Greenschist

Greenschist is an often greenish -colored rock that has undergone a metamorphosis at relatively low temperatures and low to medium pressures. Greenschist rocks are typical regionalmetamorphe arising mainly from basic igneous starting materials. The name goes back to the most significant schistose structure of the rocks and the often existing, typical greenish color. Major central European deposits are the Ophiolithzonen Zermatt -Saas -Fee and the Aosta valley.

The greenish color is caused mainly by epidote, actinolite ( an amphibole ) and minerals of the chlorite group, but they are not always predominant, so not every green shale is also really significantly colored greenish. Greenschist are the characteristic rocks of the so-called greenschist facies.

Greenschist facies

What a metamorphosis unmetamorphes rock, the so-called source rock passes, (and if it ever goes through a metamorphosis ), depends mainly on the temperature and the pressure to which it is exposed after its formation. The respective pressure and temperature conditions lead to the formation of a characteristic association of minerals, which is called metamorphic facies. It is this mineral association to a certain degree depending on the type of parent material, which may be either an igneous rock and sedimentary rocks.

The term greenschist facies originally stood for the rock composition of basalts and basalt -like rocks which a metamorphosis at about 300-500 ° C and 1 - have gone through 8 kilobars, which is within the spectrum of metamorphic facies is (abbreviated to medium pressures and temperatures: MP -MT metamorphism ).

Typical of greenschist from basaltic source rocks, called metabasites, epidote and chlorite are the minerals and the minerals actinolite, albite and quartz contained in varying proportions. In metagreywackes occur mainly albite, quartz, epidote, muscovite and in varying proportions to stilpnomelane. The typical Ti - phase of the greenschist facies is the mineral titanite. In metapelites, the composition is very diverse, with a typical mineral association of muscovite, chlorite, albite and quartz. Other conduct additional chloritoid, and in varying proportions paragonite or biotite and manganese garnet ( spessartine ).