metamorphic rock

The greenschist facies was once considered the first major facies of metamorphism proper. The name comes from the abundance of the green mineral chloritein such rocks. Because chlorite and muscovite are ubiquitous and because both exhibit a platy crystal habit, these rocks normally show a highly developed foliation and often exhibit strong metamorphic differentiation. They have been described from practically every metamorphic terrain on Earth, from earliest Precambrian to the young mountain regions. In fact, many of the Earth’s oldest rocks (about three billion years old) of the continental-shield areas are in this facies, classic examples of which are in the Appalachians, the Highlands of Scotland, New Zealand, the European Alps, Japan, and Norway.

The dominant minerals of greenschists formed from silicate-rich sediments include quartz, albite, muscovite, chlorite, epidote, calcite, actinolite, magnetite, biotite, and paragonite. Minerals less common include the manganese-rich garnet spessartine, stilpnomelane, kyanite, rutile, sphene, pyrophyllite, and chloritoid. Calcareous rocks are dominated by calcite, dolomite, and quartz; the major carbonate minerals are thermally stable. It is only when large quantities of water flush away carbon dioxide or keep its partial pressure low that carbonate-silicate reactions take place and liberate carbon dioxide. The typical minerals of this facies have low water contents as compared to the zeolite facies minerals.