page_title: Iridium
section_title: Cretaceous–Paleogene boundary presence
hierarchical_section_title: Iridium / Occurrence / Cretaceous–Paleogene boundary presence
caption_attribution_description: English: K-T boundary (red arrow) along Interstate 25, Raton Pass, ColoradoDeutsch: KT-Grenze (Roter Pfeil) entlang der Interstate 25, Raton Pass, Colorado
context_page_description: Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, iridium is considered to be the second-densest metal with a density of 22.56 g/cm³ as defined by experimental X-ray crystallography. However, at room temperature and standard atmospheric pressure, iridium has been calculated to have a density of 22.65 g/cm³, 0.04 g/cm³ higher than osmium measured the same way. Still, the experimental X-ray crystallography value is considered to be the most accurate, as such iridium is considered to be the second densest element. It is the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.
  Iridium was discovered in 1803 among insoluble impurities in natural platinum. Smithson Tennant, the primary discoverer, named iridium after the Greek goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts.
context_section_description: The Cretaceous–Paleogene boundary of 66 million years ago, marking the temporal border between the Cretaceous and Paleogene periods of geological time, was identified by a thin stratum of iridium-rich clay. A team led by Luis Alvarez proposed in 1980 an extraterrestrial origin for this iridium, attributing it to an asteroid or comet impact. Their theory, known as the Alvarez hypothesis, is now widely accepted to explain the extinction of the non-avian dinosaurs. A large buried impact crater structure with an estimated age of about 66 million years was later identified under what is now the Yucatán Peninsula (the Chicxulub crater). Dewey M. McLean and others argue that the iridium may have been of volcanic origin instead, because Earth's core is rich in iridium, and active volcanoes such as Piton de la Fournaise, in the island of Réunion, are still releasing iridium.
caption: The red arrow points to the Cretaceous–Paleogene boundary.
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