igneous
Granite
Phaneritic Acidic Igneous Rock (composed of Quartz, Alkali Feldspar, and Plagioclase)
Hardness: 6-7 on Mohs scale; Color: Grayish-brown, tan, and white speckles; Luster: Vitreous to pearly on crystal faces; Crystal structure: Phaneritic (coarse-grained visible crystals); Cleavage: Indistinct in bulk, but feldspars show two planes at 90 degrees; Specific gravity: 2.63 to 2.75.
- Hardness
- 6-7 on Mohs scale
- Color
- Grayish-brown, tan, and white speckles
- Luster
- Vitreous to pearly on crystal faces
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grayish-brown, tan, and white speckles; Luster: Vitreous to pearly on crystal faces; Crystal structure: Phaneritic (coarse-grained visible crystals); Cleavage: Indistinct in bulk, but feldspars show two planes at 90 degrees; Specific gravity: 2.63 to 2.75.
Formation & geological history
Formed from the slow crystallization of silica-rich magma beneath the Earth's surface (intrusive). Most continental granite dates from the Precambrian to Cenozoic eras, found in the roots of mountain ranges and continental shields.
Uses & applications
Extensively used in construction for countertops, floor tiles, paving stones, and memorial monuments. It is also crushed for use as an aggregate in road building and railroad ballast.
Geological facts
Granite makes up the majority of the Earth's continental crust. Because it forms deep underground and is extremely hard, its presence at the surface indicates that miles of overlying rock have been eroded away over millions of years.
Field identification & locations
Identify in the field by its 'salt and pepper' appearance and large, interlocking mineral grains. It is commonly found in mountainous regions or areas with exposed bedrock. Collectors should look for large feldspar crystals or inclusion of dark minerals like biotite.
