Iridium is a chemical element; it has the symbol Ir and atomic number 77. This very hard, brittle, silvery-white transition metal of the platinum group is considered the second-densest naturally occurring metal (after osmium), with a density of as defined by experimental X-ray crystallography.</ref> and by South American cultures Tennant, who had the advantage of a much greater amount of residue, continued his research and identified the two previously undiscovered elements in the black residue, iridium and osmium. Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.
Metalworking and applications
British scientist John George Children was the first to melt a sample of iridium in 1813 with the aid of "the greatest galvanic battery<!-- No page for "Galvanic Battery" --> that has ever been constructed" (at that time). discovered the resonant and recoil-free emission and absorption of gamma rays by atoms in a solid metal sample containing only <sup>191</sup>Ir. This phenomenon, known as the Mössbauer effect resulted in the awarding of the Nobel Prize in Physics in 1961, at the age 32, just three years after he published his discovery.
Occurrence
Along with many elements having atomic weights higher than that of iron, iridium is only naturally formed by the r-process (rapid neutron capture) in neutron star mergers and possibly rare types of supernovae.
thumb|upright=1.7|alt=Graph sowing on the x axis the elements by atomic number and on y-axis the amount in earth's crust compared to Si abundance. There is a green area with high abundance for the lighter elements between oxygen and iron. The yellow area with lowest abundant elements includes the heavier platinum group metals, tellurium and gold. The lowest abundance is clearly iridium. |Iridium is one of the least abundant elements in Earth's crust.
thumb|upright|The [[Willamette Meteorite has 4.7 ppm iridium.|alt=A large black egg-shaped boulder of porous structure standing on its top, tilted]]
Iridium is one of the nine least abundant stable elements in Earth's crust, having an average mass fraction of 0.001 ppm in crustal rock; gold is 4 times more abundant, platinum is 10 times more abundant, silver and mercury are 80 times more abundant. In contrast to its low abundance in crustal rock, iridium is relatively common in meteorites, with concentrations of 0.5 ppm or more.
<!-- upper crust in 10.1016/j.gca.2012.06.026-->
Iridium is found in nature as an uncombined element or in natural alloys, especially the iridium–osmium alloys osmiridium (osmium-rich) and iridosmium (iridium-rich). In nickel and copper deposits, the platinum group metals occur as sulfides, tellurides, antimonides, and arsenides. In all of these compounds, platinum can be exchanged with a small amount of iridium or osmium. As with all of the platinum group metals, iridium can be found naturally in alloys with raw nickel<!-- no page for "Raw Nickel" --> or raw copper. A number of iridium-dominant minerals, with iridium as the species-forming element, are known. They are exceedingly rare and often represent the iridium analogues of the above-given ones. The examples are irarsite<!-- No page for "irarsite" --> and cuproiridsite<!-- No page for "cuproiridsite" -->, to mention some.<!----> Within Earth's crust, iridium is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the Bushveld igneous complex in South Africa, and organisms is relatively low, as it does not readily form chloride complexes.
Iridium in sediments can come from cosmic dust, volcanoes, precipitation from seawater, microbial processes, or hydrothermal vents, For example, core samples from the Pacific Ocean with elevated iridium levels suggested the Eltanin impact of about 2.5 million years ago.
Cretaceous–Paleogene boundary presence
thumb|left|The red arrow points to the [[Cretaceous–Paleogene boundary.|alt=A cliff with pronounced layered structure: yellow, gray, white, gray. A red arrow points between the yellow and gray layers.]]
The Cretaceous–Paleogene boundary of 66 million years ago, marking the temporal border between the Cretaceous and Paleogene periods of geological time, was identified by a thin stratum of iridium-rich clay. Their theory, known as the Alvarez hypothesis, is now widely accepted to explain the extinction of the non-avian dinosaurs. A large buried impact crater structure with an estimated age of about 66 million years was later identified under what is now the Yucatán Peninsula (the Chicxulub crater). Dewey M. McLean and others argue that the iridium may have been of volcanic origin instead, because Earth's core is rich in iridium, and active volcanoes such as Piton de la Fournaise, in the island of Réunion, are still releasing iridium.
