metamorphic
Greenstone (Epidote/Chlorite Schist)
Metabasalt (composed primarily of Epidote Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH) and Chlorite (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6)
Hardness: 6-7 (epidote) or 2-2.5 (chlorite); Color: Pistachio green to dark forest green; Luster: Vitreous to dull/pearly; Crystal structure: Monoclinic; Cleavage: Perfect in one direction; Specific gravity: 3.2-3.5
- Hardness
- 6-7 (epidote) or 2-2
- Color
- Pistachio green to dark forest green
- Luster
- Vitreous to dull/pearly
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Physical properties
Hardness: 6-7 (epidote) or 2-2.5 (chlorite); Color: Pistachio green to dark forest green; Luster: Vitreous to dull/pearly; Crystal structure: Monoclinic; Cleavage: Perfect in one direction; Specific gravity: 3.2-3.5
Formation & geological history
Formed through regional metamorphism of mafic igneous rocks (like basalt) under low to medium temperatures and pressures. This creates 'greenstone belts' often dating back to the Archean or Proterozoic eons.
Uses & applications
Used primarily as crushed stone for road construction and aggregate. It is sometimes used for dimension stone, ornamental carvings, or high-end architectural facades due to its color.
Geological facts
The distinctive green color comes from the 'Greenschist Facies' of metamorphism. These rocks are primary indicators for geologists searching for ancient tectonic plate boundaries and gold deposits, as they are often associated with hydrothermal gold mineralization.
Field identification & locations
Field identification: Look for the characteristic pistachio-green hue and a slightly foliated or 'platy' texture. Commonly found in old mountain ranges and shield areas like the Superior Province (Canada) or Western Australia. Collectors should look for crystalline epidote veins within the matrix.
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