View source for Iridium

{{distinguish|Iron|Indium}}

{{About|the chemical element}}
{{pp-move}}
{{Infobox iridium}}

'''Iridium''' is a [[chemical element]]; it has [[Symbol (chemistry)|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 {{cvt|22.56|g/cm3}} as defined by experimental [[X-ray crystallography]].{{efn|At room temperature and standard atmospheric pressure, iridium has been calculated to have a density of {{cvt|22.65|g/cm3}}, {{cvt|0.04|g/cm3}} higher than osmium measured the same way. Still, the experimental X-ray crystallography value is considered to be the most accurate, and as such iridium is considered to be the second densest element.<ref>{{cite journal |journal=Platinum Metals Rev. |year=1989 |volume=33 |issue=1 |pages=14–16 |title=Densities of Osmium and Iridium Recalculations Based upon a Review of the Latest Crystallographic Data |last=Arblaster |first=J. W. |doi=10.1595/003214089X3311416 |s2cid=267570193 |url=https://www.technology.matthey.com/article/33/1/14-16/ |url-access=subscription }}</ref>}} <sup>191</sup>Ir and <sup>193</sup>Ir are the only two naturally occurring [[isotope]]s of iridium, as well as the only [[stable isotope]]s; the latter is the more abundant. It is one of the most [[corrosion]]-resistant metals, even at temperatures as high as {{cvt|2000|°C}}.

Iridium was discovered in 1803 in the acid-insoluble residues of [[platinum]] ores by the British chemist [[Smithson Tennant]]. The name ''iridium'', derived from the Greek word ''iris'' (rainbow), refers to the various colors of its compounds. Iridium is [[Abundance of elements in Earth's crust|one of the rarest elements]] in [[Crust (geology)#Earth's crust|Earth's crust]], with an estimated annual production of only {{convert|15,000|lb|order=flip}} in 2023.

The dominant uses of iridium and its alloys are in high-performance [[spark plug]]s, [[crucible]]s for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the [[chloralkali process]]. Important compounds of iridium are chlorides and iodides in industrial [[catalysis]]. Iridium is a component of some [[OLED]]s.

Iridium is found in [[meteorite]]s in much higher abundance than in the Earth's crust.<ref name="Becker2002">{{cite journal |first1=Luann |last1=Becker |url=http://www.miracosta.edu/home/kmeldahl/articles/blows.pdf |title=Repeated Blows |access-date=January 19, 2016 |journal=Scientific American |year=2002 |volume=286 |issue=3 |pages=77–83 |bibcode=2002SciAm.286c..76B |doi=10.1038/scientificamerican0302-76 |pmid=11857903}}</ref> For this reason, the unusually high abundance of iridium in the clay layer at the [[Cretaceous–Paleogene boundary]] gave rise to the [[Alvarez hypothesis]] that the impact of a massive extraterrestrial object caused the [[Cretaceous–Paleogene extinction event|extinction of non-avian dinosaurs and many other species 66&nbsp;million years ago]], now known to be produced by the impact that formed the [[Chicxulub crater]]. Similarly, an iridium anomaly in core samples from the Pacific Ocean suggested the [[Eltanin impact]] of about 2.5&nbsp;million years ago.<ref name="Kyte1981">{{cite journal |last=Kyte |first=Frank T. |author2=Zhiming Zhou |author3=John T. Wasson |authorlink3=John T. Wasson |date=1981 |title=High noble metal concentrations in a late Pliocene sediment |journal=Nature |volume=292 |issue=5822 |pages=417–420 |issn=0028-0836 |doi=10.1038/292417a0 |bibcode=1981Natur.292..417K |s2cid=4362591}}</ref>

== Characteristics ==

=== Physical properties ===
[[File:iridium2.jpg|left|thumb|{{convert|1|ozt|g|4|spell=In|abbr=off|lk=on}} of arc-melted iridium|alt=A flattened drop of dark gray substance]]
A member of the [[platinum group]] metals, iridium is white, resembling platinum, but with a slight yellowish cast. Because of its hardness, brittleness, and very high [[melting point]], solid iridium is difficult to machine, form, or work; thus [[powder metallurgy]] is commonly employed instead.<ref name="greenwood" /> It is the only metal to maintain good mechanical properties in air at temperatures above {{convert|1600|C|F}}.<ref name="hunt">{{cite journal |title=A History of Iridium |first=L. B. |last=Hunt |journal=Platinum Metals Review |volume=31 |issue=1 |date=1987 |pages=32–41 |doi=10.1595/003214087X3113241 |s2cid=267552692 |url=https://technology.matthey.com/documents/496120/626258/pmr-v31-i1-032-041.pdf/ |access-date=2022-09-29 |archive-date=2022-09-29 |archive-url=https://web.archive.org/web/20220929092320/https://technology.matthey.com/documents/496120/626258/pmr-v31-i1-032-041.pdf/ |url-status=dead |url-access=subscription }}</ref> It has the 10th highest [[List of elements by boiling point|boiling point among all elements]] and becomes a [[superconductor]] at temperatures below {{convert|0.14|K|°C °F|lk=in}}.<ref>{{cite book |last=Kittel |first=C.|title=[[Introduction to Solid State Physics]] |edition=7th |publisher=Wiley-India |date=2004 |isbn=978-81-265-1045-0}}</ref>

Iridium's [[modulus of elasticity]] is the second-highest among the metals, being surpassed only by [[osmium]].<ref name="hunt" /> This, together with a high [[shear modulus]] and a very low figure for [[Poisson's ratio]] (the relationship of longitudinal to lateral [[strain (chemistry)|strain]]), indicate the high degree of stiffness and resistance to deformation that have rendered its fabrication into useful components a matter of great difficulty. Despite these limitations and iridium's high cost, a number of applications have developed where mechanical strength is an essential factor in some of the extremely severe conditions encountered in modern technology.<ref name="hunt" />

The measured [[density]] of iridium is only slightly lower (by about 0.12%) than that of osmium, the [[List of elements by density|densest metal]] known.<ref>{{cite journal|title=Osmium, the Densest Metal Known |author=Arblaster, J. W. |journal=Platinum Metals Review |volume=39 |issue=4 |date=1995 |page=164 |doi=10.1595/003214095X394164164 |s2cid=267393021 |url=http://www.platinummetalsreview.com/dynamic/article/view/pmr-v39-i4-164-164 |access-date=2008-10-02 |archive-url=https://web.archive.org/web/20110927045236/http://www.platinummetalsreview.com/dynamic/article/view/pmr-v39-i4-164-164 |archive-date=2011-09-27 |url-status=dead|url-access=subscription }}</ref><ref>{{cite book |last=Cotton |first=Simon |title=Chemistry of Precious Metals |page=78 |publisher=Springer-Verlag New York, LLC |date=1997 |isbn=978-0-7514-0413-5}}</ref> Some ambiguity occurred regarding which of the two elements was denser, due to the small size of the difference in density and difficulties in measuring it accurately,<ref name="crc">{{cite book |author=Lide, D. R. |title=CRC Handbook of Chemistry and Physics. |url=https://archive.org/details/crchandbookofche00lide |url-access=registration |edition=70th |publisher=Boca Raton (FL):CRC Press |date=1990 |isbn=9780849304712}}</ref> but, with increased accuracy in factors used for calculating density, [[X-ray crystallography|X-ray crystallographic]] data yielded densities of {{cvt|22.56|g/cm3}} for iridium and {{cvt|22.59|g/cm3}} for osmium.<ref>{{cite journal|url=https://technology.matthey.com/article/33/1/14-16/|title=Densities of osmium and iridium: recalculations based upon a review of the latest crystallographic data|author=Arblaster, J. W.|journal=Platinum Metals Review|volume=33|issue=1|date=1989|pages=14–16|doi=10.1595/003214089X3311416 |s2cid=267570193 |access-date=2008-09-17|archive-date=2012-02-07|archive-url=https://web.archive.org/web/20120207064113/http://www.platinummetalsreview.com/pdf/pmr-v33-i1-014-016.pdf|url-status=dead|url-access=subscription}}</ref>

Iridium is extremely brittle, to the point of being hard to [[Welding|weld]] because the heat-affected zone cracks, but it can be made more ductile by addition of small quantities of [[titanium]] and [[zirconium]] (0.2% of each apparently works well).<ref>{{cite patent|country=US |number=3293031A|invent1=Cresswell, Peter|invent2=Rhys, David|pridate=23/12/1963|fdate=27/11/1964|pubdate=20/12/1966}}</ref>

The [[Vickers hardness]] of pure platinum is 56&nbsp;HV, whereas platinum with 50% of iridium can reach over 500&nbsp;HV.<ref>{{cite journal| url = https://technology.matthey.com/article/4/1/18-26/| journal = Platinum Metals Review| title = Iridium Platinum Alloys – A Critical Review Of Their Constitution And Properties| first = A. S.|last = Darling| date = 1960| volume =4| issue = 1| pages = 18–26| doi = 10.1595/003214060X411826| s2cid = 267392937| url-access = subscription}} Reviewed in {{Cite journal|s2cid=4211238 | doi = 10.1038/186211a0| bibcode = 1960Natur.186Q.211.| title = Iridium–Platinum Alloys| journal = Nature| year = 1960| volume = 186| issue = 4720| page = 211| doi-access = free}}</ref><ref>{{cite journal|doi = 10.1595/147106705X24409| title = The Hardening of Platinum Alloys for Potential Jewellery Application| first = T.|last = Biggs| author2=Taylor, S. S.| author3=van der Lingen, E.| journal = Platinum Metals Review| date = 2005| volume = 49| issue = 1| pages = 2–15| doi-access = free}}</ref>

