Editing Diamond (section)

== Geology ==
Diamonds are extremely rare, with concentrations of at most parts per billion in source rock.<ref name=Cartigny/> Before the 20th century, most diamonds were found in [[alluvial deposit]]s. Loose diamonds are also found along existing and ancient [[shore]]lines, where they tend to accumulate because of their size and density.<ref name=AMNH>{{cite book| vauthors = Erlich EI, Hausel WD |title=Diamond deposits: origin, exploration, and history of discovery|date=2002|publisher=Society for Mining, Metallurgy, and Exploration|location=Littleton, CO|isbn=978-0-87335-213-0}}</ref>{{rp|149}} Rarely, they have been found in [[glacial till]] (notably in [[Wisconsin]] and [[Indiana]]), but these deposits are not of commercial quality.<ref name=AMNH/>{{rp|19}} These types of deposit were derived from localized igneous [[intrusive rock|intrusions]] through [[weathering]] and [[sediment transport|transport]] by [[wind]] or [[water]].<ref name=Shirey2013>{{cite journal | vauthors = Shirey SB, Shigley JE |title=Recent Advances in Understanding the Geology of Diamonds |journal=Gems & Gemology |date=December 1, 2013 |volume=49 |issue=4 |pages=188–222 |doi=10.5741/GEMS.49.4.188 |doi-access=free}}</ref>

Most diamonds come from the [[Mantle (geology)|Earth's mantle]], and most of this section discusses those diamonds. However, there are other sources. Some blocks of the crust, or [[terrane]]s, have been buried deep enough as the crust thickened so they experienced [[ultra-high-pressure metamorphism]]. These have evenly distributed ''microdiamonds'' that show no sign of transport by magma. In addition, when meteorites strike the ground, the shock wave can produce high enough temperatures and pressures for ''microdiamonds'' and ''[[Detonation nanodiamond|nanodiamonds]]'' to form.<ref name=Shirey2013/> Impact-type microdiamonds can be used as an indicator of ancient impact craters.<ref>{{cite book|title=The Mantle and Core|vauthors=Carlson RW|url=https://books.google.com/books?id=1clZ4ABsfoAC&pg=PA248|page=248|publisher=Elsevier|year=2005|isbn=978-0-08-044848-0|access-date=November 9, 2020|archive-date=November 9, 2023|archive-url=https://web.archive.org/web/20231109173351/https://books.google.com/books?id=1clZ4ABsfoAC&pg=PA248#v=onepage&q&f=false|url-status=live}}</ref> [[Popigai impact structure]] in Russia may have the world's largest diamond deposit, estimated at trillions of carats, and formed by an asteroid impact.<ref>{{Cite journal|volume=23|pages=3–12| vauthors = Deutsch A, Masaitis VL, Langenhorst F, Grieve RA |title=Popigai, Siberia—well preserved giant impact structure, national treasury, and world's geological heritage|journal=Episodes|year=2000|issue=1|doi=10.18814/epiiugs/2000/v23i1/002|doi-access=free}}</ref>

A common misconception is that diamonds form from highly compressed [[coal]]. Coal is formed from buried prehistoric plants, and most diamonds that have been dated are far older than the first [[embryophyte|land plants]]. It is possible that diamonds can form from coal in [[subduction zone]]s, but diamonds formed in this way are rare, and the carbon source is more likely [[carbonate]] rocks and organic carbon in sediments, rather than coal.<ref>{{cite web |url=http://geology.com/articles/diamonds-from-coal/ |title=How do diamonds form? They don't form from coal! | vauthors = King H |date=2012 |work=Geology and Earth Science News and Information |publisher=geology.com |access-date=June 29, 2012 |archive-url=https://web.archive.org/web/20131030014537/http://geology.com/articles/diamonds-from-coal/ |archive-date=October 30, 2013 |url-status=live}}</ref><ref>{{cite journal |title=10 common scientific misconceptions |vauthors=Pak-Harvey A |journal=The Christian Science Monitor |date=October 31, 2013 |url=http://www.csmonitor.com/Science/2013/1031/10-common-scientific-misconceptions/Diamonds-form-from-pressurized-coal |access-date=August 30, 2017 |archive-date=January 6, 2017 |archive-url=https://web.archive.org/web/20170106012033/http://www.csmonitor.com/Science/2013/1031/10-common-scientific-misconceptions/Diamonds-form-from-pressurized-coal |url-status=live }}</ref>

=== Surface distribution ===
[[File:World geologic provinces.jpg|thumb|[[Geologic province]]s of the world. The pink and orange areas are [[Shield (geology)|shields]] and [[Platform (geology)|platforms]], which together constitute cratons.]]

