Greenschist

Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered mafic volcanic rock. 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 schistocity or some foliation or layering. An alternate term for these rocks is greenstone. The rock is derived from basalt, gabbro or similar rocks containing sodium-rich plagioclase feldspar, chlorite, epidote and quartz. Chlorite and epidote give the green colour [1].

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Petrology

Greenschist, as a rock type, is defined by the presence of the minerals chlorite + actinolite +/- albite +/- epidote. 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 prefer small, flat or acicular crystal habits.

Greenschist Facies

Greenschist Facies is determined by the particular T-P 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 + 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.

Culture

Europe

Greenschist rocks have been used to make axes across Europe. Several sites including Langdale axe industry 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

• Greenstone belt
• Metamorphism
• List of rock types
• List of minerals
• Pounamu, another type of rock frequently called greenstone

References

• 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.