=== Chemical properties ===
Iridium is the most [[corrosion-resistant]] metal known.<ref name="Emsley" /> It is not attacked by [[acid]]s, including [[aqua regia]], but it can be dissolved in concentrated hydrochloric acid in the presence of sodium perchlorate. In the presence of [[oxygen]], it reacts with [[cyanide]] salts.<ref name="emsley">{{cite book|last=Emsley|first=John|title=Nature's Building Blocks: An A–Z Guide to the Elements|edition=New|year=2011 |publisher=Oxford University Press|location=New York |isbn=978-0-19-960563-7}}</ref> Traditional [[Oxidizing agent|oxidants]] also react, including the [[halogen]]s and oxygen<ref name="perry">{{cite book|title=Handbook of Inorganic Compounds| author=Perry, D. L.| pages=203–204| date=1995| isbn=978-1439814611| publisher=CRC Press}}</ref> at higher temperatures.<ref name="lagowski">{{cite book| title=Chemistry Foundations and Applications| volume=2| editor=Lagowski, J. J.| pages=[https://archive.org/details/chemistryfoundat0000unse/page/250 250–251]| date=2004| isbn=978-0028657233| publisher=Thomson Gale| url=https://archive.org/details/chemistryfoundat0000unse/page/250}}</ref> Iridium also reacts directly with [[sulfur]] at atmospheric pressure to yield [[iridium disulfide]].<ref name = Munson-1968>{{cite journal
 |url          = https://htracyhall.org/ocr/HTH-Archives/Cabinet%208/Drawer%203%20(MATI%20-%20MOZ)/(Munson,%20R.A.)%20(Muntoni,%20C.)%20(Murase,%20K.)%20(linked)/(Munson,%20R.A.)%20(Muntoni,%20C.)%20(Murase,%20K.)-237_OCR.pdf
 |author-last  = Munson
 |author-first = Ronald A.
 |date         = February 1968
 |title        = The Synthesis of Iridium Disulfide and Nickel diarsenide having the Pyrite Structure
 |journal      = Inorganic Chemistry
 |volume       = 7
 |number       = 2
 |pages        = 389–390
 |doi          = 10.1021/ic50060a047
 |access-date  = 2019-01-19
 |archive-date = 2019-04-12
 |archive-url  = https://web.archive.org/web/20190412090001/https://htracyhall.org/ocr/HTH-Archives/Cabinet%208/Drawer%203%20(MATI%20-%20MOZ)/(Munson,%20R.A.)%20(Muntoni,%20C.)%20(Murase,%20K.)%20(linked)/(Munson,%20R.A.)%20(Muntoni,%20C.)%20(Murase,%20K.)-237_OCR.pdf
 |url-status   = dead
}}</ref>

=== Isotopes ===
{{Main|Isotopes of iridium}}
Iridium has two naturally occurring stable [[isotope]]s, <sup>191</sup>Ir and <sup>193</sup>Ir, with [[natural abundance]]s of 37.3% and 62.7%, respectively.<ref name="nubase" /> At least 37 [[radioisotope]]s have also been synthesized, ranging in [[mass number]] from 164 to 202. [[iridium-192|<sup>192</sup>Ir]], which falls between the two stable isotopes, is the most stable [[Radionuclide|radioisotope]], has a [[half-life]] of 73.82&nbsp;days, and finds application in [[brachytherapy]]<ref name="mager" /> and in industrial [[radiography]], particularly for [[nondestructive testing]] of welds in steel in the oil and gas industries; iridium-192 sources have been involved in a number of radiological accidents. Three other isotopes have half-lives of at least a day—<sup>188</sup>Ir, <sup>189</sup>Ir, and <sup>190</sup>Ir.<ref name="nubase" /> Isotopes with masses below 191 decay by some combination of [[Beta decay#β+ decay|β<sup>+</sup> decay]], [[alpha decay|α decay]], and (rare) [[proton emission]]. Synthetic isotopes heavier than 193 decay by [[Beta decay#β− decay|β<sup>−</sup> decay]], and <sup>192</sup>Ir has both, but β<sup>+</sup> decay (95.24%) dominates over electron capture (4.76%).<ref name="nubase">{{NUBASE2020}}</ref>

At least 32 [[nuclear isomer|metastable isomers]] have been characterized, ranging in mass number from 164 to 197. The most stable of these is <sup>192m2</sup>Ir, which decays by [[isomeric transition]] with a half-life of 241&nbsp;years,<ref name="nubase" /> making it more stable than any of iridium's ground-state radioisotopes.

The isotope <sup>191</sup>Ir was the first one of any element to be shown to present a [[Mössbauer effect]]. This renders it useful for [[Mössbauer spectroscopy]] for research in physics, chemistry, [[biochemistry]], [[metallurgy]], and [[mineralogy]].<ref name="ir-191">{{cite book |title=Handbook of Ceramics and Composites |author=Chereminisoff, N. P. |publisher=CRC Press |date=1990 |isbn=978-0-8247-8006-7 |page=424}}</ref>

== Chemistry ==
{{See also|Iridium compounds}}
{|class="wikitable" style="float:right;margin:1em"
|-
! colspan=2| Oxidation states{{efn|Most common oxidation states of iridium are in bold. The right column lists one representative compound for each oxidation state.}}
|-
| −3||{{chem|[Ir(CO)|3|]|3-}}
|-
| −2||{{chem2|IrVO2(-)}}
|-
| −1||{{chem2|[Ir(CO)3(PPh3)](-)}}
|-
| 0||{{chem2|Ir4(CO)12}}
|-
| '''+1'''||{{chem2|[IrCl(CO)(PPh3)2]}}
|-
| '''+2'''||{{chem2|Ir(C5H5)2}}
|-
| '''+3'''||{{chem2|IrCl3}}
|-
| '''+4'''||{{chem2|IrO2}}
|-
| +5||{{chem2|Ir4F20}}
|-
| +6||{{chem|IrF|6}}
|-
| +7||{{chem2|[Ir(O2)O2]+}}
|-
| +8||{{chem2|IrO4}}
|-
| +9||{{chem2|[IrO4]+}}<ref name="IrIX" />
|}

=== Oxidation states ===
Iridium forms compounds in [[oxidation state]]s between −3 and +9, but the most common oxidation states are +1, +2, +3, and +4.<ref name="greenwood" /> Well-characterized compounds containing iridium in the +6 oxidation state include [[Iridium(VI) fluoride|{{chem2|IrF6}}]] and the oxides {{chem2|Sr2MgIrO6}} and {{chem2|Sr2CaIrO6}}.<ref name="greenwood">{{cite book |last=Greenwood |first=N. N. |author2=Earnshaw, A. |title=Chemistry of the Elements |edition=2nd |publisher=Oxford: Butterworth–Heinemann |date=1997 |isbn=978-0-7506-3365-9 |pages=1113–1143, 1294 |oclc=213025882}}</ref><ref>{{cite journal |last1=Jung |first1=D. |title=High Oxygen Pressure and the Preparation of New Iridium (VI) Oxides with Perovskite Structure: {{chem|Sr|2|MIrO|6}} (M = Ca, Mg) |journal=Journal of Solid State Chemistry |volume=115 |issue=2 |date=1995 |pages=447–455 |doi=10.1006/jssc.1995.1158 |bibcode=1995JSSCh.115..447J |last2=Demazeau |first2=Gérard}}</ref> [[Iridium(VIII) oxide]] ({{chem2|IrO4}}) was generated under matrix isolation conditions at 6 K in [[argon]].<ref>{{cite journal|title=Formation and Characterization of the Iridium Tetroxide Molecule with Iridium in the Oxidation State +VIII|journal=Angewandte Chemie International Edition|volume=48 |date=2009|pages=7879–7883|author=Gong, Y.|author2=Zhou, M.|author3=Kaupp, M.|author4=Riedel, S. |doi=10.1002/anie.200902733|pmid=19593837|issue=42 |bibcode=2009ACIE...48.7879G }}</ref> The highest oxidation state (+9), which is also the highest recorded for ''any'' element, is found in gaseous {{chem2|[IrO4]+}}.<ref name="IrIX" />

=== Binary compounds ===
Iridium does not form [[binary compound|binary]] [[hydride]]s. Only one [[binary phase|binary oxide]] is well-characterized: [[Iridium(IV) oxide|iridium dioxide]], {{chem|IrO|2}}.  It is a blue black solid that adopts the [[fluorite structure]].<ref name="greenwood" /> A [[sesquioxide]], {{chem|Ir|2|O|3}}, has been described as a blue-black powder, which is oxidized to {{chem|IrO|2}} by {{chem|HNO|3}}.<ref name="perry" /> The corresponding [[disulfide]]s, [[diselenide]]s, [[sesquisulfide]]s, and sesquiselenides are known, as well as {{chem|IrS|3}}.<ref name="greenwood" />

Binary trihalides, {{chem|IrX|3}}, are known for all of the halogens.<ref name="greenwood" /> For oxidation states +4 and above, only the [[Iridium(IV) fluoride|tetrafluoride]], [[Iridium(V) fluoride|pentafluoride]] and [[Iridium hexafluoride|hexafluoride]] are known.<ref name="greenwood" /> Iridium hexafluoride, {{chem|IrF|6}}, is a volatile yellow solid, composed of octahedral molecules. It decomposes in water and is reduced to {{chem|link=iridium tetrafluoride|IrF|4}}.<ref name="greenwood" /> Iridium pentafluoride is also a strong oxidant, but it is a [[tetramer]], {{chem|Ir|4|F|20}}, formed by four corner-sharing octahedra.<ref name="greenwood" />

=== Complexes ===
[[File:IrCl3(aq)x.jpg|thumb|left|Hydrated [[iridium trichloride]], a common salt of iridium.]]
Iridium has extensive [[coordination chemistry]].

Iridium in its complexes is always [[low-spin]]. Ir(III) and Ir(IV) generally form [[octahedral molecular geometry|octahedral complexes]].<ref name="greenwood" />  Polyhydride complexes are known for the +5 and +3 oxidation states.<ref>{{cite book| last = Holleman| first = A. F.| author2=Wiberg, E.| author3=Wiberg, N.| title=Inorganic Chemistry| edition=1st| publisher=Academic Press| date=2001| isbn=978-0-12-352651-9| oclc =47901436}}</ref> One example is {{chem2|IrH5(P<sup>i</sup>Pr3)2}} (<sup>i</sup>Pr = [[isopropyl]]).<ref>{{cite journal |doi=10.1021/acs.chemrev.6b00080|title=Polyhydrides of Platinum Group Metals: Nonclassical Interactions and σ-Bond Activation Reactions |year=2016 |last1=Esteruelas |first1=Miguel A. |last2=López |first2=Ana M. |last3=Oliván |first3=Montserrat |journal=Chemical Reviews |volume=116 |issue=15 |pages=8770–8847 |pmid=27268136 |doi-access=free |hdl=10261/136216 |hdl-access=free }}</ref> The ternary hydride {{chem|Mg|6|Ir|2|H|11}} is believed to contain both the {{chem|IrH|5|4-}} and the 18-electron {{chem|IrH|4|5-}} anion.<ref>{{cite journal| title = {{chem|Mg|6|Ir|2|H|11}}, a new metal hydride containing saddle-like {{chem|IrH|4|5-}} and square-pyramidal {{chem|IrH|5|4-}} hydrido complexes | last = Černý| first = R.| author2=Joubert, J.-M.| author3=Kohlmann, H.| author4=Yvon, K. | journal = Journal of Alloys and Compounds| volume = 340| issue = 1–2| date = 2002|pages = 180–188| doi=10.1016/S0925-8388(02)00050-6}}</ref>

