Greenschist

Greenschist is a foliated, low-grade metamorphic rock whose green color comes from the minerals chlorite, actinolite, and epidote. It typically also contains albite, quartz, and white mica, and has a clear schistose fabric that lets it split along aligned mineral layers.

The rock defines the 'greenschist facies,' a set of relatively low temperature and pressure metamorphic conditions that are extremely common in the deeper parts of mountain belts. Greenschist most often forms from the metamorphism of basaltic and other mafic rocks.

It is one of the most widespread metamorphic rock types, found in orogenic belts and shield regions worldwide.

Greenschist forms by low-grade regional metamorphism, typically at temperatures of about 300-450 degrees C and moderate pressures. Mafic igneous rocks such as basalt and gabbro, or volcanic ash and mudstone, are recrystallized into green chlorite-, actinolite-, and epidote-bearing assemblages, with directed pressure aligning the platy and needle-like minerals into a schistose foliation.

These conditions are common in the interiors of fold-and-thrust mountain belts during continental collision and in accretionary settings. With increasing grade, greenschist gives way to amphibolite as hornblende replaces actinolite. Greenschist terrains are widespread, including in the Alps, the Appalachians, and ancient greenstone belts.

Greenschist is identified by its green to greenish-gray color, foliated schistose texture, and slightly lustrous, sometimes wavy split surfaces. The green minerals chlorite, actinolite, and epidote are usually fine to medium grained and may give a faint sheen.

It has moderate softness (the chlorite-rich parts can be scratched). Distinguish it from blueschist, which is blue and forms at higher pressure, and from amphibolite, which is darker, coarser, and richer in hornblende. Unfoliated greenstone is the massive equivalent. The combination of green color, schistosity, and low-grade mafic mineralogy is the key clue.

Greenschist has limited commercial use but is sometimes employed as a decorative or paving stone and as crushed aggregate where locally available. Attractive foliated specimens are collected for their green color and metamorphic interest.

Its greatest importance is scientific: greenschist defines a fundamental metamorphic facies and helps geologists map the temperature and pressure conditions and tectonic history of mountain belts. It can also be spatially associated with mineral deposits in greenstone belts. Greenschist has little metaphysical tradition, its significance being chiefly geological.