Greenschist Identification Guide
How to identify greenschist, a foliated low-grade metamorphic rock, by its green minerals, schistosity, and sheen, versus greenstone and serpentinite.
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What Greenschist Looks Like
Greenschist is a foliated metamorphic rock formed by low-grade metamorphism of basalt and other mafic rocks. Its green color comes from chlorite, actinolite, and epidote, often with albite and quartz. The defining feature is schistosity—a well-developed parallel alignment of platy/elongate minerals that gives the rock a layered, splitting fabric and a silky-to-shiny sheen on foliation surfaces. Color ranges from pale to dark green, frequently with a slight sparkle from chlorite.
Step-by-Step Field-ID Checklist
- Look for foliation: parallel mineral layers that the rock tends to split along (schistosity).
- Check the sheen: chlorite and actinolite give a silky to slightly glittery luster on cleavage surfaces.
- Identify green minerals: soft platy chlorite, needle-like actinolite, and pistachio-green epidote.
- Scratch test: chlorite is soft (~2–2.5); epidote and actinolite are harder (6–7).
- Note any folding or crenulation of the foliation, common in greenschist.
Key Diagnostic Tests
- Mohs hardness: variable—chlorite ~2–2.5, actinolite/epidote ~6–7
- Streak: pale green to white depending on mineral
- Fabric: schistose (foliated); splits along mineral alignment
- Specific gravity: ~2.8–3.0
- No strong acid reaction (silicate minerals)
- Generally non-magnetic, though minor magnetite may occur
Common Look-Alikes and How to Tell Them Apart
- Greenstone: Also chlorite/epidote/actinolite-rich and green, but massive and non-foliated—greenschist has clear schistosity, greenstone does not.
- Serpentinite: Smoother, soapier feel, often with slickensided polished surfaces; dominated by serpentine rather than chlorite/actinolite schistosity.
- Chlorite schist: A subtype dominated by chlorite; greenschist is a broader facies including epidote and actinolite.
- Phyllite: Finer-grained with a glossier sheen and less obvious individual minerals; greenschist grains are coarser and more visible.
- Amphibolite: Higher grade, darker, hornblende-dominated, and typically not green-toned.
Where Greenschist Is Found
Greenschist forms in subduction zones and orogenic belts at relatively low temperatures and moderate pressures ("greenschist facies"). It occurs in mountain belts and ancient shields worldwide, including the Alps, Appalachians, and Precambrian terranes.
Frequently asked questions
How can you tell if it's greenschist?
Greenschist is green from chlorite, actinolite, and epidote and shows clear schistosity, meaning aligned platy minerals that give a silky sheen and a tendency to split along foliation planes.
What is the difference between greenschist and greenstone?
Both contain the same green metamorphic minerals, but greenschist is foliated and splits along mineral layers, while greenstone is massive and lacks foliation.
What does greenschist look like?
It is a pale-to-dark green foliated rock with a layered, slightly glittery or silky fabric, often crinkled or folded, that tends to split into sheets.
What minerals are in greenschist?
The characteristic minerals are chlorite, actinolite, and epidote, commonly with albite, quartz, and sometimes minor magnetite or calcite.
Greenschist vs serpentinite: how do I tell them apart?
Serpentinite has a soapy, smooth feel and polished slickensided surfaces and is dominated by serpentine, while greenschist is schistose with visible chlorite, actinolite, and epidote.
Greenschist identified by the community
Recent Greenschist specimens identified with Rock Identifier.
