DEFINITION of 'Iridium'

One of the transition metals on the periodic table. Iridium appears as symbol Ir on the periodic table and has an atomic weight of 192.217 and a density of 22.56 g/cm³, making it the second densest known element. Because iridium is so expensive, it is typically used only in applications that require a very small amount of the element. High-grade automobile spark plugs, for example, have small iridium tips that last 100,000 miles - much longer than conventional spark plugs. The primary use for iridium is in alloys with its neighbor on the periodic table, platinum (symbol Pt, atomic number 78).


Iridium is the most corrosion-resistant metal on Earth. It is extremely difficult to melt; because of its high melting point, it is hard to form, machine or work iridium. It is also one of the rarest elements in Earth's crust. The huge asteroid that is believed to have wiped out the dinosaurs 65 million years ago is also responsible for the thin layer of iridium-rich clay found around Earth. This layer of iridium forms the Cretaceous-Tertiary boundary (also known as the K-T Boundary because the Cretaceous is represented by the letter "K").