igneous
Granite Pegmatite
Phaneritic Plutonic Igneous Rock
Hardness: 6-7 (Mohs); Color: Pink to tan potassium feldspar, smoky quartz, and black biotite mica; Luster: Vitreous to pearly; Crystal structure: Coarse-grained (phaneritic); Cleavage: Perfect in two directions for feldspar; Specific gravity: 2.6-2.7
- Hardness
- 6-7 (Mohs)
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 6-7 (Mohs); Color: Pink to tan potassium feldspar, smoky quartz, and black biotite mica; Luster: Vitreous to pearly; Crystal structure: Coarse-grained (phaneritic); Cleavage: Perfect in two directions for feldspar; Specific gravity: 2.6-2.7
Formation & geological history
Formed from the slow cooling of magma deep underground, specifically during the final stages of a magma body's crystallization where high water content allows for large crystal growth. Found in continental crustal environments.
Uses & applications
Used primarily as decorative stone, countertops, construction aggregate, and as a source for industrial minerals like feldspar and mica. Highly coarse versions are sought by mineral collectors for individual crystals.
Geological facts
Pegmatites are famous for containing the world's largest crystals. Individual crystals in pegmatites can sometimes reach over 10 meters in length, though this specimen shows more typical macroscopic crystals.
Field identification & locations
Identify in the field by the exceptionally large, interlocking crystals of feldspar and quartz. It is commonly found in mountainous regions or areas where ancient continental crust is exposed, such as the Appalachian or Rocky Mountains.
