Iridium is a remarkably rare and dense metal found in specific concentrations both on Earth and in space. As a member of the Platinum Group Metals (PGMs), it is characterized by its silvery-white appearance and extreme resistance to corrosion. This scarcity in the Earth’s accessible crust, combined with its unique chemical properties, dictates where it is naturally found and how humanity accesses it. Understanding Iridium’s distribution requires examining the planet’s geological history and the cosmic events that shaped it.
The Rarity and Geochemical History
The scarcity of Iridium in the Earth’s crust is a direct consequence of planetary differentiation, the process by which the planet formed. Iridium is a highly siderophile element (HSE), meaning it has a strong chemical affinity for iron. When the early Earth was molten, materials separated based on density and chemical preference.
The vast majority of the planet’s Iridium dissolved into the heavy, liquid iron, which then sank to form the Earth’s core. This process stripped the upper layers, including the mantle and crust, of most of their original Iridium content. Estimates suggest that over 98% of the Earth’s Iridium is sequestered deep within the core, leaving the continental crust with an extremely low abundance, typically less than one part per billion. The small amount accessible today is thought to be residual material or later additions delivered by meteorites and volcanic activity.
Primary Global Mining Sources
The limited Iridium accessible in the Earth’s crust is not mined independently. It is recovered as a secondary product from deposits rich in other metals, as Iridium is concentrated in large-scale geological formations alongside other Platinum Group Metals (PGMs). The world’s primary source is the Bushveld Igneous Complex in South Africa, a massive layered intrusion holding the largest known reserves of PGMs on Earth.
This complex provides the majority of the global Iridium supply, extracted as a byproduct during the mining of platinum and palladium. Other significant terrestrial sources include the Noril’sk deposits in Russia, rich in nickel-copper sulfide ores. The Sudbury Basin in Canada, linked to an ancient impact event, also yields Iridium from its extensive nickel and copper mining operations. In these locations, Iridium exists in minute concentrations within the primary ores, requiring a complex and costly metallurgical process for separation.
Iridium as an Extraterrestrial Tracer
Iridium’s relative abundance in space rock, particularly in meteorites, provides a powerful tool for tracing cosmic impacts throughout Earth’s history. Because Iridium is rare in the Earth’s crust but plentiful in extraterrestrial material, a sudden spike in its concentration in geological layers is known as an Iridium anomaly. The most famous example is found in the Cretaceous-Paleogene (K-Pg) boundary layer, a thin band of clay deposited 66 million years ago.
This worldwide layer contains Iridium concentrations up to 100 times higher than the surrounding rock, serving as definitive evidence of a massive extraterrestrial impact. Scientists link this Iridium-rich layer directly to the Chicxulub impact event in the Yucatán Peninsula, where a large asteroid vaporized upon impact, scattering Iridium dust globally. Iridium is also found in ferromanganese crusts on the deep-sea floor, where it accumulates over vast periods as micrometeorites rain down on Earth. This slow deposition provides a continuous record of the influx of extraterrestrial material to the planet.