metamorphic
Greenstone
Greenschist (primarily composed of chlorite, actinolite, and epidote)
Hardness: 3.5-5.5 (Mohs scale); Color: Dull green, olive green, or gray-green; Luster: Dull to waxy; Structure: Massive to weakly foliated; Cleavage: Poor; Specific Gravity: 2.8-3.0.
- Hardness
- 3
- Color
- Dull green, olive green, or gray-green
- Luster
- Dull to waxy
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Physical properties
Hardness: 3.5-5.5 (Mohs scale); Color: Dull green, olive green, or gray-green; Luster: Dull to waxy; Structure: Massive to weakly foliated; Cleavage: Poor; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed through the low-grade metamorphism of mafic igneous rocks (such as basalt or gabbro) under low temperatures and pressures (greenschist facies). This process often occurs in ancient mountain-building sites or oceanic crust subduction zones, dating from the Archean to the Cenozoic.
Uses & applications
Historically used for stone tools (axes) due to its toughness; currently used as crushed stone for road construction, decorative garden stone, and occasionally in architectural masonry.
Geological facts
The characteristic green color comes from the inclusion of the mineral chlorite. 'Greenstone belts' are some of the oldest known geological formations on Earth and are often associated with major gold and copper deposits.
Field identification & locations
Identify in the field by its distinct dull green color and relative softness compared to quartz. It lacks the grains of sandstone and the glassy texture of obsidian. Commonly found in the Appalachian Mountains, Lake Superior region, and Western Australia.
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