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greenschist
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{{Short description|Metamorphic rock}}File:Chlorite schist.jpg|thumb|right|Chlorite schistschistFile:Greenschist - Prasinite - at Cap Corse in Corsica in France.jpg|thumb|Greenschist (prasinite) at Cap CorseCap CorseFile:Piece of greenschist from Dalupirip, Itogon, Benguet, Philippines 02.jpg|thumb|Greenschist (epidote) from ItogonItogonGreenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically {{convert|300 |-| 450|°C|F|-1}} and 2â10 kilobars ({{convert|2000 |-| 10000 |bar|psi|disp=output only|-2}}).WEB, Encyclopædia Britannica, Metamorphic Rock, Greenschist Facies,www.britannica.com/EBchecked/topic/377777/metamorphic-rock/80338/Greenschist-facies, 9 April 2013, Greenschists commonly have an abundance of green minerals such as chlorite, serpentine, and epidote, and (:wikt:platy|platy) minerals such as muscovite and platy serpentine. The platiness gives the rock schistosity (a tendency to split into layers). Other common minerals include quartz, orthoclase, talc, carbonate minerals and amphibole (actinolite).Greenschist is a general field petrologic term for metamorphic or altered mafic volcanic rock. In Europe, the term prasinite is sometimes used. A greenstone is sometimes a greenschist but can also be rock types without any schistosity, especially metabasalt (spilite). However, basalts may remain quite black if primary pyroxene does not revert to chlorite or actinolite. To qualify for the name a rock must also exhibit schistosity or some foliation or layering. The rock is derived from basalt, gabbro or similar rocks containing sodium-rich plagioclase feldspar, chlorite, epidote and quartz.WEB,vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_vermont.html, America’s volcanic past: Vermont, 2006-11-25,vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_vermont.html," title="web.archive.org/web/20060923100848vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_vermont.html,">web.archive.org/web/20060923100848vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_vermont.html, 2006-09-23, - the content below is remote from Wikipedia
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Petrology
Greenschist is defined by the presence of the minerals chlorite, epidote, or actinolite, which give the rock its green color. Greenschists also have pronounced schistosity.BOOK, Jackson, Julia A., Glossary of geology., 1997, American Geological Institute, Alexandria, Virginia, 0922152349, Fourth, greenschist, Schistosity is a thin layering of the rock produced by metamorphism (a foliation) that permits the rock to easily be split into flakes or slabs less than {{convert|5 to 10|mm|1|sp=us}} thick. This arises from the presence of chlorite or other platy minerals that become aligned in layers during metamorphism.BOOK, Schmid, R., Fettes, D., Harte, B., Davis, E., Desmons, J., 2007, How to name a metamorphic rock., Metamorphic Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks, Cambridge University Press, Cambridge, 7,stuff.mit.edu/afs/athena.mit.edu/course/12/12.115/www/12.114%20Papers/BGS/Metamorphic/DetailedMetamorphic/1%20How%20to%20name%20a%20metamorphic%20rock.pdf, 28 February 2021, JOURNAL, Robertson, S., BGS Rock Classification Scheme, Volume 2: Classification of metamorphic rocks, British Geological Survey Research Report, 1999, RR 99-02,nora.nerc.ac.uk/id/eprint/3226/1/RR99002.pdf, 27 February 2021, {{harvid, British Geological Survey, 1999, |page=5}}Greenschist may also contain albite and often has a lepidoblastic, nematoblastic or schistose texture defined primarily by chlorite and actinolite. Grain size is rarely coarse, due primarily to the mineral assemblage. Chlorite and to a lesser extent actinolite typically exhibit small, flat or acicular crystal habits.Greenstone is a field term for any massive mafic volcanic rock that has been altered to a greenish color by the formation of the same minerals that give the green color to greenschist, whether or not the rock displays schistosity.{{sfn|Jackson|1997|loc=“greenstone“}} The term has also been used to describe any igneous intrusions into the Coal Measures Group of Scotland, to describe chamosite-rich mudstone of Lower Jurassic age in Great Britain, or for nephrite or other greenish gemstones.{{sfn|Jackson|1997|loc=“greenstone [ign], [mineral], [sed]“}}Greenschist facies
