Understanding Iridium from Platinum Group Ores — The Asteroid Impact Witness

Smithson Tennant discovered iridium simultaneously with osmium in 1803, finding both in the insoluble residue left after dissolving crude platinum in aqua regia. He named iridium from the Greek 'iris' (rainbow) because its salts form a striking range of colors — from deep red to green, blue, and violet. Iridium is the second-densest element (22.56 g/cm³) and the most corrosion-resistant metal known.

In 1980, Luis and Walter Alvarez discovered an iridium-rich clay layer at the Cretaceous-Paleogene boundary — 66 million years old. Iridium is rare in Earth's crust (0.001 ppm) but abundant in asteroids. The anomalous iridium concentration (30 times normal) at this boundary proved that a massive asteroid impact caused the mass extinction that killed the non-avian dinosaurs. This iridium anomaly has been found at over 100 sites worldwide.

The International Prototype of the Kilogram — a cylinder of 90% platinum and 10% iridium — defined the kilogram from 1889 to 2019. Stored under three nested bell jars at the Bureau International des Poids et Mesures in Sèvres, France, it was the last physical artifact defining an SI unit. The platinum-iridium alloy was chosen for its extreme hardness, density, oxidation resistance, and dimensional stability. The kilogram was redefined in terms of the Planck constant in 2019.

Iridium-tipped spark plugs are the premium standard in modern engines. The iridium center electrode is only 0.4 mm diameter — much thinner than conventional nickel plugs — creating a more focused spark with lower voltage requirements. Iridium's melting point (2,466°C) and hardness ensure the tip barely erodes, lasting 100,000+ miles versus 30,000 for standard plugs. Most modern high-performance and fuel-efficient engines are factory-equipped with iridium spark plugs.

Iridium crucibles withstand temperatures above 2,000°C and resist attack by virtually all chemical reagents — essential for growing high-purity single crystals of materials like sapphire (Al₂O₃), yttrium aluminum garnet (YAG), and lithium niobate for optical and electronic applications. Sapphire crystals grown in iridium crucibles are used for LED substrates and smartphone camera lens covers. Each sapphire crystal furnace uses an iridium crucible worth $50,000-100,000.

Iridium-192 (half-life 73.8 days) is the most widely used gamma source in industrial radiography and brachytherapy (internal radiation therapy for cancer). In brachytherapy, tiny Ir-192 seeds are placed directly inside or next to tumors — particularly cervical, endometrial, and head/neck cancers — delivering high radiation doses to the tumor while sparing surrounding tissue. Ir-192 produces gamma rays at energies ideal for both imaging and therapy.

Iridium oxide (IrO₂) is the most efficient and stable anode catalyst for the oxygen evolution reaction in proton exchange membrane (PEM) water electrolyzers that produce green hydrogen. No other material matches iridium oxide's combination of catalytic activity and corrosion resistance in the extremely acidic conditions inside PEM electrolyzers. The growing green hydrogen economy is creating unprecedented demand for iridium — a potential supply bottleneck for the energy transition.

Iridium is a silvery-white metal, extremely hard and brittle, with the second-highest density of any element. Melting point is 2,466°C. It is the most corrosion-resistant metal known — insoluble in all acids and acid mixtures including aqua regia at any temperature. Only molten alkali metal hydroxides and halogens at high temperatures attack it. Iridium is obtained by dissolving PGM concentrates in chlorine gas at high pressure, then selectively precipitating each metal.

Annual iridium production is approximately 7 tonnes — among the smallest of any industrial metal. South Africa produces 85% from the Bushveld Complex. Iridium is priced at $4,000-6,000 per troy ounce, making it one of the most expensive metals. The PEM electrolyzer industry's growing demand for iridium may consume more than current production within a decade. Iridium recycling from spent catalysts and crucibles recovers about 30% of annual demand.

Record iridium's key data: atomic number 77, density 22.56 g/cm³, melting point 2,466°C, silvery-white metal. Iridium's story spans from witnessing the end of the dinosaurs to potentially enabling the green energy revolution. Its extreme rarity and irreplaceable role in PEM electrolyzers for hydrogen production make it a critical bottleneck element. Research into reducing iridium loading in electrolyzers and finding non-iridium catalysts is one of the most active areas in energy materials science.