Iridium is the silent witness of a catastrophe

Iridium is the silent witness of a catastrophe

The wafer-thin iridium-rich layer that geologists worldwide find in Earth’s layers became known as the characteristic boundary between the Cretaceous and Tertiary geological eras. It has an extraterrestrial origin, created after the devastating impact of the Chicxulub meteorite 66 million years ago. The blow was so great that 85 percent of life on Earth became extinct, including the dinosaurs.

The American geologist Walter Alvarez, and almost simultaneously his Dutch colleague Jan Smit, were the first to realize that the strikingly different composition of the layer between the Cretaceous and Tertiary sediments must be a silent witness to the catastrophe. The meteorite that was pulverized by the collision released large amounts of iridium-rich dust into the atmosphere, which then settled over a large area. The discovery of the impact crater near Mexico’s Yucatan Peninsula has confirmed that this scenario indeed occurred.

Iridium is one of the rarest metals on earth. The main source from which it can be extracted is platinum and nickel ore. Of all metals, iridium is the most corrosion resistant. It has a very high melting point and does not dissolve in strong acids. The latter property also led to the discovery of the element in 1803 by the British chemist Smithson Tennant. He was intrigued by the black residue left after platinum ore was dissolved in aqua regia, a mixture of three parts hydrochloric acid and one part nitric acid. While his predecessors thought the sediment was soot or lead, Tennant discovered two new chemical elements: iridium (atomic number 77) and also osmium (76).

Pure iridium is very brittle and almost impossible to machine. Yet applications were soon found for the new metal. In 1834 it turned out to be perfect for the hard, corrosion-resistant nibs in luxury gold fountain pens. After American chemists developed a new method for processing iridium, the belief in new applications became so great that they founded a specialized company for this purpose in Cincinnati in 1881: the American Iridium Company. The special metal was processed into ultra-fine hypodermic needles, the bearing of compass needles and into robust parts of telegraph machines.

On board satellites

The high melting point later made iridium ideally suited as a material for crucibles in which chemical reactions could take place at high temperatures and under aggressive conditions. Processed in a thermocouple, it was used to measure very high temperatures. Iridium is also used in the casing of nuclear generators on board satellites, because it is extremely heat-resistant and can easily withstand the high G-forces of launches.

Because it is so corrosion resistant, in 1889 the International Bureau of Weights and Measures chose a bar of 90 percent platinum and 10 percent iridium as the size for the standard meter. Two notches on this so-called X-meter, kept in Sèvres, France, defined the standard size until 1960. Nowadays it is much more precisely defined as the distance that light enters 1/299,792,458 second.