Iridium

Category: transition metal Atomic number: 77 Colour: silvery white Melting point: 2,466°C (4,406°F) Boiling point: 4,428°C (8,002°F) First isolated: 1803

The metal iridium played an important role in confirming that the mass extinction of the dinosaurs (and other species) 65 million years ago was caused by a huge meteorite strike. It is rare on Earth but can often be found in meteorites. In 1980, the Nobel Prize-winning physicist Luis Alvarez and his colleagues from the University of California showed that there is an unusually high level of iridium in layers of the Earth’s rock strata that were formed 65 million years ago – this has become known as the K–Pg boundary (because it is the meeting point of the Cretaceous and Paleogene eras, for which the abbreviations are ‘K’ and ‘Pg’).

This layer is visible at the surface in places, such as at Badlands in Alberta, Canada, or on the island of Zealand in Denmark, but it is part of the fossil record around the world. Alvarez and his colleagues theorized that it was evidence of a massive meteorite impact at that time. The result was a long-lasting ‘impact winter’, which prevented plants from photosynthesizing and led to many species starving and disappearing. The theory was given greater weight in the 1990s with the discovery of the Chicxulub crater in the Gulf of Mexico – 180 kilometres wide; this impact crater confirmed that the K–Pg boundary was caused by debris being thrown up into the atmosphere and then settling after a huge meteorite strike.

The Goddess of the Rainbow

In Greek mythology, Iris, the goddess of the rainbow, was the daughter of the sea god Thaumas and his wife Electra. In between serving nectar to the gods and goddesses, and acting as their messenger, she is said to have gathered water from the ocean and used it to water the clouds with her pitcher. Because of the wide range of colourful salts it produces, Smithson Tennant named iridium after Iris, whose name is also the root of the word ‘iridescent’.

In its pure metallic state, iridium is a brittle, shiny, silverish metal. As we’ve learned, it was first isolated by Smithson Tennant in 1803, along with osmium. For all the ingenuity of nineteenth-century scientists, it took decades for anyone to find a use for iridium, as the extremely high melting point made it difficult to work with. However, in 1834 the inventor John Isaac Hawkins, who wanted to create a thin, hard point for fountain-pen nibs, managed to create an iridium-pointed gold pen. Over time, methods were developed to work iridium and to create alloys of it with other metals – it is very hard and corrosion-resistant, and has been used to tip spark plugs, make aircraft parts and construct crucibles for use at very high temperatures. One of its isotopes, iridium-192, is also used in radiation therapy to treat patients with cancer.