metamorphic
Greenstone
Greenschist (metamorphosed mafic igneous rock)
Hardness: 6-7 (Mohs); Color: Dark green to grayish-green; Luster: Dull to waxy; Texture: Fine-grained, often massive or slightly foliated; Composition: Chlorite, epidote, actinolite, and albite.
- Hardness
- 6-7 (Mohs)
- Color
- Dark green to grayish-green
- Luster
- Dull to waxy
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark green to grayish-green; Luster: Dull to waxy; Texture: Fine-grained, often massive or slightly foliated; Composition: Chlorite, epidote, actinolite, and albite.
Formation & geological history
Formed through low-grade metamorphism (greenschist facies) of mafic igneous rocks like basalt or gabbro. This occurs at temperatures of 300-450°C and moderate pressure, often in ancient volcanic arc environments or seafloor spreading centers.
Uses & applications
Commonly used in landscaping, as crushed stone for road construction, and historically used by indigenous cultures to craft stone tools like adzes and axes due to its toughness. Occasional use in ornamental carvings.
Geological facts
The 'green' color is specifically due to the presence of minerals like chlorite and epidote produced during the alteration of original volcanic minerals. Many greenstones date back to the Archean Eon, making them some of the oldest rocks on Earth.
Field identification & locations
Identify in the field by its distinct dull green hue, lack of visible crystals without magnification, and extreme toughness. It is commonly found in suture zones and ancient mountain belts like the Appalachians or the Canadian Shield.
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