Iridium

Iridium is a dense, corrosion-resistant transition metal with the chemical symbol Ir and atomic number 77. It belongs to the platinum group metals (PGMs) and is one of the rarest naturally occurring elements in the Earth’s crust. Exhibiting a brilliant silvery-white lustre, iridium is renowned for its exceptional hardness, high melting point, and unparalleled resistance to chemical attack. Discovered in 1803 by the English chemist Smithson Tennant, iridium was named after Iris, the Greek goddess of the rainbow, due to the colourful iridescent hues of its salts. Though it is scarce and difficult to work with, iridium plays a vital role in modern industry, advanced technology, and scientific instrumentation.

Discovery and Natural Occurrence

Iridium was identified in the residues left after dissolving crude platinum in aqua regia. Tennant’s discovery of iridium and osmium marked a significant expansion of the platinum group. Iridium occurs naturally in association with platinum and other noble metals in alluvial deposits and nickel–copper ores. Major sources of iridium are found in South Africa, Russia, and Canada, primarily as a by-product of nickel and platinum mining.
Geologically, iridium is of particular scientific interest because of its high concentration in meteorites and certain sedimentary layers, notably the Cretaceous–Palaeogene (K–Pg) boundary, where elevated iridium levels provide evidence for the asteroid impact that led to the extinction of the dinosaurs around 66 million years ago.

Physical and Chemical Properties

Iridium is one of the densest and most corrosion-resistant elements known. Its unique combination of physical and chemical properties makes it valuable for specialised industrial applications.
Key characteristics include:

• Atomic weight: 192.22
• Melting point: 2446 °C
• Boiling point: 4428 °C
• Density: 22.56 g/cm³ (second only to osmium)
• Crystal structure: face-centred cubic (FCC)
• Electrical conductivity: high
• Chemical stability: resistant to acids, bases, and oxidation even at high temperatures

Iridium forms a few stable oxidation states, mainly +3 and +4, and its compounds such as iridium(IV) oxide (IrO₂) and iridium(III) chloride (IrCl₃) are important in catalysis and electrochemistry.

Everyday and Consumer Applications

Although iridium is not encountered directly in daily life, it underpins numerous technologies that affect modern living. Its durability, conductivity, and stability make it essential in a variety of high-performance consumer and industrial products.

• Electronics and Mobile Devices: Iridium is used in electrical contacts and connectors due to its excellent resistance to wear and corrosion. These components appear in mobile phones, computers, and automotive electronics, ensuring long-lasting performance.
• Jewellery and Watches: Because of its bright lustre and resistance to tarnish, iridium is alloyed with platinum to enhance strength and hardness in fine jewellery and luxury watch cases.
• Medical Devices: Iridium is used in pacemaker electrodes, catheters, and radiation therapy sources. The radioactive isotope iridium-192 is widely employed in brachytherapy, a form of targeted cancer treatment.
• Writing Instruments: The durable tips of high-end fountain pens often contain iridium, valued for its smooth writing quality and wear resistance.

These everyday applications illustrate how iridium, despite its rarity, supports the durability, precision, and reliability expected in modern consumer goods.

Industrial and Technological Applications

Iridium’s most significant uses lie in industry and advanced technology. Its properties make it indispensable where extreme temperatures, corrosive environments, and precision engineering are required.
1. Catalysis and Chemical IndustryIridium and its compounds serve as catalysts in several crucial chemical processes, particularly in hydrogenation, dehydrogenation, and carbon–hydrogen activation reactions. Iridium catalysts are used in the production of acetic acid, fine chemicals, and pharmaceutical intermediates.
Iridium oxide (IrO₂) and mixed iridium–ruthenium oxides are employed as electrocatalysts in chlor-alkali cells and water electrolysis systems, supporting the generation of chlorine, sodium hydroxide, and hydrogen fuel. In emerging energy technologies, iridium catalysts play a vital role in proton exchange membrane (PEM) electrolysers, which produce green hydrogen—an area of growing economic importance in the global energy transition.
2. Aerospace and Automotive EngineeringDue to its exceptional heat and corrosion resistance, iridium is used in spark plugs for high-performance engines, including aircraft and premium vehicles. Iridium-tipped spark plugs last significantly longer than traditional ones and provide more efficient ignition.
In the aerospace sector, iridium coatings and alloys withstand re-entry heat and oxidation, serving in rocket engine components, satellite systems, and electrical contacts exposed to extreme conditions.
3. Scientific Instruments and Measurement DevicesIridium is a key material in crucibles used for the growth of single crystals of semiconductors, such as gallium arsenide. These crucibles must resist temperatures above 2000 °C without contamination, a requirement few materials can meet.
Historically, iridium was used in the international prototype metre bar and the kilogram standard, alloyed with platinum to ensure stability and resistance to corrosion. Though replaced by definitions based on fundamental constants, these standards demonstrate iridium’s role in precision metrology.
4. Electronics and Thin-Film TechnologyIridium films are used in microelectronic devices, data storage, and thin-film resistors due to their durability and conductivity. In OLED (organic light-emitting diode) technology, iridium complexes function as phosphorescent materials, improving the efficiency and brightness of display screens in televisions and smartphones.

Economic Importance and Market Dynamics

Iridium is one of the most expensive industrial metals, largely due to its rarity and the complexity of extraction. It occurs at concentrations of less than 0.001 parts per million in the Earth’s crust. Global annual production averages only 6 to 8 tonnes, primarily as a by-product of platinum and nickel refining.
Economic characteristics include:

• Major producers: South Africa, Russia, and Canada dominate global output.
• Market value: Prices are volatile but can range between US$4,000 and US$6,000 per troy ounce, depending on industrial demand.
• Strategic importance: Iridium is listed as a critical raw material in several countries due to its essential role in clean energy technologies and limited availability.
• Recycling: A significant portion of iridium used in industry is recycled from spent catalysts and scrap electronic components, helping offset limited natural supply.

The growth of hydrogen fuel technologies, semiconductor manufacturing, and advanced materials is expected to sustain strong demand for iridium in coming decades.

Environmental and Safety Considerations

Iridium metal and most of its compounds are considered chemically inert and non-toxic, posing minimal environmental hazard. However, some iridium salts can cause irritation or allergic reactions if mishandled. Radioactive isotopes, particularly iridium-192, require strict regulatory control due to their potential radiation hazards.
Mining and refining iridium are energy-intensive processes associated with the environmental impact of platinum group metal extraction. Consequently, recycling and reprocessing are becoming increasingly important in reducing the ecological footprint of iridium-based industries.

Strategic and Future Outlook

Iridium’s future significance lies in its expanding use across clean energy, aerospace, and digital technologies. Its role in hydrogen production through PEM electrolysers positions it at the forefront of the global transition to renewable energy. Furthermore, continued demand for high-efficiency OLED displays, wear-resistant coatings, and radiation therapy sources will sustain its economic value.
Ongoing research aims to develop iridium-based nanomaterials for catalysis, quantum devices, and next-generation electronics, where its stability and conductivity can be leveraged at the atomic scale. However, given its rarity and cost, technological innovation will also focus on reducing iridium dependency through alloy optimisation and recycling efficiency.