igneous
Granite Pegmatite
Coarse-grained Granite (SiO2 - Aluminum and Alkalies)
Hardness: 6-7 Mohs; Color: Mottle white, tan, and dark gray/green; Luster: Vitreous to pearly; Crystal structure: Phaneritic (very large crystals), often containing orthoclase, quartz, and mica; Cleavage: Variable based on mineral (perfect in micas); Specific gravity: 2.6-2.7
- Hardness
- 6-7 Mohs
- Color
- Mottle white, tan, and dark gray/green
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 6-7 Mohs; Color: Mottle white, tan, and dark gray/green; Luster: Vitreous to pearly; Crystal structure: Phaneritic (very large crystals), often containing orthoclase, quartz, and mica; Cleavage: Variable based on mineral (perfect in micas); Specific gravity: 2.6-2.7
Formation & geological history
Formed during the final stages of a magma chamber's crystallization when high water content allows for rapid crystal growth. These structures often date back to the Proterozoic or Paleozoic eras and form in deep-seated plutonic environments.
Uses & applications
Used as an industrial source for rare elements (lithium, tantalum), high-grade industrial minerals (mica and feldspar), and occasionally crushed for architectural stone or gravel. Large specimens are popular with amateur rock collectors.
Geological facts
Pegmatites are famous for growing some of the largest crystals on Earth; a single spodumene crystal in a pegmatite has been recorded at 47 feet long. They act as 'geological trash cans' where rare elements that don't fit in common minerals finally crystallize.
Field identification & locations
Identify in the field by looking for exceptionally large mineral grains (usually >1cm) compared to surrounding rock. Common in mountainous regions with exposed continental crust like the Black Hills of South Dakota or the Appalachian mountains. Tips for collectors: Check for small translucent quartz pockets which may hide 'vugs' with well-formed crystals.
