How to identify Greenschist

To correctly identify Greenschist (Greenschist), 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 (light to dark green), often with a silvery or grayish sheen due to mica/chlorite.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Often dull to sub-vitreous on fresh surfaces, but individual mineral grains (especially micas and chlorite) can exhibit a pearly or silky luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Schistose (foliated), fine- to medium-grained. Platy minerals are visibly aligned, giving the rock a layered or banded appearance. Can be slightly friable.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral crystals are typically anhedral to subhedral, forming interlocking grains. Platy minerals (chlorite, muscovite) are elongated and aligned. Porphyroblasts (e.g., albite, garnet) may be present.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Exhibits excellent schistosity, meaning it readily splits along planes defined by the parallel alignment of platy minerals. This is not true mineral cleavage but a rock fabric.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Regional metamorphic terrains, especially in ancient mountain belts, subduction zones, and areas of continental collision. Often found in association with other metamorphic rocks like phyllite, slate, and amphibolite.

Key Facts

Hardness

Variable, typically 2-6 on Mohs scale, depending on mineral composition (e.g., chlorite 2-2.5, actinolite 5-6, epidote 6-7). The rock as a whole is relatively soft due to abundant platy minerals.

Specific Gravity

2.7-3.0 g/cm³

Crystal System

Monoclinic (chlorite, actinolite, epidote), Orthorhombic (epidote), Triclinic (albite). The rock itself is polycrystalline.

Color

Green (light to dark), grayish-green, silvery-green.

Luster

Dull to sub-vitreous, pearly on foliation surfaces.

Transparency

Opaque to translucent.

Fracture

Irregular to splintery, often controlled by schistosity.

Cleavage

Excellent schistosity (rock fabric), individual minerals have distinct cleavages (e.g., chlorite perfect basal, actinolite perfect prismatic).

Composition

Dominated by chlorite, actinolite, epidote, and albite. May also contain quartz, muscovite, biotite, garnet, and minor accessory minerals like sphene, magnetite, and ilmenite.

Physical characteristics

• Crystal Habit: Platy (chlorite, muscovite), acicular/prismatic (actinolite), granular (epidote, albite, quartz).
• Cleavage Type: Excellent schistosity (rock fabric). Mineral cleavages vary (e.g., perfect basal in chlorite, perfect prismatic in actinolite).
• Fracture Type: Irregular, splintery, or platy along foliation.
• Tenacity: Brittle, but can be somewhat flexible along foliation planes if rich in mica/chlorite.
• Luster Type: Dull to sub-vitreous, pearly on foliation surfaces.

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