Iridium also forms [[oxyanion]]s with oxidation states +4 and +5.  {{chem|K|2|IrO|3}} and {{chem|KIrO|3}} can be prepared from the reaction of [[potassium oxide]] or [[potassium superoxide]] with iridium at high temperatures.  Such solids are not soluble in conventional solvents.<ref>{{cite journal|title=The chemistry of ruthenium, osmium, rhodium, iridium, palladium and platinum in the higher oxidation states|journal=Coordination Chemistry Reviews|volume=46|date=1982 |pages=1–127|author=Gulliver, D. J.|author2=Levason, W.|doi=10.1016/0010-8545(82)85001-7}}</ref>

Just like many elements, iridium forms important chloride complexes. Hexachloroiridic (IV) acid, {{chem|H|2|IrCl|6}}, and its [[ammonium]] salt are common iridium compounds from both industrial and preparative perspectives.<ref name="ullmann-pt" /> They are intermediates in the purification of iridium and used as precursors for most other iridium compounds, as well as in the preparation of [[anode]] coatings. The {{chem|IrCl|6|2-}} ion has an intense dark brown color, and can be readily reduced to the lighter-colored {{chem|IrCl|6|3-}} and vice versa.<ref name="ullmann-pt" /> [[Iridium(III) chloride|Iridium trichloride]], {{chem|IrCl|3}}, which can be obtained in [[anhydrous]] form from direct oxidation of iridium powder by [[chlorine]] at 650&nbsp;°C,<ref name="ullmann-pt" /> or in hydrated form by dissolving {{chem|Ir|2|O|3}} in [[hydrochloric acid]], is often used as a starting material for the synthesis of other Ir(III) compounds.<ref name="greenwood" /> Another compound used as a starting material is potassium hexachloroiridate(III), {{chem2|K3IrCl6}}.<ref>{{cite book |doi=10.1002/9780470132432.ch42 |chapter=Pentaammineiridium(III) Complexes |date=1970 |last1=Schmidtke |first1=Hans-Herbert  |title=Inorganic Syntheses |volume=12 |pages=243–247 |isbn=978-0-470-13171-8 }}</ref>

=== Organoiridium chemistry ===
[[File: Ir2Cl2 cod 2improved.svg|thumb|left|[[Cyclooctadiene iridium chloride dimer]] is a common complex of Ir(I).]]

[[Organoiridium compound]]s contain iridium–[[carbon]] bonds. Early studies identified the very stable [[tetrairidium dodecacarbonyl]], {{chem|Ir|4|(CO)|12}}.<ref name="greenwood" /> In this compound, each of the iridium atoms is bonded to the other three, forming a [[Tetrahedron|tetrahedral]] cluster. The discovery of [[Vaska's complex]] ({{chem|IrCl(CO)[P(C|6|H|5|)|3|]|2}}) opened the [[door]] for [[oxidative addition]] reactions, a process fundamental to useful reactions. For example, [[Crabtree's catalyst]], a [[homogeneous catalyst]] for [[hydrogenation]] reactions.<ref>{{cite journal|first = R. H.| last = Crabtree| author-link =Robert H. Crabtree| title = Iridium compounds in catalysis| journal = Accounts of Chemical Research| date = 1979| volume = 12| pages = 331–337| doi = 10.1021/ar50141a005|issue = 9}}</ref><ref>{{cite book| title=The Organometallic Chemistry of the Transition Metals| url=http://chimicibicocca.altervista.org/data/chimica_lucidi.pdf| author=Crabtree, R. H.| date=2005| publisher=Wiley| isbn=978-0471662563| oclc=224478241| author-link=Robert H. Crabtree| url-status=dead| archive-url=https://web.archive.org/web/20121119073400/http://chimicibicocca.altervista.org/data/chimica_lucidi.pdf| archive-date=2012-11-19}}</ref>

[[File:C-HactnBergGrah.png|upright=2|left|thumb|Oxidative addition to hydrocarbons in [[organoiridium chemistry]]<ref name="RGB">{{cite journal|title=Carbon-hydrogen activation in completely saturated hydrocarbons: direct observation of M + R–H → M(R)(H)|author=Janowicz, A. H.|author2=Bergman, R. G.|journal=Journal of the American Chemical Society|date=1982|volume=104|issue=1|pages=352–354|doi=10.1021/ja00365a091}}</ref><ref name="WAGG">{{cite journal|title=Oxidative addition of the carbon-hydrogen bonds of neopentane and cyclohexane to a photochemically generated iridium(I) complex|author=Hoyano, J. K.|author2=Graham, W. A. G.|journal=Journal of the American Chemical Society|date=1982|volume=104|issue=13|pages=3723–3725|doi=10.1021/ja00377a032|bibcode=1982JAChS.104.3723H }}</ref>|alt=Skeletal formula presentation of a chemical transformation. The initial compounds have a C5H5 ring on their top and an iridium atom in the center, which is bonded to two hydrogen atoms and a P–PH3 group or to two C–O groups. Reaction with alkane under UV light alters those groups.]]
Iridium complexes played a pivotal role in the development of [[C–H bond activation|carbon–hydrogen bond activation]] (C–H activation), which promises to allow functionalization of [[hydrocarbon]]s, which are traditionally regarded as [[Reactivity (chemistry)|unreactive]].<ref>{{cite journal |doi=10.1039/c0cs00156b|title=Regioselectivity of the Borylation of Alkanes and Arenes |year=2011 |last1=Hartwig |first1=John F. |journal=Chemical Society Reviews |volume=40 |issue=4 |pages=1992–2002 |pmid=21336364 }}</ref>

== History ==
=== Platinum group ===
[[File:Winged goddess Cdm Paris 392.jpg|thumb|upright|The Greek goddess [[Iris (mythology)|Iris]], after whom iridium was named.|alt=Photo of part of a black vase with brown picture on it: A woman with wings on her back hold an arrow with right hand and gives a jar to a man. A small deer is standing in front of the woman.]]

The discovery of iridium is intertwined with that of platinum and the other metals of the [[platinum group]]. The first European reference to platinum appears in 1557 in the writings of the Italian humanist [[Julius Caesar Scaliger]] as a description of an unknown noble metal found between [[Darién Province|Darién]] and Mexico, "which no fire nor any Spanish artifice has yet been able to [[Liquefaction|liquefy]]".<ref name="weeks">{{cite journal | last=Weeks | first=Mary Elvira | title=The discovery of the elements. VIII. The platinum metals | journal=Journal of Chemical Education | publisher=American Chemical Society (ACS) | volume=9 | issue=6 | year=1932 | issn=0021-9584 | doi=10.1021/ed009p1017 | pages=1017–1034| bibcode=1932JChEd...9.1017W }}{{cite book |title=Discovery of the Elements |url=https://archive.org/details/discoveryofeleme07edunse |url-access=registration |pages=[https://archive.org/details/discoveryofeleme07edunse/page/385 385]–407 |author=Weeks, M. E. |date=1968 |edition=7th |publisher=Journal of Chemical Education |isbn=978-0-8486-8579-9 |oclc=23991202}}</ref> From their first encounters with platinum, the Spanish generally saw the metal as a kind of [[impurity]] in gold, and it was treated as such. It was often simply thrown away, and there was an official decree forbidding the [[adulteration]] of gold with platinum impurities.<ref name="history">{{cite book |title=A History of Platinum and its Allied Metals |pages=7–8 |author=Donald McDonald, Leslie B. Hunt |date=1982 |publisher=Johnson Matthey Plc |isbn=978-0-905118-83-3}}</ref>
[[File:Platinum symbol.svg|thumb|left|upright=0.4|alt=A left-pointing crescent, tangent on its right to a circle containing at its center a solid circular dot|This [[alchemical symbol]] for platinum was made by joining the symbols of silver (moon) and gold (sun).]]
[[File:Almirante Antonio de Ulloa.jpg|thumb|[[Antonio de Ulloa]] is credited in European history with the discovery of platinum.]]

In 1735, [[Antonio de Ulloa]] and [[Jorge Juan y Santacilia]] saw Native Americans mining platinum while the [[Spaniards]] were travelling through [[Colombia]] and [[Peru]] for eight years. Ulloa and Juan found mines with the whitish metal nuggets and took them home to Spain. Ulloa returned to Spain and established the first [[mineralogy]] lab in Spain and was the first to systematically study platinum, which was in 1748. His historical account of the expedition included a description of platinum as being neither [[Separation process|separable]] nor [[calcination|calcinable]]. Ulloa also anticipated the discovery of platinum mines. After publishing the report in 1748, Ulloa did not continue to investigate the new metal. In 1758, he was sent to superintend [[Mercury (element)|mercury]] mining operations in [[Huancavelica]].<ref name="weeks" />

In 1741, [[Charles Wood (metallurgist)|Charles Wood]],<ref>{{cite book |url=https://books.google.com/books?id=525bAAAAQAAJ&pg=PP7 |page=52 |title=The literary life of William Brownrigg. To which are added an account of the coal mines near Whitehaven: And Observations on the means of preventing epidemic fevers |last1=Dixon |first1=Joshua |last2=Brownrigg |first2=William |date=1801 |url-status=live |archive-url=https://web.archive.org/web/20170324090058/https://books.google.com/books?id=525bAAAAQAAJ&pg=PP7 |archive-date=24 March 2017 |df=dmy-all}}</ref><!--https://books.google.com/books?id=S1lFAAAAcAAJ&pg=PA672--> a British [[metallurgy|metallurgist]], found various samples of Colombian platinum in Jamaica, which he sent to [[William Brownrigg]] for further investigation.

