metamorphic
Greenstone
Protolith of basalt/gabbro (chlorite-epidote metamorphic facies)
Hardness: 5-7 Mohs; Color: Dull green, grey-green, or dark olive; Luster: Dull to waxy; Texture: Fine-grained to aphanitic; Cleavage: None/Irregular break; Density: ~2.8-3.0 g/cm3
- Hardness
- 5-7 Mohs
- Color
- Dull green, grey-green, or dark olive
- Luster
- Dull to waxy
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Physical properties
Hardness: 5-7 Mohs; Color: Dull green, grey-green, or dark olive; Luster: Dull to waxy; Texture: Fine-grained to aphanitic; Cleavage: None/Irregular break; Density: ~2.8-3.0 g/cm3
Formation & geological history
Formed via low-grade regional metamorphism of mafic igneous rocks (basalt or gabbro) under high pressure and low temperatures, often in subduction zones or greenstone belts. Many dates back to the Archean or Proterozoic eons.
Uses & applications
Used historically for stone tools (axes) due to toughness; modern uses include crushed stone for road construction, decorative architectural stone, and garden landscaping.
Geological facts
Greenstone belts represent some of the oldest preserved crustal rocks on Earth. The green color comes from minerals like chlorite, actinolite, and epidote which replace the original dark minerals of the volcanic protolith.
Field identification & locations
Identify by its distinctive dull green hue and lack of foliation compared to schist. Often found in ancient shield areas like the Canadian Shield or Western Australia. It is frequently associated with gold mineralization.
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