Production
{|class="wikitable" style="text-align:center; float:right; margin-left:0.5em"
!Year!!Consumption<br />(tonnes)!!Price (US$)
|-
|2001|| 2.6||
|-
|2002||2.5 ||
|-
|2003|| 3.3||
|-
|2004||3.60 ||
|-
|2005||3.86 ||
|-
|2006||4.08 ||
|-
|2007||3.70 ||
|-
|2008||3.10 ||
|-
|2009||2.52||
|-
|2010||10.40||
|-
|2011||9.36||
|-
|2012||5.54||
|-
|2013||6.16||
|-
|2014||6.1||
|-
|2015||7.81||
|-
|2016||7.71||
|-
|2017||n.d.||
|-
|2018||n.d.||
|-
|2019||n.d.||
|-
|2020||n.d.||
|-
|2021||n.d.||
|-
|2022||n.d.||
|-
|2023||n.d.||
|-
|2024||n.d.||
|}
<!-- Should be updated sometime! A good source is http://www.platinum.matthey.com/prices/price-charts -->
Worldwide production of iridium was about in 2018. The price is high and varying (see table). Iridium reached a high of US$8000 (per troy ounce) in March 2026 (which is also the record high for iridium when adjusted for inflation). Illustrative factors that affect the price include oversupply of Ir crucibles
and changes in LED technology.
Platinum metals occur together as dilute ores. Iridium is one of the rarer platinum metals: for every 190 tonnes of platinum obtained from ores, only 7.5 tonnes of iridium is isolated. To separate the metals, they must first be brought into solution. Two methods for rendering Ir-containing ores soluble are (i) fusion of the solid with sodium peroxide followed by extraction of the resulting glass in aqua regia and (ii) extraction of the solid with a mixture of chlorine with hydrochloric acid. The first method is similar to the procedure Tennant and Wollaston used for their original separation. The second method can be planned as continuous liquid–liquid extraction and is therefore more suitable for industrial scale production. In either case, the product, an iridium chloride salt, is reduced with hydrogen, yielding the metal as a powder or sponge, which is amenable to powder metallurgy techniques. Iridium is also obtained commercially as a by-product from nickel and copper mining and processing. During electrorefining of copper and nickel, noble metals such as silver, gold and the platinum group metals as well as selenium and tellurium settle to the bottom of the cell as anode mud, which forms the starting point for their extraction.
|-
! Country !! 2016 !! 2017 !! 2018 !! 2019 !! 2020
|- class="static-row-header " style="font-weight:bold;" class=sorttop
| || 7,720 || 7,180 || 7,540 || 7,910 || 8,170
|-
| || 6,624 || 6,057 || 6,357 || 6,464 || 6,786
|-
| || 598 || 619 || 586 || 845 || 836
|-
| || 300 || 200 || 400 || 300 || 300
|-
| || 200 || 300 || 200 || 300 || 250
|}<!-- Year book of USGS is always two years old -->
Applications
Due to iridium's resistance to corrosion, it has industrial applications. The main areas of use are electrodes for producing chlorine and other corrosive products, OLEDs, crucibles, catalysts (e.g. acetic acid), and ignition tips for spark plugs. The crystals, such as gadolinium gallium garnet and yttrium gallium garnet, are grown by melting pre-sintered charges of mixed oxides under oxidizing conditions at temperatures up to . Osmium–iridium is used for compass bearings and for balances. and iridium-based spark plugs are particularly used in aviation.
Catalysis
Iridium compounds are used as catalysts in the Cativa process for carbonylation of methanol to produce acetic acid.
Iridium complexes are often active for asymmetric hydrogenation both by traditional hydrogenation. and transfer hydrogenation. This property is the basis of the industrial route to the chiral herbicide (S)-metolachlor. As practiced by Syngenta on the scale of 10,000 tons/year, the complex [Ir(COD)Cl]<sub>2</sub> in the presence of Josiphos ligands.