In 1750, after studying the platinum sent to him by Wood, Brownrigg presented a detailed account of the metal to the [[Royal Society]], stating that he had seen no mention of it in any previous accounts of known minerals.<ref>{{cite journal |pages = 584–596 |doi = 10.1098/rstl.1749.0110 |title = Several Papers concerning a New Semi-Metal, Called Platina; Communicated to the Royal Society by Mr. Wm. Watson F. R. S |date = 1749 |last1 = Watson |first1 = Wm |last2 = Brownrigg |first2 = William |journal = Philosophical Transactions |volume = 46 |issue = 491–496 |df = dmy-all |bibcode = 1749RSPT...46..584W |s2cid = 186213277 |doi-access = free }}</ref> Brownrigg also made note of platinum's extremely high melting point and refractory metal-like behaviour toward [[borax]]. Other chemists across Europe soon began studying platinum, including [[Andreas Sigismund Marggraf]],<ref>{{cite book |url=https://books.google.com/books?id=GWNQAAAAcAAJ |title=Versuche mit dem neuen mineralischen Körper Platina del pinto genannt |last1=Marggraf |first1=Andreas Sigismund |date=1760 |url-status=live |archive-url=https://web.archive.org/web/20170324173956/https://books.google.com/books?id=GWNQAAAAcAAJ |archive-date=24 March 2017 |df=dmy-all}}</ref> [[Torbern Bergman]], [[Jöns Jakob Berzelius]],<!--http://www.google.de/url?sa=t&rct=j&q=pmr-v23-i4-155-156&source=web&cd=4&ved=0CFoQFjAD&url=http%3A%2F%2Fwww.platinummetalsreview.com%2Fpdf%2Fpmr-v23-i4-155-156.pdf&ei=FxWTT_6YOoOLswaKy7XeBA&usg=AFQjCNFn8__okV3fK4xcNSg1bQ-Nm_NZHg--> [[William Lewis (scientist)|William Lewis]],<!--http://www.google.de/url?sa=t&rct=j&q=platina+William+Lewis&source=web&cd=1&ved=0CC4QFjAA&url=http%3A%2F%2Fwww.platinummetalsreview.com%2Fpdf%2Fpmr-v7-i2-066-069.pdf&ei=hhWTT4-YNozLsgb14LGLBA&usg=AFQjCNHCECiLbEjXypnkLTujKyMs47FANQ{{cite journal| title=The Platinum of New Granada: Mining and Metallurgy in the Spanish Colonial Empire| author=McDonald, M.| journal=Platinum Metals Review| volume=3| issue=4| date=1959| pages=140–145| url=http://www.platinummetalsreview.com/dynamic/article/view/pmr-v3-i4-140-145}}{cite journal| title=The So-Called 'Platinum' Inclusions in Egyptian Goldwork| first=J. M.| last=Ogden| journal=The Journal of Egyptian Archaeology| volume=62| date=1976| pages=138–144| jstor=3856354| doi=10.2307/3856354}}</ref> and by South American cultures<ref name="preCol">{{cite journal| journal=Platinum Metals Review| date=1980| volume=24| issue=21| pages=70–79| title=The Powder Metallurgy of Platinum| first=J. C.| last=Chaston |url=http://www.technology.matthey.com/pdf/pmr-v24-i2-070-079.pdf}}{{cite book| author=Juan, J.| author2=de Ulloa, A.| date=1748| title=Relación histórica del viage a la América Meridional| page=606| language=es| url=https://books.google.com/books?id=BdSGz0Ea7h8C&| series=Primera parte, tomo secondo}}--> and [[Pierre Macquer]]. In 1752, [[Henrik Teofilus Scheffer|Henrik Scheffer]] published a detailed scientific description of the metal, which he referred to as "white gold", including an account of how he succeeded in fusing platinum ore with the aid of [[arsenic]]. Scheffer described platinum as being less [[pliable]] than gold, but with similar resistance to [[corrosion]].<ref name="weeks" />

=== Discovery ===
[[Chemist]]s who studied platinum [[Dissolution (chemistry)|dissolved]] it in [[aqua regia]] (a mixture of [[hydrochloric acid|hydrochloric]] and [[nitric acid]]s) to create [[Solubility|soluble]] salts. They always observed a small amount of a dark, [[Solubility|insoluble]] residue.<ref name="hunt" /> [[Joseph Louis Proust]] thought that the residue was [[graphite]].<ref name="hunt" /> The French chemists [[Victor Collet-Descotils]], [[Antoine François, comte de Fourcroy]], and [[Louis Nicolas Vauquelin]] also observed the black residue in 1803, but did not obtain enough for further experiments.<ref name="hunt" />

In 1803 British scientist [[Smithson Tennant]] (1761–1815) analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternately with [[alkali]] and acids<ref name="Emsley" /> and obtained a volatile new oxide, which he believed to be of this new metal—which he named ''[[Osmium|ptene]]'', from the Greek word {{lang|el|πτηνός}} ''ptēnós'', "[[wing]]ed".<ref>{{cite book |title=A System of Chemistry of Inorganic Bodies |url=https://archive.org/details/asystemchemistr07thomgoog |author=Thomson, T. |author-link=Thomas Thomson (chemist) |publisher=Baldwin & Cradock, London; and William Blackwood, Edinburgh |date=1831 |volume=1 |page=[https://archive.org/details/in.ernet.dli.2015.32266/page/n721/mode/2up 693]}}</ref><ref name="griffith">{{cite journal |url=http://www.technology.matthey.com/article/48/4/182-189/ |title=Bicentenary of Four Platinum Group Metals. Part II: Osmium and iridium – events surrounding their discoveries |author=Griffith, W. P. |journal=Platinum Metals Review |volume=48 |issue=4 |date=2004 |pages=182–189 |doi=10.1595/147106704x4844|doi-access=free }}</ref> 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]].<ref name="hunt" /><ref name="Emsley" /> He obtained dark red crystals (probably of {{chem|Na|2|[IrCl|6}}]·''n''{{chem|H|2|O}}) by a sequence of reactions with [[sodium hydroxide]] and [[hydrochloric acid]].<ref name="griffith" /> He named iridium after [[Iris (mythology)|Iris]] ({{lang|el|Ἶρις}}), the Greek winged goddess of the [[rainbow]] and the messenger of the [[Twelve Olympians|Olympian gods]], because many of the [[Salt (chemistry)|salts]] he obtained were strongly colored.{{efn|''Iridium'' literally means "of rainbows".}}<ref>{{cite book |title=Discovery of the Elements |url=https://archive.org/details/discoveryofeleme0000week |url-access=registration |pages=[https://archive.org/details/discoveryofeleme0000week/page/414 414–418] |author=Weeks, M. E. |date=1968 |edition=7th |publisher=Journal of Chemical Education |isbn=978-0-8486-8579-9 |oclc=23991202}}</ref> Discovery of the new elements was documented in a letter to the [[Royal Society]] on June 21, 1804.<ref name="hunt"/><ref>{{cite journal |title=On Two Metals, Found in the Black Powder Remaining after the Solution of Platina |first=S. |last=Tennant |journal=Philosophical Transactions of the Royal Society of London |volume=94 |date=1804 |pages=411–418 |jstor=107152 |doi=10.1098/rstl.1804.0018 |url=https://zenodo.org/record/1432312 |doi-access=free}}</ref>

=== 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).<ref name="hunt" /> The first to obtain high-purity iridium was [[Robert Hare (chemist)|Robert Hare]] in 1842. He found it had a density of around {{cvt|21.8|g/cm3}} and noted the metal is nearly [[Ductility|immalleable]] and very hard. The first melting in appreciable quantity was done by [[Henri Sainte-Claire Deville]] and [[Jules Henri Debray]] in 1860. They required burning more than {{convert|300|L|USgal}} of pure {{chem|O|2}} and {{chem|H|2}} gas for each {{convert|1|kg}} of iridium.<ref name="hunt" />

These extreme difficulties in melting the metal limited the possibilities for handling iridium. [[John Isaac Hawkins]] was looking to obtain a fine and hard point for [[fountain pen]] [[Nib (pen)|nibs]], and in 1834 managed to create an iridium-pointed gold pen. In 1880, [[John Holland (pen maker)|John Holland]] and [[William Lofland Dudley]] were able to melt iridium by adding [[phosphorus]] and patented the process in the United States; British company [[Johnson Matthey]] later stated they had been using a similar process since 1837 and had already presented fused iridium at a number of [[World's fair|World Fairs]].<ref name="hunt" /> The first use of an [[alloy]] of iridium with [[ruthenium]] in [[thermocouple]]s was made by Otto Feussner<!-- No page for "Otto Feussner"
 --> in 1933. These allowed for the measurement of high temperatures in air up to {{convert|2000|C}}.<ref name="hunt" />

In [[Munich]], Germany in 1957 [[Rudolf Mössbauer]], in what has been called one of the "landmark experiments in twentieth-century physics",<ref>{{cite book |pages=179–190 |title=Landmark Experiments in Twentieth Century Physics |author=Trigg, G. L. |publisher=Courier Dover Publications |isbn=978-0-486-28526-9 |date=1995 |oclc=31409781 |chapter=Recoilless Emission and Absorption of Radiation |url=https://books.google.com/books?id=YOQ9fi5yQ4sC}}</ref> discovered the resonant and [[Atomic Recoil|recoil]]-free emission and absorption of [[gamma ray]]s by [[atom]]s in a solid metal sample containing only <sup>191</sup>Ir.<ref>{{cite journal |first=R. L. |last=Mössbauer |s2cid=121129342 |author-link=Rudolf Mössbauer |title=Gammastrahlung in Ir<sup>191</sup> |journal=Zeitschrift für Physik A |volume=151 |issue=2 |pages=124–143 |date=1958 |language=de |doi=10.1007/BF01344210 |bibcode=1958ZPhy..151..124M}}</ref> 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.<ref>{{cite book |title=Nobel Lectures, Physics 1942–1962 |publisher=Elsevier |date=1964 |chapter=The Nobel Prize in Physics 1961: presentation speech |first=I. |last=Waller |chapter-url=http://nobelprize.org/nobel_prizes/physics/laureates/1961/press.html}}</ref>

== Occurrence ==

Along with many elements having [[atomic weight]]s higher than that of iron, iridium is only naturally formed by the [[r-process]] (rapid [[neutron]] capture) in [[neutron star merger]]s and possibly rare types of supernovae.<ref>{{cite web |url=http://herschel.jpl.nasa.gov/chemicalOrigins.shtml |title=History/Origin of Chemicals |publisher=NASA |access-date=1 January 2013}}</ref><ref name="chen">{{cite journal | last1=Chen | first1=Hsin-Yu | last2=Vitale | first2=Salvatore | last3=Foucart | first3=Francois | title=The Relative Contribution to Heavy Metals Production from Binary Neutron Star Mergers and Neutron Star–Black Hole Mergers | journal=The Astrophysical Journal Letters | publisher=American Astronomical Society | volume=920 | issue=1 | date=2021-10-01 | issn=2041-8205 | doi=10.3847/2041-8213/ac26c6 | page=L3| arxiv=2107.02714 | bibcode=2021ApJ...920L...3C | hdl=1721.1/142310 | s2cid=238198587 | doi-access=free }}</ref><ref>{{Cite journal |last1=Arlandini |first1=Claudio |last2=Kappeler |first2=Franz |last3=Wisshak |first3=Klaus |last4=Gallino |first4=Roberto |last5=Lugaro |first5=Maria |last6=Busso |first6=Maurizio |last7=Straniero |first7=Oscar |date=1999-11-10 |title=Neutron Capture in Low-Mass Asymptotic Giant Branch Stars: Cross Sections and Abundance Signatures |url=https://iopscience.iop.org/article/10.1086/307938 |journal=The Astrophysical Journal |language=en |volume=525 |issue=2 |pages=886–900 |doi=10.1086/307938 |issn=0004-637X|arxiv=astro-ph/9906266 |bibcode=1999ApJ...525..886A }}</ref>

