Formula: Ir
Native element, transition metal
Specific gravity: 22.6 to 22.8
Hardness: 6 to 7
Streak: White
Colour: Silver-white
Solubility: Resists attack by any acid
Melting point: 2447oC
Boiling point: 4430oC
Abundance in the Earth’s crust: 0.4 parts per billion by mass, 0.05 parts per billion by moles
Abundance in the Solar System: 2 parts per billion by mass, 0.01 parts per billion by moles
Common impurities: Os,Ru,Pt,Au,Pd,Cu,Fe

Plutonic igneous environments

Iridium occurs in platinum-bearing ultra-basic rocks and in placers derived from them, always alloyed with other platinum group elements (Dana). Associated minerals include platinum and platinum-iron alloys (HOM).

Abnormally high amounts of iridium have been found in rocks dating to the boundary between the Cretaceous and Tertiary periods (65 million years ago). This has led to a widely held view that an iridium-containing comet struck the Earth at that time, leading to the extinction of the dinosaurs and many other forms of life (ChC).

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