Diamonds are far from evenly distributed over the Earth. A rule of thumb known as Clifford's rule states that they are almost always found in [[kimberlite]]s on the oldest part of [[craton]]s, the stable cores of continents with typical ages of 2.5{{nbsp}}billion years or more.<ref name=Shirey2013/><ref>{{cite book | vauthors = Pohl WL |title=Economic Geology: Principles and Practice |date=2011 |publisher=John Wiley & Sons |isbn=978-1-4443-9486-3}}</ref>{{rp |314}} However, there are exceptions. The [[Argyle diamond mine]] in [[Australia]], the largest producer of diamonds by weight in the world, is located in a ''mobile belt'', also known as an ''[[orogeny|orogenic belt]]'',<ref>{{cite encyclopedia | vauthors = Allaby M |title=mobile belt |encyclopedia=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-174433-4 |edition=4th}}</ref> a weaker zone surrounding the central craton that has undergone compressional tectonics. Instead of [[kimberlite]], the host rock is [[lamproite]]. Lamproites with diamonds that are not economically viable are also found in the United States, India, and Australia.<ref name=Shirey2013/> In addition, diamonds in the [[Algoman orogeny|Wawa belt]] of the [[Superior craton|Superior province]] in [[Canada]] and microdiamonds in the [[Northeastern Japan arc|island arc of Japan]] are found in a type of rock called [[lamprophyre]].<ref name=Shirey2013/>

[[Kimberlite]]s can be found in narrow (1 to 4 meters) dikes and sills, and in pipes with diameters that range from about 75 m to 1.5&nbsp;km. Fresh rock is dark bluish green to greenish gray, but after exposure rapidly turns brown and crumbles.<ref>{{cite book| vauthors = Kjarsgaard BA |chapter=Kimberlite pipe models: significance for exploration| veditors = Milkereit B |title=Proceedings of Exploration 07: Fifth Decennial International Conference on Mineral Exploration|date=2007|publisher=[[Decennial Mineral Exploration Conferences]], 2007|pages=667–677|chapter-url=http://www.dmec.ca/ex07-dvd/E07/pdfs/46.pdf |archive-url=https://web.archive.org/web/20121224053731/http://www.dmec.ca/ex07-dvd/E07/pdfs/46.pdf |archive-date=December 24, 2012 |url-status=live|access-date=March 1, 2018}}</ref> It is hybrid rock with a chaotic mixture of small minerals and rock fragments ([[clastic rock|clasts]]) up to the size of watermelons. They are a mixture of [[xenocryst]]s and [[xenolith]]s (minerals and rocks carried up from the lower crust and mantle), pieces of surface rock, altered minerals such as [[Serpentine subgroup|serpentine]], and new minerals that crystallized during the eruption. The texture varies with depth. The composition forms a continuum with [[carbonatite]]s, but the latter have too much oxygen for carbon to exist in a pure form. Instead, it is locked up in the mineral [[calcite]] ({{chem|[[Calcium|Ca]]|[[Carbon|C]]|[[Oxygen|O]]|3}}).<ref name=Shirey2013/>

All three of the diamond-bearing rocks (kimberlite, lamproite and lamprophyre) lack certain minerals ([[melilite]] and [[kalsilite]]) that are incompatible with diamond formation. In [[kimberlite]], [[olivine]] is large and conspicuous, while lamproite has Ti-[[phlogopite]] and lamprophyre has [[biotite]] and [[amphibole]]. They are all derived from magma types that erupt rapidly from small amounts of melt, are rich in [[Volatility (chemistry)|volatiles]] and [[magnesium oxide]], and are less [[redox|oxidizing]] than more common mantle melts such as [[basalt]]. These characteristics allow the melts to carry diamonds to the surface before they dissolve.<ref name=Shirey2013/>

