Along with many elements having
atomic weights higher than that of iron, iridium is only naturally formed by the
r-process (rapid
neutron capture) in
neutron star mergers and possibly rare types of supernovae. has 4.7 ppm iridium.|alt=A large black egg-shaped boulder of porous structure standing on its top, tilted Iridium is one of the nine least abundant stable
elements in
Earth's crust, having an average
mass fraction of 0.001
ppm in crustal rock;
gold is 4 times more abundant,
platinum is 10 times more abundant,
silver and
mercury are 80 times more abundant. In contrast to its low abundance in crustal rock, iridium is relatively common in
meteorites, with concentrations of 0.5 ppm or more. Iridium is found in nature as an uncombined element or in natural
alloys, especially the iridium–
osmium alloys
osmiridium (osmium-rich) and
iridosmium (iridium-rich). In
nickel and copper deposits, the
platinum group metals occur as
sulfides,
tellurides,
antimonides, and
arsenides. In all of these compounds,
platinum can be exchanged with a small amount of iridium or osmium. As with all of the platinum group metals, iridium can be found naturally in alloys with raw nickel or
raw copper. A number of iridium-dominant
minerals, with iridium as the species-forming element, are known. They are exceedingly rare and often represent the iridium analogues of the above-given ones. The examples are irarsite and cuproiridsite, to mention some. Within Earth's crust, iridium is found at highest concentrations in three types of
geologic structure:
igneous deposits (crustal intrusions from below),
impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the
Bushveld igneous complex in South Africa, and organisms is relatively low, as it does not readily form
chloride complexes. Iridium in sediments can come from
cosmic dust, volcanoes,
precipitation from seawater, microbial processes, or
hydrothermal vents, For example, core samples from the Pacific Ocean with elevated iridium levels suggested the
Eltanin impact of about 2.5 million years ago.
Cretaceous–Paleogene boundary presence .|alt=A cliff with pronounced layered structure: yellow, gray, white, gray. A red arrow points between the yellow and gray layers. 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. 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. == Production ==