igneous
Granic Pegmatite (Quartz and Feldspar)
Crystalline Quartz (SiO2) and Plagioclase Feldspar (NaAlSi3O8 – CaAl2Si2O8)
Hardness: 6-7 Mohs; Color: Milky white, grey, and dark flecks; Luster: Vitreous to greasy; Crystal structure: Trigonal/Triclinic; Cleavage: None in quartz, perfect in feldspar.
- Hardness
- 6-7 Mohs
- Color
- Milky white, grey, and dark flecks
- Luster
- Vitreous to greasy
Identified More igneous →
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Physical properties
Hardness: 6-7 Mohs; Color: Milky white, grey, and dark flecks; Luster: Vitreous to greasy; Crystal structure: Trigonal/Triclinic; Cleavage: None in quartz, perfect in feldspar.
Formation & geological history
Formed from the slow cooling of magma deep within the Earth's crust, often in the final stages of crystallization where large mineral grains develop in pegmatitic veins.
Uses & applications
Commonly used in construction aggregate, as a source for silica in glassmaking, or as a decorative landscaping stone. High-quality feldspar is used in ceramics.
Geological facts
Pegmatites are famous for containing the largest crystals ever found on Earth, sometimes reaching several meters in length because of the low viscosity of the leftover magma.
Field identification & locations
Identify by the interlocking large crystal grains and the absence of layering. Frequently found in mountainous regions or where ancient continental shields are exposed.