=== Exploration ===
[[File:Diavik Mine.tif|thumb|upright|[[Diavik Mine]], on an island in Lac de Gras in northern Canada]]

[[Kimberlite]] pipes can be difficult to find. They weather quickly (within a few years after exposure) and tend to have lower topographic relief than surrounding rock. If they are visible in outcrops, the diamonds are never visible because they are so rare. In any case, kimberlites are often covered with vegetation, sediments, soils, or lakes. In modern searches, [[geophysical survey|geophysical methods]] such as [[aeromagnetic survey]]s, [[electrical resistivity tomography|electrical resistivity]], and [[gravimetry]], help identify promising regions to explore. This is aided by isotopic dating and modeling of the geological history. Then surveyors must go to the area and collect samples, looking for kimberlite fragments or ''indicator minerals''. The latter have compositions that reflect the conditions where diamonds form, such as extreme melt depletion or high pressures in [[eclogite]]s. However, indicator minerals can be misleading; a better approach is [[geothermobarometry]], where the compositions of minerals are analyzed as if they were in equilibrium with mantle minerals.<ref name=Shirey2013/>

Finding kimberlites requires persistence, and only a small fraction contain diamonds that are commercially viable. The only major discoveries since about 1980 have been in Canada. Since existing mines have lifetimes of as little as 25 years, there could be a shortage of new natural diamonds in the future.<ref name=Shirey2013/>

=== Ages ===
Diamonds are dated by analyzing inclusions using the decay of radioactive isotopes. Depending on the elemental abundances, one can look at the decay of [[Rubidium–strontium dating|rubidium to strontium]], [[Samarium–neodymium dating|samarium to neodymium]], [[Uranium–lead dating|uranium to lead]], [[Argon–argon dating|argon-40 to argon-39]], or [[Rhenium–osmium dating|rhenium to osmium]]. Those found in kimberlites have ages ranging from {{nowrap|1 to 3.5 billion years}}, and there can be multiple ages in the same kimberlite, indicating multiple episodes of diamond formation. The kimberlites themselves are much younger. Most of them have ages between tens of millions and 300 million years old, although there are some older exceptions (Argyle, [[Premier Mine|Premier]] and Wawa). Thus, the kimberlites formed independently of the diamonds and served only to transport them to the surface.<ref name=Cartigny/><ref name=Shirey2013/> Kimberlites are also much younger than the cratons they have erupted through. The reason for the lack of older kimberlites is unknown, but it suggests there was some change in mantle chemistry or tectonics. No kimberlite has erupted in human history.<ref name=Shirey2013/>

=== Origin in mantle ===
[[File:Eclogite, détail de la roche.jpg|thumb|[[Eclogite]] with centimeter-size [[garnet]] crystals]]
[[File:Garnet inclusion in diamond.jpg|thumb|Red garnet inclusion in a diamond<ref name=DCOdecadal>{{cite book |last1=Deep Carbon Observatory |title=Deep Carbon Observatory: A Decade of Discovery |doi=10.17863/CAM.44064 |date=2019 |location=Washington, DC |url=https://deepcarbon.net/deep-carbon-observatory-decade-discovery |access-date=December 13, 2019 |archive-date=December 17, 2019 |archive-url=https://web.archive.org/web/20191217174901/https://deepcarbon.net/deep-carbon-observatory-decade-discovery |url-status=dead }}</ref>]]
Most gem-quality diamonds come from depths of 150–250&nbsp;km in the [[lithosphere]]. Such depths occur below cratons in ''mantle keels'', the thickest part of the lithosphere. These regions have high enough pressure and temperature to allow diamonds to form and they are not convecting, so diamonds can be stored for billions of years until a kimberlite eruption samples them.<ref name=Shirey2013/>

