Chloriteschist, a type of greenschistGreenschist (prasinite) at Cap Corse in Corsica, FranceGreenschist (epidote) from Itogon, Benguet, Philippines
Greenschist is a general field petrologic term for metamorphic or altered maficvolcanic 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 plagioclasefeldspar, chlorite, epidote and quartz.[2]
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.[3] 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 5 to 10 millimeters (0.2 to 0.4 in) thick. This arises from the presence of chlorite or other platy minerals that become aligned in layers during metamorphism.[4][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.[6] 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.[7]
Greenschist faciesEdit
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 400 to 500 °C (750 to 930 °F) and depths of about 8 to 50 kilometres (5 to 31 miles) are the typical envelope of greenschist facies rocks.
Prasinite variety of greenschist (Mont-Cenis massif, French Alps)Outcrop of amphibole epidote variety of greenschist, Philippines
The equilibrium mineral assemblage of rocks subjected to greenschist facies conditions depends on primary rock composition.[8]
In greater detail the greenschist facies is subdivided into subgreenschist, lower and upper greenschist. Lower temperatures are transitional with and overlap the prehnite-pumpellyite facies and higher temperatures overlap with and include sub-amphibolite facies.
If burial continues along Barrovian Sequence metamorphic trajectories, greenschist facies gives rise to amphibolite facies assemblages, dominated by amphibole and eventually to granulite facies. Lower pressure, normally contact metamorphism produces albite-epidote hornfels while higher pressures at great depth produces eclogite.
Oceanic basalts in the vicinity of mid-ocean ridges typically exhibit sub-greenschist alteration. The greenstone belts of the various archeancratons are commonly altered to the greenschist facies. These ancient rocks are noted as host rocks for a variety of ore deposits in Australia, Namibia and Canada.
Greenschist-like rocks can also be formed under blueschist facies conditions if the original rock (protolith) contains enough magnesium. This explains the scarcity of blueschist preserved from before the Neoproterozoic Era 1000 Ma ago when the Earth's oceanic crust contained more magnesium than today's oceanic crust.[9]
UseEdit
EuropeEdit
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.[10]
Across Europe, greenschist rocks have been used to make axes. Several sites, including Great Langdale in England, have been identified.
Eastern North AmericaEdit
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.
Pounamu, another type of rock frequently called greenstone
ReferencesEdit
^ abc"Encyclopædia Britannica, Metamorphic Rock, Greenschist Facies". Retrieved 9 April 2013.
^. Archived from the original on 2006-09-23. Retrieved 2006-11-25.
^Jackson, Julia A., ed. (1997). "greenschist". Glossary of geology (Fourth ed.). Alexandria, Virginia: American Geological Institute. ISBN0922152349.
^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(PDF). Cambridge: Cambridge University Press. p. 7. Retrieved 28 February 2021.
^Robertson, S. (1999). "BGS Rock Classification Scheme, Volume 2: Classification of metamorphic rocks" (PDF). British Geological Survey Research Report. RR 99-02: 5. Retrieved 27 February 2021.
^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): 765–784. Bibcode:1999AuJES..46..765D. doi:10.1046/j.1440-0952.1999.00744.x. ISSN 0812-0099.
^Palin, Richard M.; White, Richard W. (2016). "Emergence of blueschists on Earth linked to secular changes in oceanic crust composition". Nature Geoscience. 9 (1): 60–64. Bibcode:2016NatGe...9...60P. doi:10.1038/ngeo2605. S2CID 130847333.
^Tziligkaki, Eleni K. (2010). "Types of schist used in buildings of Minoan Crete" (PDF). Hellenic Journal of Geosciences. 45: 317–322. Retrieved December 1, 2018.
Blatt, Harvey and Robert J. Tracy (1996). Petrology; Igneous, Sedimentary, and Metamorphic, 2nd Ed., W. H. Freeman. ISBN0-7167-2438-3.
Gall, Daniel G. and Vincas P. Steponaitis, "Composition and Provenance of Greenstone Artifacts from Moundville," Southeastern Archaeology 20(2):99–117 [2001]).
Steponaitis, Vincas P. Prehistoric Archaeology in the Southeastern United States, 1970–1985. Annual Review of Anthropology, Vol. 15. (1986), pp. 363–404.
