Editing Iridium (section)

== 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]] |page=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>