igneous
Pegmatite
Granitic Pegmatite
Very coarse-grained texture (crystals >1cm); Hardness: 6-7 (Mohs); Color: Mottled pink (orthoclase), white/gray (quartz), and black/silver (mica); Luster: Vitreous to pearly; Structure: Phaneritic/Pegmatitic.
- Hardness
- 6-7 (Mohs)
- Luster
- Vitreous to pearly
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Physical properties
Very coarse-grained texture (crystals >1cm); Hardness: 6-7 (Mohs); Color: Mottled pink (orthoclase), white/gray (quartz), and black/silver (mica); Luster: Vitreous to pearly; Structure: Phaneritic/Pegmatitic.
Formation & geological history
Formed during the final stage of a magma's crystallization. Highly fluid, water-rich melts allow for rapid ion movement, resulting in exceptionally large crystals. Most common in Precambrian or Paleozoic shields.
Uses & applications
Source of rare earth elements (lithium, cesium, tantalum), industrial minerals like feldspar and mica, and gemstones such as beryl or tourmaline.
Geological facts
Pegmatites are known for containing the largest crystals found on Earth; some spodumene crystals in South Dakota have reached 42 feet in length.
Field identification & locations
Identify by exceptionally large, interlocking crystals of quartz and pink feldspar. Commonly found in mountain ranges and tectonic belts. Collectors value pieces with well-defined crystal faces or rare accessory minerals.
