igneous
Granitic Pegmatite
Granitic Pegmatite with Quartz and Feldspar
Hardness: 6-7 (Mohs scale), Color: tan, white, and translucent, Luster: vitreous to pearly, Structure: phaneritic/very coarse-grained crystals, Cleavage: good (in feldspars), Specific Gravity: ~2.6.
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Physical properties
Hardness: 6-7 (Mohs scale), Color: tan, white, and translucent, Luster: vitreous to pearly, Structure: phaneritic/very coarse-grained crystals, Cleavage: good (in feldspars), Specific Gravity: ~2.6.
Formation & geological history
Formed from the final, water-rich stages of a magma's crystallization. The high water content allows ions to move quickly, forming exceptionally large crystals. These are typically found in batholiths and surrounding metamorphic rocks.
Uses & applications
Source of high-purity quartz and feldspar for glass/ceramics; some yield rare earth elements and gems like tourmaline or beryl. Often used as decorative stone.
Geological facts
Pegmatites can host the largest crystals known on Earth, sometimes reaching over 10 meters in length. They are the 'garbage cans' of magmatic systems, concentrating rare elements like Lithium, Beryllium, and Tantalum.
Field identification & locations
Identify by exceptionally large crystal size compared to the surrounding rock. Common in geologically old continental crust areas like the Appalachians or the Black Hills. In the field, look for sparkling mica flakes and glassy quartz veins.
