight). Worldwide, there is a very thin layer of iridium sediment within the Earth’s crust that was put down at the end of Earth’s crustaceous period. Asteroids containing a high percentage of the element struck the planet and dispersed iridium dust across the globe. Scientists believe that the asteroid that dispersed the iridium across the Earth was the same as the one that struck at the time of the extinction of the dinosaurs. Iridium can also be found in natural alloys with platinum and osmium in alluvial deposits. Recovery of the element is a byproduct of the nickel mining industry.

Since iridium is very hard and brittle, it is difficult to machine and work compared to other metals. Iridium’s primary use is a hardening agent for platinum. Other uses for iridium include fountain-pen nibs (alloyed with osmium), compass bearings, high-temperature crucibles and heavy-duty electrical contacts.

Here are a few important facts about Iridium,[2,5]

  • Atomic number: 77
  • Atomic weight: 192.217
  • Melting point: 2719 K (2446°C or 4435°F)
  • Boiling point: 4701 K (4428°C or 8002°F)
  • Density: 22.42 grams per cubic centimeter
  • Phase at room temperature: Solid
  • Element classification: Metal
  • Period number: 6  
  • Group number: 9   
  • Group name: none
  • Electron configuration: [Xe] 4f145d76s2



  1. KnowledgeDoor (www.knowledgedoor.com)
  2. Jefferson Lab (https://www.jlab.org)
  3. Chemicool (www.chemicool.com/)
  4. Wikipedia (www.wikipedia.org)
  5. Royal Society of Chemistry (www.rsc.org)