igneous
Pegmatite (with Quartz and Feldspar)
Granitic Pegmatite
Hardness: 6-7 (Mohs scale); Color: Tan, white, and grey with orange-red iron staining; Luster: Vitreous to pearly; Crystal structure: Massive/large interlocking crystals; Cleavage: Perfect on feldspar grains, none on quartz grains; Specific Gravity: 2.5-2.8.
- Hardness
- 6-7 (Mohs scale)
- Color
- Tan, white, and grey with orange-red iron staining
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Tan, white, and grey with orange-red iron staining; Luster: Vitreous to pearly; Crystal structure: Massive/large interlocking crystals; Cleavage: Perfect on feldspar grains, none on quartz grains; Specific Gravity: 2.5-2.8.
Formation & geological history
Formed during the final stages of a magma's crystallization. Highly fluid, water-rich magma allows for the growth of exceptionally large crystals. These are typically associated with large granitic intrusions and can date from billions of years ago to the Cenozoic era.
Uses & applications
Primary source of industrial minerals like quartz and feldspar (for glass and ceramics). It is also mined for rare elements (lithium, tantalum) and gemstones like tourmaline or beryl.
Geological facts
Pegmatites are famous for containing the world's largest individual crystals. Some beryl crystals in pegmatites have been recorded at over 14 meters (46 feet) in length.
Field identification & locations
Identify by exceptionally large grain size (usually >1cm). Look for interlocking translucent quartz and opaque feldspar. Commonly found in mountain belts and Precambrian shield areas. Use a scratch test to confirm hard minerals.
