igneous
Granitic Pegmatite
Granite Pegmatite (principally KAlSi3O8, SiO2, and KAl2(AlSi3O10)(OH)2)
Hardness: 6-7 (Mohs scale); Color: White, cream, tan, and smoky grey; Luster: Vitreous to pearly; Crystal Structure: Very large interlocking crystals (>1cm); Cleavage: Perfect in mica and feldspar components; Specific Gravity: 2.6-2.7
- Hardness
- 6-7 (Mohs scale)
- Color
- White, cream, tan, and smoky grey
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 6-7 (Mohs scale); Color: White, cream, tan, and smoky grey; Luster: Vitreous to pearly; Crystal Structure: Very large interlocking crystals (>1cm); Cleavage: Perfect in mica and feldspar components; Specific Gravity: 2.6-2.7
Formation & geological history
Formed during the final stages of a magma's crystallization, usually in hydrothermal veins or at the margins of large plutons. The high water content allows for rapid crystal growth resulting in massive mineral sizes. Most often associated with Precambrian or Paleozoic geological eras.
Uses & applications
Primary source of industrial minerals like sheet mica, feldspar for ceramics/glass, and rare elements (lithium, tantalum). High-quality specimens are mined for gemstones like tourmaline, beryl, and spodumene.
Geological facts
Pegmatites can contain the world's largest crystals; beryl crystals over 30 feet long have been found in pegmatite deposits. They are sometimes called 'nature's treasure chest' because they concentrate rare earth elements.
Field identification & locations
Identify in the field by the exceptionally large crystal size compared to surrounding rock. Look for the 'graphic' texture (intergrown quartz and feldspar) and the presence of sheet-like books of mica. Found in mountainous or shield regions like the Black Hills (SD), Brazil, and Ontario.
