Greenschist
Encyclopedia
Greenschist is a general field petrologic
term applied to metamorphic or altered mafic
volcanic rock
. The term greenstone
is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite
, actinolite
and epidote
minerals that dominate the rock. 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
. http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_vermont.html.
Greenschist often has a lepidoblastic, nematoblastic or schistose texure defined primarily by chlorite and actinolite. Greenschists often have some foliation
resulting in mineral alignment, especially of 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.
The equilibrium mineral assemblage of rocks subjected to greenschist facies conditions depends on primary rock composition.
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 ridge
s typically exhibit sub-greenschist alteration. The greenstone belt
s of the various archean
craton
s 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
.
have been identified.
, 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.
Petrology
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....
term applied to metamorphic or altered mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
volcanic rock
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...
. The term greenstone
Greenstone
Pounamu is several types of hard, durable and highly valued nephrite jade, bowenite, or serpentinite found in New Zealand. Pounamu is the Māori name. The rocks are also generically known as "greenstone" in New Zealand English....
is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
, actinolite
Actinolite
Actinolite is an amphibole silicate mineral with the chemical formula .-Etymology:The name actinolite is derived from the Greek word aktis , meaning "beam" or "ray", because of the mineral's fibrous nature...
and epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...
minerals that dominate the rock. 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
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
or similar rocks containing sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
-rich plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, chlorite, epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...
and quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
. http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_vermont.html.
Petrology
Greenschist, as a rock type, is defined by the presence of the minerals chlorite and actinolite and may contain albite or epidote.Greenschist often has a lepidoblastic, nematoblastic or schistose texure defined primarily by chlorite and actinolite. Greenschists often have some foliation
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
resulting in mineral alignment, especially of 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.
Greenschist facies
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 and depths of about 8 to 50 kilometers are the typical envelope of greenschist facies rocks.The equilibrium mineral assemblage of rocks subjected to greenschist facies conditions depends on primary rock composition.
- Basalt: chlorite + actinoliteActinoliteActinolite is an amphibole silicate mineral with the chemical formula .-Etymology:The name actinolite is derived from the Greek word aktis , meaning "beam" or "ray", because of the mineral's fibrous nature...
+ albite +/- epidote - Ultramafic: chlorite + serpentine +/- talcTalcTalc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
+/- tremoliteTremoliteTremolite is a member of the amphibole group of silicate minerals with composition: Ca2Mg5Si8O222. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades...
+/- diopsideDiopsideDiopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class...
+/- bruciteBruciteBrucite is the mineral form of magnesium hydroxide, with the chemical formula Mg2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and formed during serpentinization of dunites... - Pelites: quartzQuartzQuartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
+/- albiteAlbiteAlbite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
+/- k-feldspar +/- chlorite, muscoviteMuscoviteMuscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
, garnetGarnetThe garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
, pyrophyllitePyrophyllitePyrophyllite is a phyllosilicate mineral composed of aluminium silicate hydroxide: Al2Si4O102. It occurs in two more or less distinct varieties, namely, as crystalline folia and as compact masses; distinct crystals are not known....
+/- graphiteGraphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal... - Calc-silicates: calciteCalciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
+/- dolomiteDolomiteDolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
+/- quartz +/- micas, scapoliteScapoliteScapolite , is a group of rock-forming silicate minerals composed of aluminium, calcium, and sodium silicate with chlorine, carbonate and sulfate.-Properties:...
, wollastoniteWollastoniteWollastonite is a calcium inosilicate mineral that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids...
, 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
Prehnite-pumpellyite facies
The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres....
and higher temperatures overlap with and include sub-amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
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
Granulite
Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure...
. Lower pressure, normally contact metamorphism produces albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
-epidote hornfels
Hornfels
Hornfels is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses and have been rendered...
while higher pressures at great depth produces eclogite
Eclogite
Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...
.
Oceanic basalts in the vicinity of mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
s typically exhibit sub-greenschist alteration. The greenstone belt
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
s of the various archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
s are commonly altered to the greenschist facies. These ancient rocks are noted as host rocks for a variety of ore deposits in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
and Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
.
Europe
Greenschist rocks have been used to make axes across Europe. Several sites including Langdale axe industryLangdale axe industry
The Langdale axe industry is the name given by archaeologists to the centre of a specialised stone tool manufacturing at Great Langdale in England's Lake District during the Neolithic period .The area has outcrops of fine-grained greenstone suitable for making polished axes which have been...
have been identified.
Eastern North America
A form of chlorite schist was popular in prehistoric Native American communities for the production of axes and celtsCelt (tool)
Celt is an archaeological term used to describe long thin prehistoric stone or bronze adzes, other axe-like tools, and hoes.-Etymology:The term "celt" came about from what was very probably a copyist's error in many medieval manuscript copies of Job 19:24 in the Latin Vulgate Bible, which became...
, as well as ornamental items. In the Middle Woodland period
Woodland period
The Woodland period of North American pre-Columbian cultures was from roughly 1000 BCE to 1000 CE in the eastern part of North America. The term "Woodland Period" was introduced in the 1930s as a generic header for prehistoric sites falling between the Archaic hunter-gatherers and the...
, greenschist was one of the many trade items that were part of the Hopewell culture
Hopewell culture
The Hopewell tradition is the term used to describe common aspects of the Native American culture that flourished along rivers in the northeastern and midwestern United States from 200 BCE to 500 CE. The Hopewell tradition was not a single culture or society, but a widely dispersed set of related...
exchange network, sometimes transported over thousands of kilometers.
During the time of the Mississippian culture
Mississippian culture
The Mississippian culture was a mound-building Native American culture that flourished in what is now the Midwestern, Eastern, and Southeastern United States from approximately 800 CE to 1500 CE, varying regionally....
, the polity of Moundville
Moundville Archaeological Site
Moundville Archaeological Site, also known as the Moundville Archaeological Park, is a Mississippian site on the Black Warrior River in Hale County, near the town of Tuscaloosa, Alabama...
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
- Greenstone beltGreenstone beltGreenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
- MetamorphismMetamorphismMetamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
- List of rock types
- List of minerals
- Pounamu, another type of rock frequently called greenstone