How to identify Greenschist

To correctly identify Greenschist (Metamorphic rock (Greenschist facies)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

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

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

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