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Tourmaline is a complex borosilicate mineral group characterized by a trigonal crystal system and a highly variable chemical composition. Its general formula is XY3Z6(T6O18)(BO3)3V3W, where X, Y, Z, T, V, and W represent various cations and anions. This variability leads to a wide range of colors and properties, making it one of the most diverse mineral groups. Tourmaline crystals are typically prismatic, often with characteristic rounded triangular cross-sections and vertical striations. It exhibits strong pleochroism and can be piezoelectric and pyroelectric.
How to Identify
- Color
- Extremely variable, including black (schorl), pink to red (rubellite), blue (indicolite), green (verdelite), yellow, brown (dravite), colorless (achroite), and multi-colored (watermelon tourmaline).
- Luster
- Vitreous to resinous.
- Texture
- Typically smooth on crystal faces, but can be rough if etched or weathered.
- Crystal Form
- Prismatic crystals, often elongated, with characteristic rounded triangular cross-sections. Vertical striations are common on prism faces. Can also be massive or columnar.
- Cleavage
- Indistinct to poor, often appearing as conchoidal fracture rather than true cleavage.
- Geological Environment
- Granitic pegmatites, aplites, hydrothermal veins, metamorphic schists, gneisses, and quartzites. Also found in detrital sediments due to its resistance to weathering.
Key Facts
- Hardness: 7-7.5 on the Mohs scale
- Specific Gravity: 2.9-3.2 (varies with composition)
- Crystal System: Trigonal
- Color: Extremely variable, often pleochroic
- Luster: Vitreous to resinous
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Indistinct to poor (often appears as fracture)
- Composition: Complex borosilicate of aluminum, iron, magnesium, sodium, lithium, and other elements
Quick Check
- Color: Highly variable (black, pink, red, green, blue, brown, yellow, colorless, multi-colored)
- Luster: Vitreous to resinous
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, columnar, acicular, massive, compact. Crystals often show a characteristic rounded triangular cross-section and vertical striations.
- Cleavage Type: Indistinct to poor, basal {0001} and rhombohedral {1011}. Fracture is more common than cleavage.
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to resinous
Formation
Tourmaline forms primarily in igneous and metamorphic environments. In igneous rocks, it is common in granitic pegmatites and aplites, where it crystallizes from late-stage, boron-rich hydrothermal fluids. In metamorphic rocks, it forms during regional and contact metamorphism of boron-rich sediments, often associated with schists, gneisses, and quartzites. Hydrothermal alteration processes can also lead to tourmaline formation in various rock types.
Usage
Gemstone (Elbaite varieties like rubellite, indicolite, verdelite, watermelon tourmaline), piezoelectric and pyroelectric applications (less common now but historically significant), flux in metallurgy, pressure gauges, and as a source of boron in some industrial processes. Schorl is sometimes used as an ornamental stone.
Age Distribution
Tourmaline minerals are found in rocks ranging from Precambrian to Cenozoic, with significant occurrences in Mesozoic and Cenozoic pegmatites and metamorphic rocks.
Where to Find
Brazil
Minas Gerais is a world-renowned source for gem-quality tourmaline, including rubellite, indicolite, and watermelon tourmaline, primarily from pegmatite deposits.
United States
Maine and California (especially San Diego County) are historically significant for gem tourmaline, particularly pink and green varieties from pegmatites.
Afghanistan
Known for fine quality rubellite and indicolite, often found in pegmatite deposits in the Nuristan and Kunar provinces.
Pakistan
Produces excellent gem-quality tourmaline, including pink, green, and bi-color varieties, from pegmatites in the Gilgit-Baltistan region.
Africa (e.g., Nigeria, Mozambique, Madagascar)
Various African countries are significant producers of gem tourmaline, including Paraíba-type tourmaline (copper-bearing elbaite) from Mozambique and Nigeria, and a wide range of colors from Madagascar.
Russia
The Ural Mountains are known for schorl and some gem-quality tourmaline.
Finding Tips
Look for Pegmatites
Tourmaline is most commonly found in granitic pegmatites. Look for coarse-grained igneous rocks with large crystals of quartz, feldspar, and mica, as these are prime environments for tourmaline.
Identify Triangular Cross-Sections
A key identifying feature of tourmaline is its characteristic rounded triangular cross-section when broken or viewed perpendicular to the crystal length. This helps distinguish it from amphiboles.
Check for Striations
Many tourmaline crystals exhibit prominent vertical striations (parallel lines) on their prism faces. This is a good indicator.
Observe Color and Pleochroism
While color is highly variable, the presence of strong pleochroism (different colors when viewed from different angles) is a diagnostic feature for many gem-quality tourmalines.
Examine Metamorphic Rocks
In metamorphic terrains, search for tourmaline in schists, gneisses, and quartzites, especially those associated with boron-rich protoliths or hydrothermal alteration.
Similar Rocks
Amphibole
Amphibole Group
Also known as: Hornblende, Tremolite, Actinolite
Epidote
Epidote
Also known as: Pistacite
Staurolite
Staurolite
Also known as: Fairy Cross
Scientific Classification
- Mineral Class
- Silicates
- Group
- Cyclosilicates (specifically, the Tourmaline Group)
- Crystal System
- Trigonal
- Chemical Formula
- XY3Z6(T6O18)(BO3)3V3W (general formula, where X=Na, Ca, K, vacancy; Y=Li, Mg, Fe2+, Mn2+, Al, Fe3+, Cr3+, V3+; Z=Al, Fe3+, Cr3+, V3+, Mg; T=Si, Al, B; B=Boron; V=OH, O; W=OH, F, O)
- Composition
- Complex borosilicate, with significant variations in the proportions of aluminum, iron, magnesium, sodium, lithium, and other elements, leading to numerous end-member species within the group.
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