[[File:Elemental abundances.svg|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.]]
[[File:Willamette Meteorite AMNH.jpg|thumb|upright|The [[Willamette Meteorite]], the sixth-largest meteorite found in the world, has 4.7&nbsp;ppm iridium.<ref>{{cite journal |title=The chemical classification of iron meteorites—VII. A reinvestigation of irons with Ge concentrations between 25 and 80 ppm |author=Scott, E. R. D. |author2=Wasson, J. T. |author3=Buchwald, V. F. |journal=Geochimica et Cosmochimica Acta |date=1973 |volume=37 |pages=1957–1983 |doi=10.1016/0016-7037(73)90151-8 |bibcode= 1973GeCoA..37.1957S |issue=8}}</ref>|alt=A large black egg-shaped boulder of porous structure standing on its top, tilted]]
Iridium is one of the nine least abundant stable [[Periodic table|elements]] in [[Earth's crust]], having an average [[Mass fraction (chemistry)|mass fraction]] of 0.001&nbsp;[[parts per million|ppm]] in crustal rock; [[gold]] is 4 times more abundant, [[platinum]] is 10 times more abundant, [[silver]] and [[Mercury (element)|mercury]] are 80 times more abundant.<ref name="greenwood" /> [[Osmium]], [[tellurium]], [[ruthenium]], [[rhodium]] and [[rhenium]] are about as abundant as iridium.<ref>{{cite book |editor-last1=Haynes |editor-first1=W. M. |editor-last2=Lide |editor-first2=David R. |editor-last3=Bruno |editor-first3=Thomas J. |year=2017 |chapter=Abundance of Elements in the Earth's Crust and in the Sea |chapter-url=https://archive.org/details/CRCHandbookOfChemistryAndPhysics97thEdition2016/page/n2401 |title=[[CRC Handbook of Chemistry and Physics]] |edition=97th |publisher=[[CRC Press]] |pages=14–17 |isbn=978-1-4987-5429-3}}</ref> In contrast to its low abundance in crustal rock, iridium is relatively common in [[meteorite]]s, with concentrations of 0.5&nbsp;ppm or more.<ref name="argonne" /> The overall concentration of iridium on Earth is thought to be much higher than what is observed in crustal rocks, but because of the density and [[Goldschmidt classification|siderophilic]] ("iron-loving") character of iridium, it descended below the crust and into [[Earth's core]] when the planet was still [[Lava|molten]].<ref name="ullmann-pt">{{cite book |display-authors=8 |author=Renner, H. |author2=Schlamp, G. |author3=Kleinwächter, I. |author4=Drost, E. |author5=Lüschow, H. M. |author6=Tews, P. |author7=Panster, P. |author8=Diehl, M. |author9=Lang, J. |author10=Kreuzer, T. |author11=Knödler, A. |author12=Starz, K. A. |author13=Dermann, K. |author14=Rothaut, J. |author15=Drieselman, R. |chapter=Platinum group metals and compounds |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley |date=2002 |doi=10.1002/14356007.a21_075 |isbn=978-3527306732}}</ref>
<!-- upper crust in 10.1016/j.gca.2012.06.026-->

Iridium is found in nature as an uncombined element or in natural [[alloy]]s, especially the iridium–[[osmium]] alloys [[osmiridium]] (osmium-rich) and [[iridosmium]] (iridium-rich).<ref name="Emsley">{{cite book| title=Nature's Building Blocks: An A–Z Guide to the Elements| last=Emsley| first=J.| publisher=[[Oxford University Press]]| date=2003| location=Oxford, England, UK| isbn=978-0-19-850340-8| chapter=Iridium| pages=[https://archive.org/details/naturesbuildingb0000emsl/page/201 201–204]| chapter-url=https://archive.org/details/naturesbuildingb0000emsl/page/201}}</ref> In [[nickel]] and copper deposits, the [[platinum group]] metals occur as [[sulfide]]s, [[telluride (chemistry)|tellurides]], [[antimonide]]s, and [[arsenide]]s. 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 [[native copper|raw copper]].<ref>{{cite journal| doi=10.1016/j.mineng.2004.04.001| journal=Minerals Engineering| volume=17| date=2004| pages=961–979| title=Characterizing and recovering the platinum group minerals—a review| first1=Z.| last1=Xiao| last2=Laplante| first2=A. R.| issue=9–10| bibcode=2004MiEng..17..961X}}</ref> A number of iridium-dominant [[mineral]]s, 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.<!--<ref>{{cite web |url=https://www.mindat.org/min-2042.html |title=Irarsite: Mineral information, data and localities |website=Mindat.org |access-date=27 September 2022}}</ref><ref>{{cite web| url=https://www.mindat.org/element/Iridium| title=Iridium: The mineralogy of Iridium| website=Mindat.org| access-date=27 September 2022}}</ref><ref>{{cite web |url=http://nrmima.nrm.se/|title=International Mineralogical Association – Commission on New Minerals, Nomenclature and Classification|website=nrmima.nrm.se|access-date=2018-10-06|archive-url=https://web.archive.org/web/20190810195707/http://nrmima.nrm.se//|archive-date=2019-08-10|url-status=dead}}</ref>--><ref>{{cite web|url=http://www.handbookofmineralogy.org/pdfs/cuproiridsite.pdf|title= Cuproiridsite CuIr<sub>2</sub>S<sub>4</sub>| website=Handbook of mineralogy.org|access-date=3 March 2022}}</ref><ref>{{cite journal| url=https://www.fmm.ru/images/8/89/NDM_2010_45_Stepanov_eng.pdf |journal=New Data on Minerals|date=2010|volume=45|page=23|title=Irasite Discovery in Copper-Nickel Ores of Shanuch Deposit (KAMCHATKA) |author1=Vitaly A. Stepanov|author2=Valentina E. Kungurova|author3=Vitaly I. Gvozdev|access-date=3 March 2022}}</ref><ref>{{cite journal|url=https://rruff.info/uploads/CM33_509.pdf|journal=The Canadian Mineralogist|date=1995|volume=33|pages=509–520|title=Iridium, Rhodium, and Platinum Sulfides in Chromitites from the Ultramafic Massifs of Finero, Italy, and Ojen, Spain |first1=Giorgio  |last1= Garuti | first2 = Moreno | last2 = Gazzotti | first3= Jose | last3 = Torres-Ruiz |access-date=2 November 2022}}</ref> Within Earth's crust, iridium is found at highest concentrations in three types of [[Geology|geologic]] structure: [[Igneous rock|igneous]] deposits (crustal intrusions from below), [[impact crater]]s, and deposits reworked from one of the former structures. The largest known primary reserves are in the [[Bushveld igneous complex]] in South Africa,<ref name="kirk-pt" /> (near the largest known impact structure, the [[Vredefort impact structure]]) though the large copper–[[nickel]] deposits near [[Norilsk#Norilsk-Talnakh nickel deposits|Norilsk]] in Russia, and the [[Sudbury Basin]] (also an impact crater) in Canada are also significant sources of iridium. Smaller reserves are found in the United States.<ref name="kirk-pt" /> Iridium is also found in secondary deposits, combined with [[platinum]] and other [[platinum group]] metals in [[alluvium|alluvial]] deposits. The alluvial deposits used by [[pre-Columbian]] people in the [[Chocó Department]] of [[Colombia]] are still a source for platinum-group metals. <!-- The second large alluvial deposit was found in the [[Ural mountain]]s, Russia, which is still mined.{{Citation needed|date=September 2008}} --> As of 2003, world reserves have not been estimated.<ref name="Emsley" />

=== Marine oceanography ===
Iridium is found within marine organisms, [[sediment]]s, and the water column. The abundance of iridium in seawater<ref name="Goldberg">{{cite journal |last1=Goldberg |first1=Hodge |last2=Kay |first2=V |last3=Stallard |first3=M |last4=Koide |first4=M |title=Some comparative marine chemistries of platinum and iridium |journal=Applied Geochemistry |date=1986 |volume=1 |issue=2 |pages=227–232 |doi=10.1016/0883-2927(86)90006-5|bibcode=1986ApGC....1..227G }}</ref> and organisms<ref name="Wells">{{cite journal |last1=Wells |first1=Boothe |title=Iridium in marine organisms |journal=Geochimica et Cosmochimica Acta |date=1988 |volume=52 |issue=6 |pages=1737–1739 |doi=10.1016/0016-7037(88)90242-6|bibcode=1988GeCoA..52.1737W }}</ref> is relatively low, as it does not readily form [[Transition metal chloride complex|chloride complexes]].<ref name="Wells"/> The abundance in organisms is about 20 parts per trillion, or about five [[orders of magnitude]] less than in [[sedimentary rock]]s at the [[Cretaceous–Paleogene boundary|Cretaceous–Paleogene (K–T) boundary]].<ref name="Wells" /> The concentration of iridium in seawater and marine sediment is sensitive to [[Oxygenation (environmental)|marine oxygenation]], seawater temperature, and various geological and biological processes.<ref name="Sawlowicz">{{cite journal |last1=Sawlowicz |first1=Z |title=Iridium and other platinum-group elements as geochemical markers in sedimentary environments |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=1993 |volume=104 |issue=4 |pages=253–270 |doi=10.1016/0031-0182(93)90136-7|bibcode=1993PPP...104..253S }}</ref>

Iridium in sediments can come from [[cosmic dust]], volcanoes, [[precipitation (chemistry)|precipitation]] from seawater, microbial processes, or [[hydrothermal vent]]s,<ref name="Sawlowicz" /> and its abundance can be strongly indicative of the source.<ref name="Macdougall">{{cite journal |last1=Crocket |first1=Macdougall |last2=Harriss |first2=R |title=Gold, palladium and iridium in marine sediments |journal=Geochimica et Cosmochimica Acta |date=1973 |volume=37 |issue=12 |pages=2547–2556 |doi=10.1016/0016-7037(73)90264-0|bibcode=1973GeCoA..37.2547C }}</ref><ref name="Sawlowicz" /> It tends to associate with other ferrous metals in [[manganese nodule]]s.<ref name="Goldberg" /> Iridium is one of the characteristic elements of extraterrestrial rocks, and, along with osmium, can be used as a tracer element for meteoritic material in sediment.<ref name="Peucker-Ehrenbrink">{{cite book |last1=Peucker-Ehrenbrink |first1=B |title=Accretion of Extraterrestrial Matter Throughout Earth's History |chapter=Iridium and Osmium as Tracers of Extraterrestrial Matter in Marine Sediments |date=2001 |pages=163–178 |doi=10.1007/978-1-4419-8694-8_10|isbn=978-1-4613-4668-5 }}</ref><ref name="Barker">{{cite journal |last1=Barker |first1=J |last2=Edward |first2=A |title=Accretion rate of cosmic matter from iridium and osmium contents of deep-sea sediments |journal=Geochimica et Cosmochimica Acta |date=1968 |volume=32 |issue=6 |pages=627–645 |doi=10.1016/0016-7037(68)90053-7|bibcode=1968GeCoA..32..627B }}</ref> For example, core samples from the Pacific Ocean with elevated iridium levels suggested the [[Eltanin impact]] of about 2.5&nbsp;million years ago.<ref name="Kyte1981"/>

Some of the [[mass extinction]]s, such as the [[Cretaceous extinction]], can be identified by anomalously high concentrations of iridium in sediment, and these can be linked to major [[asteroid impact]]s.<ref name="Colodner">{{cite journal |last1=Colodner |first1=D |last2=Edmond |first2=J |title=Post-depositional mobility of platinum, iridium and rhenium in marine sediments |journal=Nature |date=1992 |volume=358 |issue=6385 |pages=402–404 |doi=10.1038/358402a0|bibcode=1992Natur.358..402C |s2cid=37386975 }}</ref>

=== Cretaceous–Paleogene boundary presence ===
[[File:K-T boundary.jpg|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.]]