October 15, 2023
greenschist, metamorphic, rocks, that, formed, under, lowest, temperatures, pressures, usually, produced, regional, metamorphism, typically, kilobars, commonly, have, abundance, green, minerals, such, chlorite, serpentine, epidote, platy, minerals, such, musco. Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism typically 300 450 C 570 840 F and 2 10 kilobars 29 000 145 000 psi 1 Greenschists commonly have an abundance of green minerals such as chlorite serpentine and epidote and platy minerals such as muscovite and platy serpentine 1 The platiness gives the rock schistosity a tendency to split into layers Other common minerals include quartz orthoclase talc carbonate minerals and amphibole actinolite 1 Chlorite schist a type of greenschistGreenschist prasinite at Cap Corse in Corsica FranceGreenschist epidote from Itogon Benguet PhilippinesGreenschist 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 2 Contents 1 Petrology 2 Greenschist facies 3 Use 3 1 Europe 3 2 Eastern North America 4 See also 5 ReferencesPetrology EditGreenschist is defined by the presence of the minerals chlorite epidote or actinolite which give the rock its green color Greenschists also have pronounced schistosity 3 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 5 to 10 millimeters 0 2 to 0 4 in thick This arises from the presence of chlorite or other platy minerals that become aligned in layers during metamorphism 4 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 6 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 7 Greenschist facies Edit nbsp Graph of metamorphic facies temperature and pressure rangesGreenschist 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 400 to 500 C 750 to 930 F and depths of about 8 to 50 kilometres 5 to 31 miles are the typical envelope of greenschist facies rocks nbsp Prasinite variety of greenschist Mont Cenis massif French Alps nbsp Outcrop of amphibole epidote variety of greenschist PhilippinesThe equilibrium mineral assemblage of rocks subjected to greenschist facies conditions depends on primary rock composition 8 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 In greater detail the greenschist facies is subdivided into subgreenschist lower and upper greenschist Lower temperatures are transitional with and overlap the prehnite pumpellyite facies and higher temperatures overlap with and include sub amphibolite facies If burial continues along Barrovian Sequence metamorphic trajectories greenschist facies gives rise to amphibolite facies assemblages dominated by amphibole and eventually to granulite facies Lower pressure normally contact metamorphism produces albite epidote hornfels while higher pressures at great depth produces eclogite Oceanic basalts in the vicinity of mid ocean ridges typically exhibit sub greenschist alteration The greenstone belts of the various archean cratons are commonly altered to the greenschist facies These ancient rocks are noted as host rocks for a variety of ore deposits in Australia Namibia and Canada Greenschist like rocks can also be formed under blueschist facies conditions if the original rock protolith contains enough magnesium This explains the scarcity of blueschist preserved from before the Neoproterozoic Era 1000 Ma ago when the Earth s oceanic crust contained more magnesium than today s oceanic crust 9 Use EditEurope Edit 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 10 Across Europe greenschist rocks have been used to make axes Several sites including Great Langdale in England have been identified Eastern North America Edit 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 EditList of minerals List of rock types Metamorphism Pounamu another type of rock frequently called greenstoneReferences Edit a b c Encyclopaedia Britannica Metamorphic Rock Greenschist Facies Retrieved 9 April 2013 America s volcanic past Vermont Archived from the original on 2006 09 23 Retrieved 2006 11 25 Jackson Julia A ed 1997 greenschist Glossary of geology Fourth ed Alexandria Virginia American Geological Institute ISBN 0922152349 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 PDF Cambridge Cambridge University Press p 7 Retrieved 28 February 2021 Robertson S 1999 BGS Rock Classification Scheme Volume 2 Classification of metamorphic rocks PDF British Geological Survey Research Report RR 99 02 5 Retrieved 27 February 2021 Jackson 1997 greenstone Jackson 1997 greenstone ign mineral sed 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 5 765 784 Bibcode 1999AuJES 46 765D doi 10 1046 j 1440 0952 1999 00744 x ISSN 0812 0099 Palin Richard M White Richard W 2016 Emergence of blueschists on Earth linked to secular changes in oceanic crust composition Nature Geoscience 9 1 60 64 Bibcode 2016NatGe 9 60P doi 10 1038 ngeo2605 S2CID 130847333 Tziligkaki Eleni K 2010 Types of schist used in buildings of Minoan Crete PDF Hellenic Journal of Geosciences 45 317 322 Retrieved December 1 2018 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 99 117 2001 Steponaitis Vincas P Prehistoric Archaeology in the Southeastern United States 1970 1985 Annual Review of Anthropology Vol 15 1986 pp 363 404 Retrieved from https en wikipedia org w index php title Greenschist amp oldid 1175999411, wikipedia, wiki, book, books, library,
