iridium

iridium. A member of the platinum group metals, it is a remarkably dense, brittle, and corrosion-resistant transition metal. It is the second-densest naturally occurring element, following osmium, and is renowned for its exceptional resistance to attack by most acids, molten metals, and silicates. Discovered in the early 19th century, its primary modern applications are in high-performance spark plugs, crucibles for crystal growth, and as a hardening agent in specialized alloys, while its geological signature plays a crucial role in astrogeology.

Properties

Iridium is characterized by its extreme hardness and high melting point, which exceeds that of platinum. It possesses a very high density and a pronounced resistance to corrosion, even at elevated temperatures; it is not attacked by any acid, including aqua regia, though it can be dissolved by certain molten salts like sodium cyanide. The element exhibits a face-centered cubic crystal structure over a wide temperature range. Its most common oxidation states are +3 and +4, with compounds such as iridium(IV) chloride and iridium(III) chloride being well-studied. Complexes of iridium, including Vaska's complex and those used in OLED technology, are significant in organometallic chemistry and catalysis, particularly for processes like Catalytic reforming.

Occurrence and production

Native iridium is rarely found in pure form and is most commonly obtained as a by-product of nickel and copper mining and refining. It is one of the least abundant elements in the Earth's crust, with its concentration in the planetary crust being exceptionally low. Commercially, it is recovered from ores containing other platinum group metals, primarily from deposits in the Bushveld Igneous Complex in South Africa and the Norilsk region in Russia. A globally significant, though geologically rare, source is the Cretaceous–Paleogene boundary layer, where an anomalously high concentration provides key evidence for the Alvarez hypothesis of an asteroid impact event. The refining process typically involves complex steps of dissolution, precipitation, and reduction, often carried out by companies like Impala Platinum and Anglo American Platinum.

Applications

The primary use of iridium is in the manufacture of high-performance spark plug electrodes for aviation and high-efficiency engines due to its durability and high melting point. It is indispensable in the chemical industry for crucibles used in the Czochralski process to grow large, high-quality single crystals of oxides like sapphire. As a hardening agent, it forms alloys with osmium for fountain pen nibs and with platinum for standard mass prototypes, such as the former International Prototype of the Kilogram housed at the International Bureau of Weights and Measures. Its compounds serve as catalysts in the Cativa process for acetic acid production and in electrolysis for chloralkali process cells. Radioactive iridium-192 is a vital gamma-ray source in industrial radiography and brachytherapy for treating cancers.

History

The element was discovered in 1803 by Smithson Tennant in London while working with insoluble residues left after dissolving crude platinum in aqua regia. He named it for Iris, the Greek goddess of the rainbow, due to the striking and varied colors of its salts. Its first major application was in alloy form for the tips of fountain pen nibs. The scientific importance of iridium expanded dramatically in 1980 when a team led by Luis Alvarez and Walter Alvarez discovered a global layer enriched with the element coinciding with the Cretaceous–Paleogene extinction event, providing pivotal support for the theory that the extinction was caused by a massive impact event.

Precautions

Iridium in bulk solid form poses little health risk due to its corrosion resistance and inertness. However, iridium compounds should be handled with care as they can be strong oxidizing agents or irritants, and fine iridium powder is a potential fire hazard. The primary safety concern involves the radioisotope iridium-192, a powerful gamma emitter used in industrial radiography; its use is strictly regulated by agencies like the Nuclear Regulatory Commission to prevent radiation poisoning and radioactive contamination. Disposal of iridium-containing materials, particularly radioactive sources, must follow protocols established by bodies such as the International Atomic Energy Agency.

Category:Chemical elements Category:Transition metals Category:Platinum group metals