{{Main|Cretaceous–Paleogene extinction event}}

The [[Cretaceous–Paleogene boundary]] of 66 million years ago, marking the temporal border between the [[Cretaceous]] and [[Paleogene]] periods of [[Geologic time scale|geological time]], was identified by a thin [[stratum]] of [[iridium anomaly|iridium-rich clay]].<ref name="Alvarez" /> A team led by [[Luis Walter Alvarez|Luis Alvarez]] proposed in 1980 an extraterrestrial origin for this iridium, attributing it to an [[asteroid]] or [[comet]] impact.<ref name="Alvarez">{{cite journal|title=Extraterrestrial cause for the Cretaceous–Tertiary extinction|author=Alvarez, L. W.|author-link=Luis Walter Alvarez|author2=Alvarez, W.|author3=Asaro, F.|author4=Michel, H. V.|s2cid=16017767|date=1980|journal=Science|volume=208|issue=4448|pages=1095–1108|doi=10.1126/science.208.4448.1095|pmid=17783054|bibcode = 1980Sci...208.1095A |url=http://chaos.swarthmore.edu/courses/soc26/bak-sneppan/13_alverez.pdf|citeseerx=10.1.1.126.8496}}</ref> 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]]).<ref>{{cite journal |last=Hildebrand |first=A. R. |author2=Penfield, Glen T. |author3=Kring, David A. |author4=Pilkington, Mark |author5=Zanoguera, Antonio Camargo |author6=Jacobsen, Stein B. |author7= Boynton, William V. |title=Chicxulub Crater; a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico |date=1991 |volume=19 |issue=9 |journal=[[Geology (journal)|Geology]] |pages=867–871 |doi=10.1130/0091-7613(1991)019<0867:CCAPCT>2.3.CO;2 |bibcode=1991Geo....19..867H}}</ref><ref>{{cite book|author=Frankel, C.|title=The End of the Dinosaurs: Chicxulub Crater and Mass Extinctions|date=1999|publisher=Cambridge University Press|isbn=978-0-521-47447-4|oclc=40298401|url-access=registration |url=https://archive.org/details/endofdinosaursch00fran}}</ref> Dewey M. McLean and others argue that the iridium may have been of [[volcano|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.<ref>{{cite book|title=The Cretaceous-Tertiary Event and Other Catastrophes in Earth History|author=Ryder, G.|author2=Fastovsky, D. E.|author3=Gartner, S.|publisher=Geological Society of America|date=1996|isbn=978-0-8137-2307-5|page=47}}</ref><ref>{{cite journal |author=Toutain, J.-P.|author2=Meyer, G.|date=1989|title=Iridium-Bearing Sublimates at a Hot-Spot Volcano (Piton De La Fournaise, Indian Ocean)|journal=Geophysical Research Letters|volume=16|issue=12|pages=1391–1394|doi=10.1029/GL016i012p01391|bibcode=1989GeoRL..16.1391T}}</ref>

== Production ==
{|class="wikitable" style="text-align:center; float:right; margin-left:0.5em"
!Year!!Consumption<br />(tonnes)!!Price (US$)<ref name="usgs">[https://minerals.usgs.gov/minerals/pubs/commodity/platinum/ Platinum-Group Metals]. U.S. Geological Survey Mineral Commodity Summaries</ref>
|-
|2001|| 2.6||{{convert|415.25|$/ozt|$/g|abbr=on|lk=in}}
|-
|2002||2.5 ||{{convert|294.62|$/ozt|$/g|abbr=on|lk=in}}
|-
|2003|| 3.3||{{convert|93.02|$/ozt|$/g|abbr=on|lk=in}}
|-
|2004||3.60 ||{{convert|185.33|$/ozt|$/g|abbr=on|lk=in}}
|-
|2005||3.86 ||{{convert|169.51|$/ozt|$/g|abbr=on|lk=in}}
|-
|2006||4.08 ||{{convert|349.45|$/ozt|$/g|abbr=on|lk=in}}
|-
|2007||3.70 ||{{convert|444.43|$/ozt|$/g|abbr=on|lk=in}}
|-
|2008||3.10 ||{{convert|448.34|$/ozt|$/g|abbr=on|lk=in}}
|-
|2009||2.52||{{convert|420.4|$/ozt|$/g|abbr=on|lk=in}}
|-
|2010||10.40||{{convert|642.15|$/ozt|$/g|abbr=on|lk=in}}
|-
|2011||9.36||{{convert|1035.87|$/ozt|$/g|abbr=on|lk=in}}
|-
|2012||5.54||{{convert|1066.23|$/ozt|$/g|abbr=on|lk=in}}
|-
|2013||6.16||{{convert|826.45|$/ozt|$/g|abbr=on|lk=in}}
|-
|2014||6.1||{{convert|556.19|$/ozt|$/g|abbr=on|lk=in}}
|-
|2015||7.81||{{convert|544|$/ozt|$/g|abbr=on|lk=in}}
|-
|2016||7.71||{{convert|586.90|$/ozt|$/g|abbr=on|lk=in}}
|-
|2017||n.d.||{{convert|908.35|$/ozt|$/g|abbr=on|lk=in}}
|-
|2018||n.d.||{{convert|1293.27|$/ozt|$/g|abbr=on|lk=in}}
|-
|2019||n.d.||{{convert|1485.80|$/ozt|$/g|abbr=on|lk=in}}
|-
|2020||n.d.||{{convert|1633.51|$/ozt|$/g|abbr=on|lk=in}}
|-
|2021||n.d.||{{convert|5400.00|$/ozt|$/g|abbr=on|lk=in}}
|-
|2022||n.d.||{{convert|3980.00|$/ozt|$/g|abbr=on|lk=in}}
|-
|2023||n.d.||{{convert|4652.38|$/ozt|$/g|abbr=on|lk=in}}
|-
|2024||n.d.||{{convert|5000.00|$/ozt|$/g|abbr=on|lk=in}}
|}
<!-- Should be updated sometime! A good source is http://www.platinum.matthey.com/prices/price-charts -->

Worldwide production of iridium was about {{convert|7300|kg}} in 2018.<ref name="usgs2018">{{cite book |last1=Singerling |first1=Sheryl A. |url=https://d9-wret.s3.us-west-2.amazonaws.com/assets/palladium/production/atoms/files/myb1-2018-plati.pdf |title=2018 Minerals Yearbook |last2=Schulte |first2=Ruth F. |date=August 2021 |publisher=USGS |page=57.11 |chapter=Platinum-Group Metals}}</ref> The price is high and varying (see table). Illustrative factors that affect the price include oversupply of Ir crucibles<ref name="usgs" /><ref>{{cite journal|author=Hagelüken, C. |journal=Metall |volume=60 |issue=1–2 |date=2006 |pages=31–42 |title=Markets for the catalysts metals platinum, palladium, and rhodium |url=http://www.preciousmetals.umicore.com/publications/articles_by_umicore/general/show_Metal_PGMmarkets_200602.pdf |url-status=dead |archive-url=https://web.archive.org/web/20090304195307/http://www.preciousmetals.umicore.com/publications/articles_by_umicore/general/show_Metal_PGMmarkets_200602.pdf |archive-date=March 4, 2009 }}</ref>
and changes in [[LED]] technology.<ref>
{{cite web
| url = http://www.platinum.matthey.com/media/1631250/other_pgm.pdf
| title = Platinum 2013 Interim Review
| website = Platinum Today
| publisher = [[Johnson Matthey]]
| access-date = 2014-01-10}}
</ref>

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.<ref name=JM>{{cite web |url=https://matthey.com/en/iridium-supply-hydrogen-electrolysers|title=Recycling and thrifting: the answer to the iridium question in electrolyser growth|first1=Marge |last1=Ryan |date=2022-11-16}}</ref> To separate the metals, they must first be brought into [[Solution (chemistry)|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]].<ref name="ullmann-pt" /><ref name="kirk-pt" /> From soluble extracts, iridium is separated by precipitating solid [[ammonium hexachloroiridate]] ({{chem|(NH|4|)|2|IrCl|6}}) or by extracting {{chem|IrCl|6|2-}} with organic amines.<ref>{{cite journal| title = The Platinum Metals| first = Raleigh| last = Gilchrist| journal = Chemical Reviews| date = 1943| volume = 32| issue = 3| pages = 277–372| doi = 10.1021/cr60103a002| s2cid = 96640406}}</ref> 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 ''[[metal sponge|sponge]]'', which is amenable to [[powder metallurgy]] techniques.<ref>{{cite journal| title =Processing of Iridium and Iridium Alloys| first = E. K.| last = Ohriner| journal = Platinum Metals Review| volume = 52| issue = 3| date = 2008| pages = 186–197| doi =10.1595/147106708X333827| doi-access = free}}</ref><ref>{{cite journal| first = L. B.| last = Hunt| author2 = Lever, F. M.| journal = Platinum Metals Review| volume = 13| issue = 4| date = 1969| pages = 126–138| title = Platinum Metals: A Survey of Productive Resources to industrial Uses| doi = 10.1595/003214069X134126138| s2cid = 267561907| url = http://www.platinummetalsreview.com/pdf/pmr-v13-i4-126-138.pdf| access-date = 2008-10-01| archive-date = 2008-10-29| archive-url = https://web.archive.org/web/20081029205825/http://www.platinummetalsreview.com/pdf/pmr-v13-i4-126-138.pdf| url-status = dead}}</ref> Iridium is also obtained commercially as a by-product from [[nickel]] and copper mining and processing. During [[Copper extraction techniques#Electrorefining|electrorefining of copper]] and nickel, noble metals such as silver, gold and the [[platinum group metal]]s as well as [[selenium]] and [[tellurium]] settle to the bottom of the cell as ''anode mud'', which forms the starting point for their extraction.<ref name="usgs" />