Host rocks in a mantle keel include [[harzburgite]] and [[lherzolite]], two type of [[peridotite]]. The most dominant rock type in the [[upper mantle (Earth)|upper mantle]], peridotite is an [[igneous rock]] consisting mostly of the minerals [[olivine]] and [[pyroxene]]; it is low in [[Silicon dioxide|silica]] and high in [[magnesium]]. However, diamonds in peridotite rarely survive the trip to the surface.<ref name=Shirey2013/> Another common source that does keep diamonds intact is [[eclogite]], a [[metamorphic]] rock that typically forms from [[basalt]] as an oceanic plate plunges into the mantle at a [[Subduction|subduction zone]].<ref name=Cartigny/>

A smaller fraction of diamonds (about 150 have been studied) come from depths of 330–660&nbsp;km, a region that includes the [[Transition zone (Earth)|transition zone]]. They formed in eclogite but are distinguished from diamonds of shallower origin by inclusions of [[majorite]] (a form of [[garnet]] with excess silicon). A similar proportion of diamonds comes from the lower mantle at depths between 660 and 800&nbsp;km.<ref name=Cartigny/>

Diamond is thermodynamically stable at high pressures and temperatures, with the phase transition from [[graphite]] occurring at greater temperatures as the pressure increases. Thus, underneath continents it becomes stable at temperatures of 950{{nbsp}}degrees Celsius and pressures of 4.5 gigapascals, corresponding to depths of 150{{nbsp}}kilometers or greater. In subduction zones, which are colder, it becomes stable at temperatures of 800&nbsp;°C and pressures of 3.5{{nbsp}}gigapascals. At depths greater than 240&nbsp;km, iron–nickel metal phases are present and carbon is likely to be either dissolved in them or in the form of [[carbide]]s. Thus, the deeper origin of some diamonds may reflect unusual growth environments.<ref name=Cartigny/><ref name=Shirey2013/>

In 2018 the first known natural samples of a phase of ice called [[Ice VII]] were found as inclusions in diamond samples. The inclusions formed at depths between 400 and 800&nbsp;km, straddling the upper and lower mantle, and provide evidence for water-rich fluid at these depths.<ref>{{cite journal| vauthors = Cartier K |title=Diamond Impurities Reveal Water Deep Within the Mantle|journal=Eos|date=April 2, 2018|volume=99|doi=10.1029/2018EO095949|doi-access=free}}</ref><ref name=Perkins>{{cite journal|vauthors=Perkins S|title=Pockets of water may lie deep below Earth's surface|journal=Science|date=March 8, 2018|url=https://www.science.org/content/article/pockets-water-may-lay-deep-below-earth-s-surface|access-date=June 30, 2022|archive-date=March 8, 2018|archive-url=https://web.archive.org/web/20180308220310/http://www.sciencemag.org/news/2018/03/pockets-water-may-lay-deep-below-earth-s-surface|url-status=live}}</ref>

=== Carbon sources ===
The mantle has roughly one billion [[tonne|gigatonnes]] of carbon (for comparison, the atmosphere-ocean system has about 44,000 gigatonnes).<ref>{{cite book | vauthors = Lee CA, Jiang H, Dasgupta R, Torres M  |chapter=A Framework for Understanding Whole-Earth Carbon Cycling |pages=313–357 |doi=10.1017/9781108677950.011 | veditors = Orcutt BN, Daniel I, Dasgupta R |title=Deep carbon: past to present |date=2019 |publisher=Cambridge University Press |isbn=978-1-108-67795-0|s2cid=210787128 }}</ref> Carbon has two [[stable nuclide|stable isotopes]], [[carbon-12|<sup>12</sup>C]] and [[carbon-13|<sup>13</sup>C]], in a ratio of approximately 99:1 by mass.<ref name=Shirey2013/> This ratio has a wide range in meteorites, which implies that it also varied a lot in the early Earth. It can also be altered by surface processes like [[photosynthesis]]. The fraction is generally compared to a standard sample using a ratio [[isotopic signature#Carbon isotopes|δ<sup>13</sup>C]] expressed in parts per thousand. Common rocks from the mantle such as basalts, carbonatites, and kimberlites have ratios between −8 and −2. On the surface, organic sediments have an average of −25 while carbonates have an average of 0.<ref name=Cartigny/>