(File:Metamorphic facies EN.svg|thumb|upright=1.35|Graph of metamorphic facies temperature and pressure ranges) Greenschist facies is determined by the particular temperature and pressure conditions required to metamorphose basalt to form the typical greenschist facies minerals chlorite, actinolite, and albite. Greenschist facies results from low temperature, moderate pressure metamorphism. Metamorphic conditions which create typical greenschist facies assemblages are called the Barrovian Facies Sequence, and the lower-pressure Abukuma Facies Series. Temperatures of approximately {{convert|400| to| 500|°C|F|-1}} and depths of about {{convert|8| to| 50|km|mi|abbr=off|0}} are the typical envelope of greenschist facies rocks. File:Roche verte Mont-Cenis2.jpg|thumb|Prasinite variety of greenschist (Mont-Cenis massif, French Alps)]](File:Dalupirip schist 02.jpg|thumb|Outcrop of amphibole epidote variety of greenschist, Philippines)The equilibrium mineral assemblage of rocks subjected to greenschist facies conditions depends on primary rock composition.JOURNAL, Dalstra, H. J., Ridley, J. R., Bloem, E. J. M., Groves, D. I., 1999-10-01, Metamorphic evolution of the central Southern Cross Province, Yilgarn Craton, Western Australia, Australian Journal of Earth Sciences, 46, 5, 765â784, 10.1046/j.1440-0952.1999.00744.x, 1999AuJES..46..765D, 0812-0099,- Basalt: chlorite + actinolite + albite +/- epidote
- Ultramafic: chlorite + serpentine +/- talc +/- tremolite +/- diopside +/- brucite
- Pelites: quartz +/- albite +/- k-feldspar +/- chlorite, muscovite, garnet, pyrophyllite +/- graphite
- Calc-silicates: ‘’calcite +/- dolomite +/- quartz +/- micas, scapolite, wollastonite, etc.
Use
Europe
In Minoan Crete, greenschist and blueschist were used to pave streets and courtyards between 1650 and 1600 BC. These rocks were likely quarried in Agia Pelagia on the north coast of central Crete.JOURNAL, Tziligkaki, Eleni K., 2010, Types of schist used in buildings of Minoan Crete,www.hellenjgeosci.geol.uoa.gr/45/Tziligkaki.pdf, Hellenic Journal of Geosciences, 45, 317â322, December 1, 2018, Across Europe, greenschist rocks have been used to make axes. Several sites, including Great Langdale in England, have been identified.Eastern North America
A form of chlorite schist was popular in prehistoric Native American communities for the production of axes and celts, as well as ornamental items. In the Middle Woodland period, greenschist was one of the many trade items that were part of the Hopewell culture exchange network, sometimes transported over thousands of kilometers.During the time of the Mississippian culture, the polity of Moundville apparently had some control over the production and distribution of greenschist. The Moundville source has been shown to be from two localities in the Hillabee Formation of central and eastern Alabama.See also
- List of minerals
- List of rock types
- Metamorphism
- Pounamu, another type of rock frequently called greenstone
References
{{Reflist}}- Blatt, Harvey and Robert J. Tracy (1996). Petrology; Igneous, Sedimentary, and Metamorphic, 2nd Ed., W. H. Freeman. {{ISBN|0-7167-2438-3}}.
- Gall, Daniel G. and Vincas P. Steponaitis, “Composition and Provenance of Greenstone Artifacts from Moundville,” Southeastern Archaeology 20(2):99â117 [2001]).
- Steponaitis, Vincas P. Prehistoric Archaeology in the Southeastern United States, 1970â1985. Annual Review of Anthropology, Vol. 15. (1986), pp. 363â404.
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