By Ron B. Davis Jr., Georgetown University
Osmium and iridium, two exceptionally rare elements here on Earth, are the true gold standard of durability and heat resistance in engineering. But probably the greatest impact of iridium on science was Walter Alvarez’s breakthrough idea to use this element to understand what caused the extinction of the dinosaurs.
Osmium and Iridium
All six of the platinum group elements tend to cluster together, both on the table and in nature, but there is one pair that is especially cozy—osmium and iridium. Osmium and iridium mix so well and so easily that they are often found in nature as an alloy.
When the alloy has a greater percentage of iridium, it has been called ‘osmiridium’. When osmium is dominant, the alloy takes the name ‘iridosmine’. Both osmium and iridium have a relatively low abundance compared to many of the lighter transition metals.
And at Earth’s surface, iridium is remarkably rare compared to its average abundance in the solar system. Both are far less common than even the precious metals platinum or gold. Most of the iridium and osmium that was part of the early Earth more than four billion years ago sank into the iron-rich core along with the other siderophilic elements. Odd-numbered iridium is even more scarce then even-numbered osmium, as the Oddo-Harkins rule predicts.
Specialty Applications
Despite their scarcity, osmium and iridium are sought-after metals for specialty applications. Their position deep within the d-block in the sixth row explains their extreme hardness and high melting points. Combine that with their high resistance to corrosion, and they both make a great choice for the manufacture of mechanical bearings and electrical contacts in spark plugs.
But if iridium and osmium are a celebrity couple, iridium is definitely the A-lister. Because of its extreme scarcity on Earth’s surface, iridium offered a critical clue to unravelling the greatest murder mystery in Earth’s history.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Luis and Walter Alverez
The detectives who solved the case were the physics Nobel laureate and Manhattan Project veteran, Luis Alverez and his son, Walter—a geologist. Together, they formulated the theory that a tremendous spike in iridium levels found in a layer of rock from 65 million years ago might explain the extinction of the dinosaurs.
The elder Alverez had already made a name for himself in the world of nuclear physics, winning his Nobel Prize in 1968.
Alvarez’s greatest contribution to science, post-Nobel Prize, might just be his brilliant discovery that evidence for a great mass extinction on Earth some 65 million years ago was hidden in rocks all across the planet. That evidence was in the form of iridium. His remarkable ability to merge physics with other branches of science wasn’t lost on Alvarez, who once quipped about his most famous project, “you might say that I’m practicing geology without a license”.
Iridium, the Evidence of the Impact
Although iridium is a siderophile that is rare in the Earth’s crust, it is actually much more abundant in our solar system. Not only is there likely to be lots more iridium trapped in the core of the planet, there is also a ‘lot’ more iridium in our solar system that didn’t begin on our planet at all.
Countless meteoroids are in motion in our solar system. The individual pieces of these heavenly bodies can have elemental profiles that are often quite different from Earth’s surface. Those that are metallic in composition are much, much higher in iridium content than the surface of our planet.
This means that when metallic meteoroids fall to Earth as meteorites, they can deliver huge quantities of scarce metals like iridium to Earth’s surface. If such an impact were of sufficient energy, that iridium could be scattered across the entire globe, leaving evidence of the impact that might endure in the geologic record for millions of years.
Neutron-activation Analysis
The Alvarez’s chose to take samples from a stretch of limestone in central Italy that was known to have been deposited about 65 million years ago, the time period during which the dinosaurs suddenly vanished from the face of the Earth. Fortunately, it turned out not to matter that they were about six thousand miles from the actual impact site.
Alvarez married his knowledge of physics with his passion for geoscience, prospecting for iridium in their samples using a technique called neutron-activation analysis. This process is strikingly similar to that by which cobalt made Szilard’s doomsday bomb operate. By bombarding the samples with a neutron beam, iridium-191 is converted to iridium-192, a radioactive isotope.
As the neutron-rich iridium-192 isotope undergoes beta decay to form the stable isotope platinum-192, it also releases gamma radiation. By measuring the intensity of this specific wavelength of gamma radiation, Alvarez was able to determine the iridium content of his geological samples.
Vindication of the Alvarez Theory
The results of their 1980 study were jaw-dropping. The iridium levels in the narrow band of clay dated to 65 million years ago, were one hundred times higher—or more—than nearby rocks formed just before and after the event was thought to have taken place.
In the following years, others joined the search and found high levels of iridium in rocks from the same time period in many other places around the world.
The final vindication of the Alvarez theory would come in 1991, when evidence connected the worldwide iridium anomaly to a massive crater, just off the Yucatan peninsula in Mexico. The meteorite that killed the dinosaurs had been tracked down, and a 65-million-year-old cold case solved, thanks to clues left by iridium.
Common Questions about Osmium and Iridium
When the alloy has a greater percentage of iridium, it has been called ‘osmiridium’. When osmium is dominant, the alloy takes the name ‘iridosmine’.
By measuring the intensity of this specific wavelength of gamma radiation, Walter Alvarez was able to determine the iridium content of his geological samples.
The final vindication of the Alvarez theory would come in 1991, when evidence connected the worldwide iridium anomaly to a massive crater, just off the Yucatan peninsula in Mexico.