{{mw-datatable}}{{static row numbers}}
{| class="wikitable sortable mw-datatable static-row-numbers" style=text-align:right;
|+Leading iridium-producing countries (kg)<ref>{{Cite web|last=|first=|date=|title=Mineral Yearbook 2020 tables-only release|url=https://www.usgs.gov/centers/national-minerals-information-center/platinum-group-metals-statistics-and-information|url-status=|archive-url=|archive-date=|access-date=|website=USGS}}</ref>
|- 
! Country !!  2016 !! 2017 !! 2018 !! 2019 !! 2020
|- class="static-row-header " style="font-weight:bold;" class=sorttop
| {{left}} {{noflag|World}} || 7,720 || 7,180 || 7,540 || 7,910 || 8,170
|-
|{{flagg|us*eft|South Africa|pref=Natural resources of}} || 6,624 || 6,057 || 6,357 || 6,464 || 6,786
|-
|{{flagg|us*eft|Zimbabwe|pref=Natural resources of}} || 598 || 619 || 586 || 845 || 836
|-
|{{flagg|us*eft|Canada|pref=Natural resources of}} || 300 || 200 || 400 || 300 || 300
|-
|{{flagg|us*eft|Russia|pref=Natural resources of}} || 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, [[OLED]]s, crucibles, [[Cativa process|catalysts]] (e.g. [[acetic acid]]), and ignition tips for spark plugs.<ref name=JM/>

=== Metal and alloys ===
Resistance to heat and corrosion are the bases for several uses of iridium and its alloys.

Owing to its high melting point, hardness, and [[corrosion resistance]], iridium is used to make crucibles. Such [[crucible]]s are used in the [[Czochralski process]] to produce oxide single-crystals (such as [[sapphire]]s) for use in computer memory devices and in solid state lasers.<ref name="Handley">{{cite journal|title= Increasing Applications for Iridium| first = J. R.| last = Handley| journal = Platinum Metals Review| volume = 30| issue = 1| date = 1986| pages = 12–13| doi = 10.1595/003214086X3011213| url = https://technology.matthey.com/article/30/1/12-13/| url-access = subscription}}</ref><ref>{{cite journal|title= On the Use of Iridium Crucibles in Chemical Operations| first = W.| last = Crookes|author-link=William Crookes| journal = Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character| volume = 80| issue = 541| date = 1908| pages = 535–536| jstor = 93031|doi= 10.1098/rspa.1908.0046|bibcode = 1908RSPSA..80..535C | doi-access = free}}</ref> 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 {{cvt|2100|°C}}.<ref name="hunt" />

Certain long-life aircraft engine parts are made of an iridium alloy, and an iridium–[[titanium]] alloy is used for deep-water pipes because of its corrosion resistance.<ref name="Emsley" /> Iridium is used for [[multi-pored]] [[Spinneret (polymers)|spinnerets]], through which a plastic polymer melt is extruded to form fibers, such as [[rayon]].<ref>{{cite journal|title= Spinnerets for viscose rayon cord yarn| journal = Fibre Chemistry| volume =10| issue = 4| date = 1979| doi = 10.1007/BF00543390| pages = 377–378| first = R. V.| last = Egorova|author2=Korotkov, B. V. |author3=Yaroshchuk, E. G. |author4=Mirkus, K. A. |author5=Dorofeev N. A. |author6= Serkov, A. T. | s2cid = 135705244}}</ref> Osmium–iridium is used for compass bearings and for balances.<ref name="hunt" />

Because of their resistance to arc erosion, iridium alloys are used by some manufacturers for the centre electrodes of [[spark plug]]s,<ref name="Handley" /><ref>{{cite book | last1=Graff | first1=Muriel | last2=Kempf | first2=Bernd | last3=Breme | first3=Jürgen | title=Materials for Transportation Technology | chapter=Iridium Alloy for Spark Plug Electrodes | publisher=Wiley-VCH Verlag GmbH & Co. KGaA | publication-place=Weinheim, FRG | date=2005-12-23 | pages=1–8 | doi=10.1002/3527606025.ch1| isbn=9783527301249 }}</ref> and iridium-based spark plugs are particularly used in aviation.

===Catalysis===
Iridium compounds are used as [[catalysis|catalysts]] in the [[Cativa process]] for [[carbonylation]] of [[methanol]] to produce [[acetic acid]].<ref name="ullmann-acetic">{{cite book|first=H.|last= Cheung| author2=Tanke, R. S.| author3=Torrence, G. P.|chapter=Acetic acid|title=Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley|date=2000|doi=10.1002/14356007.a01_045|isbn= 978-3527306732}}</ref><ref name = "Jones">{{cite journal | last1 = Jones | first1 = Jane H. | title = The cativa™ process for the manufacture of acetic acid. | journal = Platinum Metals Review | volume = 44 | issue = 3 | year = 2000 | pages= 94–105 | doi = 10.1595/003214000X44394105 | url = https://technology.matthey.com/article/44/3/94-105/| doi-access = free }}</ref>

Iridium complexes are often active for [[asymmetric hydrogenation]] both by traditional [[hydrogenation]].<ref>{{cite journal|doi=10.1021/ar700113g|date=2007|author=Roseblade, S. J.|author2=Pfaltz, A.|title=Iridium-catalyzed asymmetric hydrogenation of olefins|volume=40|issue=12|pages=1402–1411|pmid=17672517|journal=[[Accounts of Chemical Research]]}}</ref> and [[transfer hydrogenation]].<ref>{{cite journal |doi=10.1021/ar700134q|title=Asymmetric Transfer Hydrogenation of Ketones with Bifunctional Transition Metal-Based Molecular Catalysts† |year=2007 |last1=Ikariya |first1=Takao |last2=Blacker |first2=A. John |journal=Accounts of Chemical Research |volume=40 |issue=12 |pages=1300–1308 |pmid=17960897 }}</ref>  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]].<ref>{{cite book|editor=Matthias Beller, Hans-Ulrich Blaser|series=Topics in Organometallic Chemistry|volume=42|publisher=Springer|location=Berlin, Heidelberg|year=2012|isbn=978-3-642-32832-9|title=Organometallics as Catalysts in the Fine Chemical Industry}}</ref>

=== Medical imaging ===
The radioisotope [[iridium-192]] is one of the two most important sources of energy for use in industrial [[Industrial radiography#Radioisotope sources|γ-radiography]] for [[non-destructive testing]] of metals.<ref>{{cite journal| title=The use and scope of Iridium 192 for the radiography of steel| first=R.| last=Halmshaw| date=1954| journal=British Journal of Applied Physics| volume=5| issue=7| pages=238–243| doi=10.1088/0508-3443/5/7/302| bibcode=1954BJAP....5..238H}}</ref><ref name="Hellier">{{cite book| last=Hellier| first=Chuck| title=Handbook of Nondestructive Evlaluation| publisher=The McGraw-Hill Companies| date=2001| isbn=978-0-07-028121-9}}</ref> Additionally, {{SimpleNuclide|Ir|192}} 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.<ref name="Emsley" /> [[Iridium-192]] is normally produced by neutron activation of isotope [[iridium-191]] in natural-abundance iridium metal.<ref name="iridium-192">{{cite book |author1=Jean Pouliot |author2=Luc Beaulieu |chapter=13 – Modern Principles of Brachytherapy Physics: From 2-D to 3-D to Dynamic Planning and Delivery |editor1=Richard T. Hoppe |editor2=Theodore Locke Phillips |editor3=Mack Roach |title=Leibel and Phillips Textbook of Radiation Oncology |edition=3rd |publisher=W.B. Saunders |year=2010 |pages=224–244 |isbn=9781416058977 |doi=10.1016/B978-1-4160-5897-7.00013-5 |url=https://www.sciencedirect.com/topics/medicine-and-dentistry/iridium-192}}</ref>

=== Photocatalysis and OLEDs ===
Iridium complexes are key components of white [[OLED]]s.  Similar complexes are used in [[photocatalysis]].<ref>{{cite journal |doi=10.1002/adma.200803537|title=Recent Developments in the Application of Phosphorescent Iridium(III) Complex Systems |year=2009 |last1=Ulbricht |first1=Christoph |last2=Beyer |first2=Beatrice |last3=Friebe |first3=Christian |last4=Winter |first4=Andreas |last5=Schubert |first5=Ulrich S. |journal=Advanced Materials |volume=21 |issue=44 |pages=4418–4441 |bibcode=2009AdM....21.4418U |s2cid=96268110 }}</ref>

=== Scientific ===
[[File:Platinum-Iridium meter bar.jpg|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#International prototype kilogram|kilogram]] mass, kept by the [[Bureau International des Poids et Mesures|International Bureau of Weights and Measures]] near Paris.<ref name="Emsley" /> The meter bar was replaced as the definition of the fundamental unit of length in 1960 by a line in the [[atomic spectrum]] of [[Krypton#Metric role|krypton]],{{efn|The definition of the meter was changed again in 1983. The meter is currently defined as the distance traveled by light in a vacuum during a time interval of {{frac|299,792,458}} of a second.}}<ref name="meter">{{cite web| url=https://www.nist.gov/document/museum-timelinepdf| publisher = National Institute for Standards and Technology|first = W. B.| last = Penzes|title=Time Line for the Definition of the Meter|date=2001|access-date=2008-09-16}}</ref> but the kilogram prototype remained the international standard of mass [[2019 revision of the SI|until 20 May 2019]], when the kilogram was redefined in terms of the [[Planck constant]].<ref>General section citations: ''Recalibration of the U.S. National Prototype Kilogram'', R.{{nbsp}}S.{{nbsp}}Davis, Journal of Research of the National Bureau of Standards, '''90''', No. 4, {{nowrap|July–August}} 1985 ([http://nvlpubs.nist.gov/nistpubs/jres/090/jresv90n4p263_A1b.pdf 5.5{{nbsp}}MB PDF] {{Webarchive|url=https://web.archive.org/web/20170201170330/http://nvlpubs.nist.gov/nistpubs/jres/090/jresv90n4p263_A1b.pdf |date=2017-02-01 }}); and ''The Kilogram and Measurements of Mass and Force'', Z.{{nbsp}}J.{{nbsp}}Jabbour ''et al.'', J. Res. Natl. Inst. Stand. Technol. '''106''', 2001, {{nowrap|25–46}} ([https://www.nist.gov/sites/default/files/documents/calibrations/j61jab.pdf 3.5{{nbsp}}MB PDF])<sub>{{nbsp}}</sub></ref>
<!--
Iridium is often used as a coating for non-conductive materials in preparation for observation in [[scanning electron microscopes]] (SEM).
The addition of a {{cvt|2|to|20|nm}} 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.<ref>{{cite journal |last1=Höflinger |first1=Gisela |title=Brief Introduction to Coating Technology for Electron Microscopy |url=https://www.leica-microsystems.com/science-lab/brief-introduction-to-coating-technology-for-electron-microscopy/ |website=Leica Microsystems |publisher=Leica Microsystems |access-date=22 April 2019|date=2013-08-28 }}</ref> 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.<ref>{{cite journal|doi=10.1016/j.memsci.2014.03.048|date=2014|author=Abdullah, S. Z.|author2=Bérubé, Pierre R. |author3=Horne, D.J.|title=SEM imaging of membranes: Importance of sample preparation and imaging parameters|volume=463|pages=113–125|journal=[[Journal of Membrane Science]]}}</ref>

