Greenschist

Description

Greenschist is a fine- to medium-grained metamorphic rock characterized by its distinctive green color and schistose texture. The green hue is imparted by the dominant presence of chlorite, actinolite, and epidote. It exhibits a strong foliation (schistosity) due to the parallel alignment of platy minerals like chlorite and muscovite, allowing it to split into thin layers. Quartz and albite are also common constituents. The rock represents a specific metamorphic facies, indicating the pressure and temperature conditions under which it formed.

How to Identify

Color

Typically various shades of green, from light to dark, often with a dull to slightly greasy appearance.

Luster

Dull to sub-vitreous, sometimes silky on foliation surfaces due to mica/chlorite.

Texture

Schistose, meaning it has a well-developed foliation (parallel alignment of platy minerals) that gives it a layered or platy appearance. Fine- to medium-grained.

Crystal Form

Individual mineral grains are typically anhedral (lacking well-formed crystal faces) and intergrown. Platy minerals like chlorite and muscovite are elongated and aligned.

Cleavage

Exhibits schistosity, which is a type of rock cleavage where the rock splits along planes defined by the aligned platy minerals. This is distinct from mineral cleavage.

Geological Environment

Forms in regional metamorphic settings, typically associated with convergent plate boundaries (orogenic belts) where mafic igneous rocks or pelitic sediments are subjected to low-to-medium grade metamorphism. Common in ancient shield areas and mountain ranges.

Key Facts

• Hardness: Variable, typically 2-6 on Mohs scale, depending on mineral composition (chlorite is soft, quartz/actinolite are harder).
• Specific Gravity: 2.7-3.0 g/cm
• Crystal System: Not applicable for a rock; constituent minerals have their own crystal systems (e.g., chlorite is monoclinic, actinolite is monoclinic, quartz is trigonal).
• Color: Green (pale to dark)
• Luster: Dull to sub-vitreous, silky on foliation surfaces
• Transparency: Opaque
• Fracture: Irregular to splintery, often controlled by schistosity
• Cleavage: Schistosity (rock cleavage), not mineral cleavage, though constituent minerals have their own cleavages.
• Composition: Predominantly chlorite, actinolite, epidote, albite, and quartz. May also contain muscovite, biotite, garnet, magnetite, and calcite.

Quick Check

• Color: Green (various shades)
• Luster: Dull to sub-vitreous, sometimes silky on foliation surfaces
• Streak: White to pale green

Physical Characteristics

• Crystal Habit: Fine- to medium-grained, anhedral, with platy minerals (chlorite, muscovite) showing preferred orientation.
• Cleavage Type: Schistosity (rock cleavage), well-developed, allowing the rock to split into thin, often wavy layers.
• Fracture Type: Irregular to splintery, often controlled by the schistosity.
• Tenacity: Brittle, but can be somewhat flexible in very thin layers due to schistosity.
• Luster Type: Dull to sub-vitreous, with a silky sheen on foliation surfaces due to aligned micas/chlorite.

Formation

Greenschist forms under conditions of the greenschist facies, which is a low-temperature (300-500 C) and low-to-medium pressure (2-10 kilobars) metamorphic environment. It typically results from the metamorphism of mafic igneous rocks (like basalt or gabbro) or pelitic sedimentary rocks (like shale). The characteristic green color is due to the abundance of chlorite, actinolite, and epidote. Protoliths undergo recrystallization and mineralogical changes, with original minerals like pyroxene and plagioclase transforming into these hydrous green minerals.

Usage

Historically, greenschist has been used locally as a building stone, for flagstones, and in some areas for carving due to its relatively soft nature and schistosity. Its economic importance is generally limited, though it can host certain mineral deposits, particularly gold and sulfide ores, in some regions.

Age Distribution

Common in ancient orogenic belts, often Precambrian to Paleozoic, but can form in any age where appropriate metamorphic conditions are met.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Metamorphic Rock

Group

Schist

Crystal System

Not applicable (rock)

Chemical Formula

Variable, reflecting the mineral assemblage (e.g., chlorite: (Mg,Fe)Al(SiAl)O(OH); actinolite: Ca(Mg,Fe)SiO(OH))

Composition

Dominated by hydrous ferromagnesian minerals (chlorite, actinolite, epidote) and feldspar (albite) and quartz. The exact composition varies depending on the protolith and specific metamorphic conditions.

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