Populations of diamonds from different sources have distributions of δ<sup>13</sup>C that vary markedly. Peridotitic diamonds are mostly within the typical mantle range; eclogitic diamonds have values from −40 to +3, although the peak of the distribution is in the mantle range. This variability implies that they are not formed from carbon that is ''primordial'' (having resided in the mantle since the Earth formed). Instead, they are the result of tectonic processes, although (given the ages of diamonds) not necessarily the same tectonic processes that act in the present.<ref name=Shirey2013/> Diamond-forming carbon originates in the top 700 kilometers (430 mi) or so of the upper mantle closest to the surface, known as the [[asthenosphere]].<ref name=Cartigny/>

=== Formation and growth ===
[[File:Diamond age zones.jpg|thumb|Age zones in a diamond<ref name=DCOdecadal/>]]
Diamonds in the mantle form through a ''[[metasomatism|metasomatic]]'' process where a C–O–H–N–S fluid or melt dissolves minerals in a rock and replaces them with new minerals. (The vague term C–O–H–N–S is commonly used because the exact composition is not known.) Diamonds form from this fluid either by reduction of oxidized carbon (e.g., CO<sub>2</sub> or CO<sub>3</sub>) or oxidation of a reduced phase such as [[methane]].<ref name=Cartigny/>

Using probes such as polarized light, [[photoluminescence]], and [[cathodoluminescence]], a series of growth zones can be identified in diamonds. The characteristic pattern in diamonds from the lithosphere involves a nearly concentric series of zones with very thin oscillations in luminescence and alternating episodes where the carbon is resorbed by the fluid and then grown again. Diamonds from below the lithosphere have a more irregular, almost polycrystalline texture, reflecting the higher temperatures and pressures as well as the transport of the diamonds by convection.<ref name=Shirey2013/>

=== Transport to the surface ===
[[File:VolcanicPipe.jpg|thumb|upright=1.2|Diagram of a volcanic pipe]]
Geological evidence supports a model in which kimberlite magma rises at 4–20 meters per second, creating an upward path by [[hydraulic fracturing]] of the rock. As the pressure decreases, a vapor phase [[exsolution|exsolves]] from the magma, and this helps to keep the magma fluid. At the surface, the initial eruption explodes out through fissures at high speeds (over {{cvt|200|m/s|mph}}). Then, at lower pressures, the rock is eroded, forming a pipe and producing fragmented rock ([[breccia]]). As the eruption wanes, there is [[Pyroclastic rock|pyroclastic]] phase and then metamorphism and hydration produces [[serpentinite]]s.<ref name=Shirey2013/>

=== Double diamonds ===
In rare cases, diamonds have been found that contain a cavity within which is a second diamond. The first double diamond, the [[Matryoshka (diamond)|Matryoshka]], was found by [[Alrosa]] in [[Sakha Republic|Yakutia]], Russia, in 2019.<ref>{{cite web|url=https://www.nationalgeographic.com/science/article/rare-diamond-diamond-found-siberia|title=Bizarre 'nesting doll' diamond found inside another diamond|publisher=[[National Geographic]]|vauthors=Wei-Haas M|date=October 10, 2019|access-date=November 27, 2021|archive-date=November 27, 2021|archive-url=https://web.archive.org/web/20211127050127/https://www.nationalgeographic.com/science/article/rare-diamond-diamond-found-siberia|url-status=dead}}</ref> Another one was found in the [[Ellendale Diamond Field]] in [[Western Australia]] in 2021.<ref>{{cite news|url=https://www.abc.net.au/news/2021-11-26/ellendale-discovery-comes-as-race-to-restart-production-heats-up/100648088|title=Rare 'double diamond' discovery comes as race to restart mothballed Ellendale mine heats up|publisher=[[Australian Broadcasting Corporation]]|vauthors=Fowler C|date=November 26, 2021|access-date=November 27, 2021|archive-date=November 26, 2021|archive-url=https://web.archive.org/web/20211126223633/https://www.abc.net.au/news/2021-11-26/ellendale-discovery-comes-as-race-to-restart-production-heats-up/100648088|url-status=live}}</ref>