Iridium has been used in the [[radioisotope thermoelectric generator]]s of unmanned spacecraft such as the ''[[Voyager program|Voyager]]'', ''[[Viking program|Viking]]'', ''[[Pioneer program|Pioneer]]'', ''[[Cassini-Huygens|Cassini]]'', ''[[Galileo (spacecraft)|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 {{cvt|2000|°C}} and for its great strength.<ref name="hunt" />

Another use concerns X-ray optics, especially X-ray telescopes.<ref name="Ziegler">{{cite journal| doi=10.1016/S0168-9002(01)00533-2| title=High-efficiency tunable X-ray focusing optics using mirrors and laterally-graded multilayers| first1=E.| last1=Ziegler| last2=Hignette| first2=O.| last3=Morawe |first3=Ch.| last4=Tucoulou |first4=R.| journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment| volume=467–468| issue=Part 2| date=2001| pages=954–957| bibcode=2001NIMPA.467..954Z}}</ref> The mirrors of the [[Chandra X-ray Observatory]] are coated with a layer of iridium {{cvt|60|nm|lk=in}} 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 [[vacuum|high vacuum]] on a base layer of [[chromium]].<ref>{{cite web|title=Face-to-Face with Jerry Johnston, CXC Program Manager & Bob Hahn, Chief Engineer at Optical Coating Laboratories, Inc., Santa Rosa, CA|publisher=Harvard-Smithsonian Center for Astrophysics; Chandra X-ray Center|date=1995 |url=http://chandra.harvard.edu/press/bios/johnston.html|access-date=2008-09-24}}</ref>

Iridium is used in [[particle physics]] for the production of [[antiproton]]s, 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 wave]]s induced by the temperature rise due to the incident beam.<ref>{{cite journal| title=Production of low-energy antiprotons| journal=Zeitschrift Hyperfine Interactions| volume=109| issue=1–4| date=1997| doi=10.1023/A:1012680728257| pages=33–41| first=D.| last=Möhl| s2cid=118043983 |bibcode=1997HyInt.109...33M}}</ref>

Iridium forms a variety of [[Complex (chemistry)|complexes]] of fundamental interest in triplet harvesting.<ref>{{cite journal|title = Electrophosphorescence from substituted poly(thiophene) doped with iridium or platinum complex| doi = 10.1016/j.tsf.2004.05.095| date = 2004| journal = Thin Solid Films| volume = 468| issue = 1–2| pages = 226–233| first = X.| last = Wang| author2=Andersson, M. R.| author3=Thompson, M. E.| author4=Inganäsa, O.|bibcode = 2004TSF...468..226W }}</ref><ref>{{cite journal|url=http://sa.rochester.edu/jur/issues/fall2002/tonzetich.pdf|title=Organic Light Emitting Diodes—Developing Chemicals to Light the Future|publisher=Rochester University|first=Zachary J.|last=Tonzetich|journal=Journal of Undergraduate Research|volume=1|issue=1|date=2002|access-date=2008-10-10}}</ref><ref>{{cite journal| title=New Trends in the Use of Transition Metal-Ligand Complexes for Applications in Electroluminescent Devices| author = Holder, E.| author2=Langefeld, B. M. W.| author3=Schubert, U. S.| journal = Advanced Materials| volume = 17| issue = 9| pages = 1109–1121| date = 2005-04-25|doi=10.1002/adma.200400284| bibcode = 2005AdM....17.1109H| s2cid = 98683000}}</ref>

As part of the cationic nucleic acid [[Intercalation (biochemistry)|intercalators]], iridium is used to detect nucleic acids in [[CyTOF]] experiments to analyse the presence or viability of nucleated cells in biological samples.<ref name="Biver Secco Venturini 2008 pp. 1163–1177">{{cite journal | last1=Biver | first1=Tarita | last2=Secco | first2=Fernando | last3=Venturini | first3=Marcella | title=Mechanistic aspects of the interaction of intercalating metal complexes with nucleic acids | journal=Coordination Chemistry Reviews | publisher=Elsevier BV | volume=252 | issue=10–11 | year=2008 | issn=0010-8545 | doi=10.1016/j.ccr.2007.10.008 | pages=1163–1177}}</ref>
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=== Historical ===
[[File:Gama Supreme Flat Top ebonite eyedropper fountain pen 3.JPG|right|thumb|[[Fountain pen]] nib labelled ''Iridium Point'']]

Iridium–osmium alloys were previously used in [[fountain pen]] [[Nib (pen)#Nib tipping|nib tip]]s. The first major use of iridium was in 1834 in nibs mounted on gold.<ref name="hunt" /> Starting in 1944, the [[Parker 51]] fountain pen was fitted with a nib tipped by a ruthenium and iridium alloy (with 3.8% iridium). The tip material in modern fountain pens is still conventionally called "iridium", although there is seldom any iridium in it; other metals such as [[ruthenium]], [[osmium]], and [[tungsten]] have taken its place.<ref>{{cite journal|url=https://www.nibs.com/blog/nibster-writes/wheres-iridium|journal=The PENnant|volume=XIII|issue=2|date=1999|title=Notes from the Nib Works—Where's the Iridium?|author=Mottishaw, J.|archive-date=2023-07-13|access-date=2022-09-24|archive-url=https://web.archive.org/web/20230713205938/https://www.nibs.com/blog/nibster-writes/wheres-iridium|url-status=dead}}</ref>

An iridium–platinum alloy was used for the [[touch hole]]s or vent pieces of [[cannon]]. According to a report of the [[Exposition Universelle (1867)|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".<ref>{{cite journal|editor=Crookes, W.|volume=XV|date=1867|journal=The Chemical News and Journal of Physical Science|title=The Paris Exhibition|page=182 | url = https://en.wikisource.org/w/index.php?title=File:The_chemical_news._Volume_15,_January_-_June_1867._(IA_s713id13683370).pdf&page=188}}</ref>

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".<ref>{{cite book|title=The Playbook of Metals: Including Personal Narratives of Visits to Coal, Lead, Copper, and Tin Mines, with a Large Number of Interesting Experiments Relating to Alchemy and the Chemistry of the Fifty Metallic Elements|url=https://archive.org/details/playbookmetalsi00peppgoog|author=Pepper, J. H.|publisher=Routledge, Warne, and Routledge|date=1861|page=[https://archive.org/details/playbookmetalsi00peppgoog/page/n469 455]}}</ref>

== 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&nbsp;[[parts per notation|parts per trillion]] of iridium in human tissue.<ref name="Emsley" /> Like most metals, finely divided iridium powder can be hazardous to handle, as it is an irritant and may ignite in air.<ref name="kirk-pt">{{cite book|title=Kirk Othmer Encyclopedia of Chemical Technology|first = R. J.| last = Seymour|author2=O'Farrelly, J. I.|chapter=Platinum-Group Metals|doi=10.1002/0471238961.1612012019052513.a01.pub3|date=2012|publisher=Wiley| isbn=978-0471238966 }}</ref> Iridium is relatively unhazardous otherwise, with the only effect of Iridium ingestion being irritation of the [[Gastrointestinal tract|digestive tract]].<ref>{{Cite web |title=Iridium (Ir) – Chemical properties, Health and Environmental effects |url=https://www.lenntech.com/periodic/elements/ir.htm#:~:text=not%20been%20estimated.-,Health%20effects%20of%20iridium,irritation%20of%20the%20digestive%20tract. |access-date=2024-07-27 |website=www.lenntech.com}}</ref> However, soluble salts, such as the iridium halides, could be hazardous due to elements other than iridium or due to iridium itself.<ref name="mager" /> At the same time, most iridium compounds are insoluble, which makes absorption into the body difficult.<ref name="Emsley" />

A radioisotope of iridium, {{chem|192|Ir}}, is dangerous, like other radioactive isotopes. The only reported injuries related to iridium concern accidental exposure to radiation from {{chem|192|Ir}} used in [[brachytherapy]].<ref name="mager">{{cite book|title=Encyclopaedia of Occupational Health and Safety|first=J.|last=Mager Stellman|chapter=Iridium|isbn=978-92-2-109816-4|date=1998|publisher=International Labour Organization|pages=[https://archive.org/details/encyclopaediaofo0003unse/page/63 63.19]|chapter-url=https://books.google.com/books?id=nDhpLa1rl44C&pg=PT125|oclc=35279504|url=https://archive.org/details/encyclopaediaofo0003unse/page/63}}</ref> High-energy gamma radiation from {{chem|192|Ir}} 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.<ref>{{cite web| title = Radioisotope Brief: Iridium-192 (Ir-192)| work = Radiation Emergencies| publisher = Centers for Disease Control and Prevention| date = 2004-08-18| url = http://emergency.cdc.gov/radiation/isotopes/pdf/iridium.pdf| access-date = 2008-09-20|archive-url=https://web.archive.org/web/20081029205825/https://emergency.cdc.gov/radiation/isotopes/pdf/iridium.pdf|archive-date=29 October 2008|url-status=dead}}</ref> <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 radiation|gamma]] and [[Beta particle|beta]] radiation.<ref name="argonne">{{cite web|title=Iridium |work=Human Health Fact Sheet |publisher=Argonne National Laboratory |date=2005 |url=http://www.ead.anl.gov/pub/doc/Iridium.pdf |access-date=2008-09-20 |url-status=dead |archive-url=https://web.archive.org/web/20120304005456/http://www.ead.anl.gov/pub/doc/Iridium.pdf |archive-date=March 4, 2012 }}</ref>

== Notes ==
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== References ==
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{{reflist|30em}}

== External links ==
{{Commons}}
{{Wiktionary|iridium}}
* [http://www.periodicvideos.com/videos/077.htm Iridium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [https://www.britannica.com/EBchecked/topic/293985/iridium-Ir Iridium in Encyclopædia Britannica]

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{{Iridium compounds}}
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[[Category:Iridium| ]]
[[Category:Chemical elements]]
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[[Category:Chemical elements with face-centered cubic structure]]
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