Iridium
A refractory metal found in tiny nuggets in the oldest inclusions, and the tell-tale element of impacts.
Key facts
- Formula
- (Ir,Os,Ru)
- Family
- Metals and alloys
- Crystal system
- Cubic
What it is
Native iridium is a dense, very hard platinum-group metal that melts at about 2,450 °C. In meteorites it occurs alloyed with osmium, ruthenium, platinum and other refractory metals, which condensed from hot solar gas before almost anything else.
Where it is found
It forms refractory metal nuggets, often less than a micrometre across, in calcium-aluminium-rich inclusions of carbonaceous chondrites such as Allende. Iridium is far more abundant in chondrites than in Earth's crust, which is why a layer enriched in it marks the impact that ended the age of the dinosaurs.
How it is recognised
The nuggets are found with the scanning electron microscope, where their heavy elements shine brightly in backscattered electron images.
Did you know?
The iridium anomaly at the boundary between the Cretaceous and Paleogene was found in 1980 and led to the idea of the Chicxulub impact.
Iridium is one of the rarest elements in Earth's crust.