Element 77 in our International Year of the Periodic Table series is iridium. Iridium is used in pen tips, compasses and spark plugs, and provides a key clue to what wiped out the dinosaurs.

We saw for our previous element, osmium, how osmium and iridium are often used in alloys together and used in pen tips. These alloys can also be used in compass pivot points due to their corrosion resistance.

Iridium has a high melting point and low reactivity. For these reasons, it’s used in spark plugs in engines, particularly in aviation – though they can also be used in cars.

Iridium also provides a key clue as to what wiped out the dinosaurs 66 million years ago. Though iridium is relatively rare in the Earth’s crust, a thin, worldwide layer of iridium-rich clay that corresponds to 66 million years ago suggests an asteroid impact. Iridium is much more abundant in meteorites and asteroids than it is on Earth. The asteroid hypothesis was endorsed as being the cause of the extinction in 2010.

Remember, you can keep track of all of the previous entries in this series on the site here, or on the Royal Society of Chemistry’s dedicated page.