=== In space ===
{{Main|Extraterrestrial diamonds}}
Although diamonds on [[Earth]] are rare, they are very common in space. In [[meteorite]]s, about three percent of the carbon is in the form of [[nanodiamond]]s, having diameters of a few nanometers. Sufficiently small diamonds can form in the cold of space because their lower [[surface energy]] makes them more stable than graphite. The isotopic signatures of some nanodiamonds indicate they were formed outside the Solar System in stars.<ref>{{cite journal| vauthors = Tielens AG |title=The molecular universe|journal=Reviews of Modern Physics|date=July 12, 2013|volume=85|issue=3|pages=1021–1081|doi=10.1103/RevModPhys.85.1021|bibcode=2013RvMP...85.1021T}}</ref>

High pressure experiments predict that large quantities of diamonds condense from [[methane]] into a "diamond rain" on the ice giant planets [[Uranus]] and [[Neptune]].<ref>{{cite journal | vauthors = Kerr RA | title = Neptune may crush methane into diamonds | journal = Science | volume = 286 | issue = 5437 | pages = 25 | date = October 1999 | pmid = 10532884 | doi = 10.1126/science.286.5437.25a | s2cid = 42814647 }}</ref><ref>{{cite journal| vauthors = Scandolo S, Jeanloz R |author-link2=Raymond Jeanloz |title=The Centers of Planets: In laboratories and computers, shocked and squeezed matter turns metallic, coughs up diamonds and reveals Earth's white-hot center|journal=American Scientist|date=November–December 2003|volume=91|issue=6|pages=516–525|jstor=27858301|bibcode=2003AmSci..91..516S|doi=10.1511/2003.38.905|s2cid=120975663 }}</ref><ref>{{cite news |vauthors=Kaplan S |title=It rains solid diamonds on Uranus and Neptune |url=https://www.washingtonpost.com/news/speaking-of-science/wp/2017/08/25/it-rains-solid-diamonds-on-uranus-and-neptune/ |access-date=October 16, 2017 |newspaper=[[The Washington Post]] |date=August 25, 2017 |archive-date=August 27, 2017 |archive-url=https://web.archive.org/web/20170827011901/https://www.washingtonpost.com/news/speaking-of-science/wp/2017/08/25/it-rains-solid-diamonds-on-uranus-and-neptune/ |url-status=live }}</ref> Some extrasolar planets may be almost entirely composed of diamond.<ref>{{cite news|last1=Max Planck Institute for Radio Astronomy|title=A planet made of diamond|url=http://www.astronomy.com/news/2011/08/a-planet-made-of-diamond|access-date=September 25, 2017|work=Astronomy magazine|date=August 25, 2011|archive-date=May 14, 2023|archive-url=https://web.archive.org/web/20230514174530/https://astronomy.com/news/2011/08/a-planet-made-of-diamond|url-status=live}}</ref>

Diamonds may exist in carbon-rich stars, particularly [[white dwarf]]s. One theory for the origin of [[carbonado]], the toughest form of diamond, is that it originated in a white dwarf or [[supernova]].<ref>{{cite journal | vauthors = Heaney PJ, de Vicenzi EP |title=Strange Diamonds: the Mysterious Origins of Carbonado and Framesite |journal=Elements |volume=1 |pages=85–89 |year=2005 |doi=10.2113/gselements.1.2.85 |issue=2|bibcode=2005Eleme...1...85H }}</ref><ref>{{cite journal | vauthors = Shumilova T, Tkachev S, Isaenko S, Shevchuk S, Rappenglück M, Kazakov V |title=A "diamond-like star" in the lab. Diamond-like glass |journal=Carbon |date=April 2016 |volume=100 |pages=703–709 |doi=10.1016/j.carbon.2016.01.068|doi-access=free |bibcode=2016Carbo.100..703S }}</ref> Diamonds formed in stars may have been the first minerals.<ref>{{cite news |vauthors=Wei-Haas M |title=Life and Rocks May Have Co-Evolved on Earth |url=http://www.smithsonianmag.com/science-nature/life-and-rocks-may-have-co-evolved-on-earth-180957807/ |access-date=September 26, 2017 |work=[[Smithsonian (magazine)|Smithsonian]] |language=en |archive-date=September 2, 2017 |archive-url=https://web.archive.org/web/20170902203717/http://www.smithsonianmag.com/science-nature/life-and-rocks-may-have-co-evolved-on-earth-180957807/ |url-status=live }}</ref>