Geologic Distribution: Iridium has not been reported in any specific minerals in coal. Iridium was detected in only one sample of 546 analyzed. This bench of the No.2 Gas coal contained 0.945 ppm on a whole coal basis. Iridium is known geologically as an indicator of cosmic impact events such as the asteroid impact 65 million years ago that resulted in the demise of the dinosaurs. Volcanic eruptions also release Ir with the ash. Iridium is the most corrosion-resistant metal known and is not attacked by any acids, therefore once it is introduced into a coal forming peat swamp it is very likely to remain in the coal.
Economic Impact: Iridium is used in making crucibles and other apparatus for use at high temperatures, electrical contacts and as a hardening agent in platinum 1. An alloy of Ir and osmium is used to make compass bearings and ball point pen tips 1.
Environmental Impact: Iridium and other platinum group elements in coal have no environmental impact in coal mining and use 3.
1. CRC Handbook of Chemistry and Physics
2. Oman, C. L., Finkelman, R.B., and Tewalt, S.J., 1997
3. Swaine, D.J., (1990)
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