Element of the day: Iridium

Chemical Properties of Iridium

Pure Iridium can be observed to be highly lustrous and shiny.

Iridium (Ir) is a transition metal with an atomic number of 77, meaning it has 77 protons in its nucleus. It is the second-most dense element with a density of 22.56 g/cm³ (0.815 lbs/in³). In its pure form, iridium is brittle, lustrous, and silvery. It can be found in the iridium-osmium alloys, osmiridium, and iridosmine, although the majority of the element is obtained as a by-product of platinum refining. Iridium is almost universaslly rare in the Earth's rocks, with a concentration in Earth's crust of just one part per billion (ppb). It is the most corrosive-resistant metal, even at temperatures greater than 2,000 degrees Celsius.

Discovery of Iridium and Osmium

Iridium was discovered at the same time as osmium, in 1803 by the British chemist Smithson Tennant. He had dissolved crude platinum in aqua regia, which is a particularly strong mixture of acids. He noticed that a black residue remained and began experimenting on it with other types of acids and bases. The result of his efforts was the discovery of two previously unknown elements. He named one osmium due to its peculiar odor and the other iridium after Iris, the Greek goddess of the rainbow, because iridium salts were very colorful.

Confirming the Extinction of the Dinosaurs

This image marks the Cretaceous-Paleogene boundary (previously known as the Cretaceous-Tertiary boundary). This thin layer contains high amounts of iridium that originated from the 10-kilometer wide asteroid that impacted Mexico and caused the mass extinction of the dinosaurs.

Although iridium is extremely rare in Earth's crust, it is abundant in meteorites and asteroids. In 1980, Nobel Prize-winning physicist Luis Alvarez, his son Walter Alvarez, and nuclear chemists Frank Asaro and Helen Michael from the University of California, Berkeley found unusually high amounts of iridium in a layer of rock deep in the Earth's crust that formed 65 million years ago. This is at the same time that the cataclysmic event that wiped out the dinosaurs occurred. An estimated 10-kilometer wide asteroid smashed into Mexico that resulted in their mass extinction. It is believed that the debris of the asteroid impact shrouded the Earth in a cloud of dust that darkened the sky for so long that plants and some animals could not survive. This dust then settled onto the surface to form a thin layer of quartz dust that contains abnormally high levels of iridium. This geological landmark became known as the K-Pg boundary (K being the abbreviation of the Cretaceous period and Pg for the Paleogene period). Both the high amounts of iridium in asteroids and deep in Earth's crust at the same time that the dinosaurs went extinct helps confirm the theory that an asteroid impact is what resulted in the mass extinction of the dinosaurs. The K-Pg boundary is part of the fossil record and is visible in select locations around the globe, including the Badlands in Alberta, Canada, and at Stevns in south Denmark. Two other meteorite impacts that occurred 36 million years ago in Chesapeake Bay, USA and Popigai, Russia also left deposits of iridium, but these are not believed to have led to any mass extinctions.

Brachytherapy beads can be seen here for the treatment of prostate cancer where the radioactive source is placed right next to the tumor.

Modern Applications of Iridium

The high melting point, corrosion resistance, and hardness of iridium and its alloys have determined the vast majority of its uses today. However, due to its rarity, iridium is an expensive element with a price tag of about $1,000 per troy ounce (31.1 g or 0.0686 lbs) that has been quickly increasing in this last decade. This increase in price is related to the rise in demand of iridium for use in electronics-related applications. In the past, iridium alloys were utilized for coating the nibs of fountain pens and in compass bearings. In modern times, it has a wide range of uses, such as to produce long-life aircraft engine parts and to make crucibles that can withstand extremely high temperatures. Some high-grade automobile spark plugs are equipped with minuscule iridium tips that last 100,000 miles, far longer than conventional spark plugs. The radioactive isotope Ir-192 is a gamma-ray emitter that is used in radiation therapy for cancer patients through a process called brachytherapy. This type of radiotherapy involves placing a sealed radioactive source inside or right next to the area requiring treatment. Brachytherapy is used in prostate cancer, bile duct cancer (also called cholangiocarcinoma), cervical cancer, and breast cancer.