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Chemical Formula
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The simple chemical formula, which covers the main forms of Tourmaline (Elbaite, Schorl, and Dravite), is as follows:
(Na,Ca)(Mg,Li,Al,Fe2+)3Al6(BO3)3Si6O18(OH)4
The expanded formula, which additionally covers Uvite, Liddicoatite, and Buergerite, is as follows:
(Na,Ca)(Mg,Li,Al,Fe2+,Fe3+)3(Al,Mg)6(BO3)3Si6O18(OH,O,F)4
The formula for the Tourmaline group is very complex. See The chemical formula of Tourmaline for more details.
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Composition
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See The chemical formula of Tourmaline.
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Color
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Tourmaline is extremely varied in color. Colors include black, brown, green, red, pink, blue, and gray. White, colorless, yellow, orange, and purple colors are less common. Crystals are frequently multicolored, containing two or more distinct colors. Some specimens are pleochroic.
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Streak
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Whie
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Hardness
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7 - 7.5
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Crystal System
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Hexagonal
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Crystal Forms
and Aggregates
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Usually as elongated prismatic crystals that are heavily striated. Also as short, stubby, prismatic crystals. Most Tourmaline crystals have a rounded, triangular cross-section. Seldom in tabular crystals. Aggregates include columnar, radiating, botryoidal, stalactitic, in dense groups of tiny, elongated needles, and in compact masses.
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Transparency
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Transparent to opaque
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Specific Gravity
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2.9 - 3.3
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Luster
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Vitreous. Some black and brown specimens may be dull.
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Cleavage
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3,2
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Fracture
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Conchoidal to uneven
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Tenacity
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Brittle
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Other ID Marks
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1) Strongly pyroelectric.
2) Piezoelectric.
3) A few forms of Tourmaline fluoresce yellow in shortwave ultraviolet light.
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In Group
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Silicates; Cyclosilicates; Tourmaline Group
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Striking Features
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Color, crystal form, hardness, and deep vertical striations.
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Environment
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Elbaite, Schorl, and Liddicoatite are almost exclusively from granite pegmatites, while Dravite and Uviteor mostly from metamorphic environments such as marbles. Buergerite is from igneous rhyolite deposits.
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Rock Type
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Igneous, Metamorphic
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Popularity (1-4)
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1
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Prevalence (1-3)
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2
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Demand (1-3)
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1
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