metamorphic
Granite Gneiss
Felsic Gneiss (composed of Quartz, Feldspar, and Biotite)
Hardness: 6-7 on Mohs scale. Color: Pinkish-orange (orthoclase), grey (quartz), and black (biotite/hornblende). Luster: Dull to vitreous. Structure: Phaneritic with faint foliation or banding. Specific Gravity: 2.6-2.7.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
Identified More metamorphic →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 on Mohs scale. Color: Pinkish-orange (orthoclase), grey (quartz), and black (biotite/hornblende). Luster: Dull to vitreous. Structure: Phaneritic with faint foliation or banding. Specific Gravity: 2.6-2.7.
Formation & geological history
Formed through high-grade regional metamorphism of an igneous protolith (granite). Deep burial and high pressure/temperature cause the minerals to recrystallize and align into bands. Many specimens date to the Precambrian era.
Uses & applications
Used in heavy construction, as road aggregate, architectural stone, for monuments, and occasionally as landscaping stone.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. Because of its hardness, it is highly resistant to erosion.
Field identification & locations
Identify by its 'salt and pepper' or banded appearance and extreme hardness (cannot be scratched by a steel knife). Look for it in glaciated regions as 'glacial erratics' or in mountain belts like the Appalachians or Canadian Shield.
More like this
