Editing Diamond (section)

=== 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/>