Greenschists

Greenschists are a group of metamorphic rocks that form under low-grade metamorphic conditions, typically at temperatures between 300 and 500 degrees Celsius and pressures up to 6 kilobars. They are characterized by the presence of fine-grained green minerals, hence their name. Key minerals that define greenschists include chlorite, actinolite, epidote, and albite. These minerals are derived from the alteration of pre-existing mafic or intermediate igneous rocks, such as basalt or gabbro, or sometimes from sedimentary rocks like greywacke. The characteristic green color comes primarily from chlorite and actinolite.

The texture of greenschists is often defined by a fabric where the platy or acicular minerals are aligned, creating a schistosity. This foliation is a result of the directed pressure during metamorphism. Greenschists are commonly found in orogenic belts, which are regions where tectonic plates collide and mountains are formed. They are significant indicators of the metamorphic history of a region, providing clues about the temperature, pressure, and fluid conditions that prevailed during their formation. Their relatively low metamorphic grade means they represent an early stage of metamorphism before rocks transition to higher-grade amphibolites or granulites.