igneous
Granitic Pegmatite
Granitic Pegmatite (primarily SiO2 with KAlSi3O8)
Hardness: 6-7 on Mohs scale; Color: Pink, reddish-brown, and grey; Luster: Vitreous to dull; Crystal structure: Macro-crystalline with large interlocking grains of quartz and feldspar; Cleavage: Good in two directions (for feldspar components); Specific Gravity: 2.6-2.7
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, reddish-brown, and grey
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, reddish-brown, and grey; Luster: Vitreous to dull; Crystal structure: Macro-crystalline with large interlocking grains of quartz and feldspar; Cleavage: Good in two directions (for feldspar components); Specific Gravity: 2.6-2.7
Formation & geological history
Formed during the final stages of a magma's crystallization, where high water content allows for the growth of exceptionally large crystals. These typically form in dikes or veins within or around larger plutonic bodies.
Uses & applications
Used as a source of rare earth elements, industrial minerals like feldspar and mica, and sometimes as architectural stone or for gemstone extraction if high-quality crystals are present.
Geological facts
Pegmatites are famous for hosting the world's largest crystals, sometimes reaching several meters in length. They are the primary source of many gemstones including tourmaline, aquamarine, and topaz.
Field identification & locations
Identify in the field by its exceptionally large grain size compared to surrounding rock and the presence of quartz, feldspar, and mica. Commonly found in shield areas and mountain belts like the Appalachians or the Black Hills.
