metamorphic
Greenstone
Greenschist / Meta-basalt
Hardness: 6-7 (Mohs scale); Color: Dull green, forest green, or grayish-green; Luster: Dull to waxy; Crystal structure: Microcrystalline (often showing chlorite and epidote minerals); Cleavage: Poorly developed, often fractures unevenly; Specific gravity: 2.8-3.3.
- Hardness
- 6-7 (Mohs scale)
- Color
- Dull green, forest green, or grayish-green
- Luster
- Dull to waxy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dull green, forest green, or grayish-green; Luster: Dull to waxy; Crystal structure: Microcrystalline (often showing chlorite and epidote minerals); Cleavage: Poorly developed, often fractures unevenly; Specific gravity: 2.8-3.3.
Formation & geological history
Formed through the low-grade metamorphism (greenschist facies) of mafic igneous rocks like basalt or gabbro. This typically occurs at temperatures between 300 to 450 degrees Celsius at depths of several kilometers.
Uses & applications
Used as crushed stone for road construction, fill material, and occasionally as architectural stone or for carving in ornamental crafts. Historically used for making stone tools (celts) due to its toughness.
Geological facts
The characteristic green color is produced by minerals such such as chlorite, actinolite, and epidote. Vast belts of greenstone (Greenstone Belts) are among the oldest crustal fragments on Earth, dating back to the Archean Eon.
Field identification & locations
Identify in the field by its distinctive green hue, lack of obvious foliation compared to schist, and its hardness (it will not be scratched by a copper penny). It is commonly found in ancient mountain belts and subduction zones.
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