igneous
Granitic Pegmatite with Schorl Tourmaline
Granitic Pegmatite (Schorl: NaFe3Al6(BO3)3Si6O18(OH)4)
Hardness: 6-7 (Pegmatite), 7-7.5 (Tourmaline); Color: White/cream matrix with black needle-like or prismatic crystals; Luster: Vitreous to greasy; Crystal structure: Trigonal/Hexagonal (Tourmaline), Monoclinic/Triclinic (Feldspar/Quartz); Cleavage: None/Indistinct; Specific gravity: 2.6-3.2
- Hardness
- 6-7 (Pegmatite), 7-7
- Luster
- Vitreous to greasy
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Physical properties
Hardness: 6-7 (Pegmatite), 7-7.5 (Tourmaline); Color: White/cream matrix with black needle-like or prismatic crystals; Luster: Vitreous to greasy; Crystal structure: Trigonal/Hexagonal (Tourmaline), Monoclinic/Triclinic (Feldspar/Quartz); Cleavage: None/Indistinct; Specific gravity: 2.6-3.2
Formation & geological history
Formed during the final stages of a magma's crystallization, where high water content allows for the growth of very large crystals. These typically occur in granite plutons and date from various geological periods, often within stable continental shields.
Uses & applications
Source of industrial minerals like quartz, feldspar, and mica; Schorl is used in grounding jewelry and as a specimen for mineral collectors.
Geological facts
Pegmatites are known as 'nature's treasure chest' because they can contain giant crystals and rare elements. Schorl is the most common variety of tourmaline and is naturally pyroelectric, meaning it can generate an electric charge when heated.
Field identification & locations
Identify by very large grain size (phaneritic) and the presence of black, needle-like tourmaline (Schorl). Commonly found in pegmatite veins in mountainous regions like the Appalachians, Brazil, or Namibia. Look for black striations on the crystal faces.
