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Tourmaline is a complex borosilicate mineral group, characterized by a hexagonal-trigonal crystal system and a distinctive prismatic habit, often with vertical striations. Its chemical formula is highly variable, leading to a wide range of colors and properties. The general formula is XY3Z6(T6O18)(BO3)3V3W, where X, Y, Z, T, V, and W represent different cation and anion sites. The most common varieties are Schorl (iron-rich, black), Elbaite (lithium-aluminum-rich, often colorful), and Dravite (magnesium-rich, brown to yellow). Tourmaline is known for its strong pleochroism and pyroelectric/piezoelectric properties.
How to Identify
- Color
- Extremely variable; black (Schorl), pink to red (Rubellite Elbaite), green (Verdelite Elbaite, Chromdravite), blue (Indicolite Elbaite), yellow, brown (Dravite), colorless (Achroite Elbaite), or multi-colored (Watermelon Tourmaline).
- Luster
- Vitreous to resinous.
- Texture
- Typically prismatic crystals, often with distinct vertical striations on prism faces. Can be massive or granular.
- Crystal Form
- Hexagonal-trigonal system, typically elongated prismatic crystals with a characteristic rounded triangular cross-section. Terminations can be complex, often hemimorphic.
- Cleavage
- Indistinct to poor basal cleavage {0001}, often appearing as conchoidal to uneven fracture. This lack of distinct cleavage is a key differentiating feature.
- Geological Environment
- Granitic pegmatites, granites, aplites, hydrothermal veins, metamorphic rocks (schists, gneisses, marbles), and detrital sediments.
Key Facts
- Hardness: 7-7.5 on the Mohs scale
- Specific Gravity: 2.82-3.32 (varies with composition, Schorl is higher, Elbaite is lower)
- Crystal System: Trigonal
- Color: Extremely variable, often pleochroic
- Luster: Vitreous to resinous
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Indistinct to poor basal cleavage {0001}
- Composition: Complex borosilicate of aluminum, iron, magnesium, sodium, lithium, or other elements
Quick Check
- Color: Highly variable (black, pink, green, blue, brown, yellow, colorless, multi-colored)
- Luster: Vitreous to resinous
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, often elongated, with vertical striations on prism faces; also massive, columnar, or granular. Cross-section is typically a rounded triangle.
- Cleavage Type: Indistinct to poor basal cleavage {0001}. Fracture is more prominent than cleavage.
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to resinous
Formation
Tourmaline forms in a wide range of geological environments, most notably in granitic pegmatites, hydrothermal veins, and metamorphic rocks. It crystallizes from boron-rich fluids, often at elevated temperatures and pressures. In pegmatites, it can grow to very large sizes. Schorl, the iron-rich variety, is common in granites and granite pegmatites. Elbaite, the lithium-rich variety, is characteristic of complex pegmatites. Dravite, the magnesium-rich variety, is found in metamorphic rocks (e.g., schists, gneisses) and some igneous rocks.
Usage
Gemstone (Elbaite varieties like Rubellite, Indicolite, Verdelite, Achroite, Watermelon Tourmaline), piezoelectric and pyroelectric applications (though less common now), flux in metallurgy (Schorl), and as a significant indicator mineral in geological exploration for certain ore deposits.
Age Distribution
Precambrian to Cenozoic, commonly associated with granitic intrusions and metamorphic events.
Where to Find
Brazil
Minas Gerais is a world-renowned source for gem-quality Tourmaline, particularly Elbaite varieties (pink, green, blue, multi-colored).
United States
Maine and California (especially San Diego County) are historically significant for gem-quality Elbaite, including pink and green varieties.
Afghanistan
Known for fine gem-quality Tourmaline, including pink, green, and blue varieties, often found in pegmatites.
Pakistan
Produces excellent gem-quality Tourmaline, similar to Afghanistan, from pegmatite deposits.
Africa (e.g., Nigeria, Mozambique, Madagascar, Tanzania)
Significant sources of various gem-quality Tourmaline colors, including pink, green, and Paraíba-type (copper-bearing) Tourmaline.
Sri Lanka
Historically a source of various Tourmaline colors, often found in alluvial deposits.
Finding Tips
Look in Pegmatites
Tourmaline, especially gem-quality Elbaite, is most commonly found in granitic pegmatites. Look for areas with coarse-grained igneous rocks.
Check Hydrothermal Veins
Some Tourmaline varieties can be found in hydrothermal veins associated with granitic intrusions.
Examine Metamorphic Rocks
Dravite (brown Tourmaline) is often found in magnesium-rich metamorphic rocks like schists and gneisses.
Identify Crystal Habit
Look for elongated prismatic crystals, often with vertical striations and a characteristic rounded triangular cross-section.
Consider Color and Luster
Tourmaline exhibits a wide range of colors and a vitreous to resinous luster. Black Schorl is very common in granitic rocks.
Similar Rocks
Hornblende
Hornblende Group
Also known as: Amphibole
Biotite
Biotite Group
Also known as: Black Mica
Epidote
Epidote Group
Also known as: Pistacite
Actinolite
Actinolite
Also known as: Amphibole
Scientific Classification
- Mineral Class
- Silicates
- Group
- Cyclosilicates (Tourmaline Group)
- Crystal System
- Trigonal
- Chemical Formula
- XY3Z6(T6O18)(BO3)3V3W (general formula, highly variable)
- Composition
- Complex borosilicate, with varying amounts of Na, Ca, K, Fe, Mg, Mn, Al, Li, Cr, V, F, OH. Specific varieties: Elbaite (Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3(OH)3(OH)), Schorl (NaFe3Al6(Si6O18)(BO3)3(OH)3(OH)), Dravite (NaMg3Al6(Si6O18)(BO3)3(OH)3(OH)).
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