igneous
Tourmaline-bearing pegmatite
Quartz–feldspar pegmatite with black schorl tourmaline (NaFe3Al6(BO3)3Si6O18(OH)4)
AI-generated identification · may be incorrectCoarse white quartz and feldspar host a black, prismatic tourmaline crystal. Quartz hardness 7 and vitreous; feldspar hardness 6–6.5 with cleavage; schorl hardness 7–7.5, commonly striated and strongly pleochroic.
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Physical properties
Coarse white quartz and feldspar host a black, prismatic tourmaline crystal. Quartz hardness 7 and vitreous; feldspar hardness 6–6.5 with cleavage; schorl hardness 7–7.5, commonly striated and strongly pleochroic.
Formation & geological history
Formed from water-rich, silica-rich granitic magma during late-stage crystallization, allowing exceptionally large crystals to grow in pegmatite cavities and veins. Most examples are Precambrian to Mesozoic, depending on locality.
Uses & applications
Collected as a mineral specimen; attractive tourmaline-bearing pegmatite may be used decoratively, though this piece is unsuitable for construction aggregate.
Geological facts
Black tourmaline is typically schorl, the most common tourmaline species. Tourmaline crystals can become electrically polarized when heated or compressed (pyroelectric and piezoelectric effects).
Field identification & locations
Identify the pale, coarse-grained quartz–feldspar matrix and dark, elongated, often vertically striated tourmaline. Common sources include Brazil, Afghanistan, Madagascar, Namibia, and the United States.
