metamorphic
Greenstone (Epidisite / Chlorite Schist)
Metabasalt (often contains Epidote Ca2Al2(Fe3+,Al)(SiO4)(Si2O7)O(OH) and Chlorite)
Hardness: 6-7 on Mohs scale; Color: Dull grayish-green to forest green; Luster: Dull to waxy; Structure: Fine-grained, sometimes foliated or massive; Cleavage: Poor.
- Hardness
- 6-7 on Mohs scale
- Color
- Dull grayish-green to forest green
- Luster
- Dull to waxy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dull grayish-green to forest green; Luster: Dull to waxy; Structure: Fine-grained, sometimes foliated or massive; Cleavage: Poor.
Formation & geological history
Formed through low-grade regional metamorphism of mafic igneous rocks (like basalt) under high pressure and low temperatures. This process usually occurs at subduction zones or during mountain building (orogeny).
Uses & applications
Historically used for making stone tools (axes/celts) due to its toughness. Today it is used as crushed stone for construction, road base, and occasionally as decorative garden stone.
Geological facts
Greenstone belts are some of the oldest known geological formations on Earth, dating back to the Archean Eon (over 2.5 billion years ago). They are often associated with major gold and copper deposits.
Field identification & locations
Identify by its distinctive 'pistachio green' or grayish-green hue that does not rub off. It is commonly found in ancient shield areas like the Upper Midwest US, Canada, and Western Australia. It is harder than a steel knife but brittle when struck.
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