igneous
Pegmatite (Granite Pegmatite)
Intrusive Igneous Rock (Granite Pegmatite)
Hardness: 6-7 (Mohs scale), Color: White, gray, and dark brown to black, Luster: Vitreous to pearly, Crystal structure: Large phaneritic/porphyritic crystals, Cleavage: Varies by mineral constituent, Specific gravity: 2.6-2.7
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Physical properties
Hardness: 6-7 (Mohs scale), Color: White, gray, and dark brown to black, Luster: Vitreous to pearly, Crystal structure: Large phaneritic/porphyritic crystals, Cleavage: Varies by mineral constituent, Specific gravity: 2.6-2.7
Formation & geological history
Formed by the slow cooling of water-rich magma in the final stages of crystallization of an intrusive igneous body. The presence of water allows for the growth of exceptionally large crystals.
Uses & applications
Used as a source of rare minerals such as spodumene, lepidolite, and tourmaline. Also used in construction as crushed stone and for architectural stone decorative pieces.
Geological facts
Pegmatites are famous for containing some of the largest crystals in the world, sometimes reaching several meters in length. They are the primary source of many gemstones.
Field identification & locations
Identify in the field by looking for exceptionally large mineral grains (over 1 cm) within a granitic matrix. Commonly found in shield areas of continents like the Black Hills of South Dakota or the Scandinavian Shield.