Medical imaging
The radioisotope iridium-192 is one of the two most important sources of energy for use in industrial γ-radiography for non-destructive testing of metals. Additionally, is used as a source of gamma radiation for the treatment of cancer using brachytherapy, a form of radiotherapy where a sealed radioactive source is placed inside or next to the area requiring treatment. Specific treatments include high-dose-rate prostate brachytherapy, biliary duct brachytherapy, and intracavitary cervix brachytherapy.
Photocatalysis and OLEDs
Iridium complexes are key components of white OLEDs. Similar complexes are used in photocatalysis.
Scientific
thumb|[[International Prototype Meter bar|alt=NIST Library US Prototype meter bar]]
An alloy of 90% platinum and 10% iridium was used in 1889 to construct the International Prototype Meter and kilogram mass, kept by the International Bureau of Weights and Measures near Paris. but the kilogram prototype remained the international standard of mass until 20 May 2019, when the kilogram was redefined in terms of the Planck constant.
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Iridium is often used as a coating for non-conductive materials in preparation for observation in scanning electron microscopes (SEM).
The addition of a layer of iridium helps especially organic materials survive electron beam damage and reduces static charge build-up within the target area of the SEM beam's focal point. A coating of iridium also increases the signal to noise ratio associated with secondary electron emission which is essential to using SEMs for X-Ray spectrographic composition analysis. While other metals can be used for coating objects for SEM use, iridium is the preferred coating when samples will be studied with a wide variety of imaging parameters.
Iridium has been used in the radioisotope thermoelectric generators of unmanned spacecraft such as the Voyager, Viking, Pioneer, Cassini, Galileo, and New Horizons. Iridium was chosen to encapsulate the plutonium-238 fuel in the generator because it can withstand the operating temperatures of up to and for its great strength. The mirrors of the Chandra X-ray Observatory are coated with a layer of iridium thick. Iridium proved to be the best choice for reflecting X-rays after nickel, gold, and platinum were also tested. The iridium layer, which had to be smooth to within a few atoms, was applied by depositing iridium vapor under high vacuum on a base layer of chromium.
Iridium is used in particle physics for the production of antiprotons, a form of antimatter. Antiprotons are made by shooting a high-intensity proton beam at a conversion target, which needs to be made from a very high density material. Although tungsten may be used instead, iridium has the advantage of better stability under the shock waves induced by the temperature rise due to the incident beam.
Iridium forms a variety of complexes of fundamental interest in triplet harvesting.
As part of the cationic nucleic acid intercalators, iridium is used to detect nucleic acids in CyTOF experiments to analyse the presence or viability of nucleated cells in biological samples.
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Historical
right|thumb|[[Fountain pen nib labelled Iridium Point]]
Iridium–osmium alloys were previously used in fountain pen nib tips. The first major use of iridium was in 1834 in nibs mounted on gold.
An iridium–platinum alloy was used for the touch holes or vent pieces of cannon. According to a report of the Paris Exhibition of 1867, one of the pieces being exhibited by Johnson and Matthey "has been used in a Whitworth gun for more than 3000 rounds, and scarcely shows signs of wear yet. Those who know the constant trouble and expense which are occasioned by the wearing of the vent-pieces of cannon when in active service, will appreciate this important adaptation".
The pigment iridium black, which consists of very finely divided iridium, is used for painting porcelain an intense black; it was said that "all other porcelain black colors appear grey by the side of it".
Precautions and hazards
Iridium in bulk metallic form is not biologically important or hazardous to health due to its lack of reactivity with tissues; there are only about 20 parts per trillion of iridium in human tissue. Iridium is relatively non-hazardous otherwise, with the only effect of Iridium ingestion being irritation of the digestive tract. However, soluble salts, such as the iridium halides, could be hazardous due to elements other than iridium or due to iridium itself. High-energy gamma radiation from can increase the risk of cancer. External exposure can cause burns, radiation poisoning, and death. Ingestion of <sup>192</sup>Ir can burn the linings of the stomach and the intestines. <sup>192</sup>Ir, <sup>192m</sup>Ir, and <sup>194m</sup>Ir tend to deposit in the liver, and can pose health hazards from both gamma and beta radiation.
Notes
References
External links
- Iridium at The Periodic Table of Videos (University of Nottingham)
- Iridium in Encyclopædia Britannica