metamorphic
Greenstone (Greenschist)
Greenschist (composed of Chlorite, Epidote, and Actinolite)
Hardness: 3.5-5 (Mohs scale); Color: Gray-green to dark forest green; Luster: Dull to silky; Crystal structure: Usually foliated or fine-grained; Cleavage: Good in one direction (slaty/schistose); Specific gravity: 2.8-3.3
- Hardness
- 3
- Color
- Gray-green to dark forest green
- Luster
- Dull to silky
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Physical properties
Hardness: 3.5-5 (Mohs scale); Color: Gray-green to dark forest green; Luster: Dull to silky; Crystal structure: Usually foliated or fine-grained; Cleavage: Good in one direction (slaty/schistose); Specific gravity: 2.8-3.3
Formation & geological history
Formed through low-grade regional metamorphism of mafic igneous rocks (like basalt or gabbro) under low temperatures and pressures (300-450°C). Many greenstones date back to the Archean or Proterozoic eons.
Uses & applications
Used in construction as crushed stone or building veneer. Historically used by indigenous cultures for tool-making (axes and adzes) because of its toughness. Sometimes used for ornamental carvings or as decorative garden stone.
Geological facts
The 'Greenstone Belts' are some of the oldest parts of the Earth's crust and often contain significant deposits of gold and copper. The green color is primarily due to the presence of the mineral chlorite.
Field identification & locations
Identify in the field by its distinct green hue, fine-grained texture, and subtle foliation (layering). It often feels slightly soapy or smooth compared to basalt. Found in ancient mountain belts and glacial till in areas like the Great Lakes or New England.
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