metamorphic
Gneiss (specifically Epidote-Quartz Gneiss)
Gneiss containing Epidote (Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)) and Quartz (SiO2)
Hardness: 6-7 (Mohs scale); Color: Banded white (quartz/feldspar) and olive green (epidote); Luster: Vitreous to dull; Crystal structure: Foliated/Banded; Specific gravity: 2.6-3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded white (quartz/feldspar) and olive green (epidote)
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded white (quartz/feldspar) and olive green (epidote); Luster: Vitreous to dull; Crystal structure: Foliated/Banded; Specific gravity: 2.6-3.0
Formation & geological history
Forms through high-grade regional metamorphism of igneous or sedimentary rocks. The presence of green epidote suggests metamorphism under greenschist or epidote-amphibolite facies conditions.
Uses & applications
Used as crushed stone for road construction, building stone for landscaping, and occasionally decorative facing for walls.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dated at 4.03 billion years old. The distinctive green coloring in this specimen is caused by the mineral epidote.
Field identification & locations
Identify in the field by its characteristic 'gneissic banding' where minerals are segregated into distinct dark and light layers. Commonly found in the cores of mountain ranges and ancient continental shields.
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