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The Tourmaline Group is a complex borosilicate mineral group characterized by a hexagonal-trigonal crystal system and a highly variable chemical composition. It is known for its wide range of colors, often within a single crystal (zonation), and its distinctive prismatic crystal habit with characteristic striations parallel to the c-axis. Tourmaline is both piezoelectric and pyroelectric, meaning it can generate an electrical charge when subjected to mechanical stress or temperature changes, respectively. It is a common accessory mineral in many rock types and a significant gem mineral.
How to Identify
- Color
- Extremely variable; black (Schorl), pink to red (Rubellite), blue (Indicolite), green (Verdelite), yellow, brown (Dravite), colorless (Achroite), bi-colored, tri-colored (Watermelon Tourmaline).
- Luster
- Vitreous to resinous.
- Texture
- Typically prismatic crystals, often with a triangular cross-section and prominent vertical striations.
- Crystal Form
- Elongated prismatic crystals, often terminated by rhombohedral or pedion faces. Cross-sections are typically rounded triangular.
- Cleavage
- Poor to indistinct, often appearing absent. Fracture is conchoidal to uneven.
- Geological Environment
- Granitic pegmatites, high-temperature hydrothermal veins, greisens, metamorphic schists, gneisses, and quartzites. Also found in detrital sediments.
Key Facts
- Hardness: 7-7.5 on the Mohs scale
- Specific Gravity: 2.90-3.26 (varies with composition)
- Crystal System: Trigonal
- Color: Extremely variable (black, pink, green, blue, brown, colorless, multi-colored)
- Luster: Vitreous to resinous
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Poor to indistinct (often appears absent)
- Composition: Complex borosilicate, (Na,Ca)(Li,Mg,Fe,Mn,Al)3Al6(BO3)3Si6O18(OH,F)4. The general formula is XY3Z6(T6O18)(BO3)3V3W, where X, Y, Z, T, V, W represent various cation and anion sites.
Quick Check
- Color: Highly variable (black, pink, green, blue, brown, colorless, multi-colored)
- Luster: Vitreous to resinous
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, often elongated, with vertical striations on prism faces; typically terminated by rhombohedral or pedion faces. Cross-section often rounded triangular.
- Cleavage Type: Poor to indistinct, typically not observed.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous to resinous.
Formation
Tourmaline forms primarily in igneous and metamorphic rocks. It is common in granitic pegmatites, where it crystallizes from late-stage, volatile-rich magmatic fluids. It also forms in high-temperature hydrothermal veins, greisens, and metasomatic rocks. In metamorphic environments, it can be found in schists, gneisses, and quartzites, often as a result of boron metasomatism. Detrital tourmaline is also common in sedimentary rocks due to its high resistance to weathering.
Usage
Gemstone (especially Elbaite varieties like Rubellite, Indicolite, Verdelite, Watermelon Tourmaline), piezoelectric and pyroelectric applications (though less common now), pressure gauges, and as a source of boron in some industrial processes. Schorl is sometimes used as an ornamental stone.
Age Distribution
Found in rocks of various ages, from Precambrian to Cenozoic, depending on the specific geological environment of formation.
Where to Find
Brazil
Minas Gerais is a world-renowned source for gem-quality tourmaline, including Elbaite varieties like Rubellite, Indicolite, and Watermelon Tourmaline.
United States
Maine and California (especially San Diego County) are historically significant for gem tourmaline, particularly pink and green varieties.
Afghanistan and Pakistan
Known for fine gem-quality tourmaline, including vibrant greens, blues, and pinks, often found in pegmatites.
Africa
Countries like Nigeria, Mozambique, Madagascar, and Namibia are important sources of various tourmaline colors, including Paraíba-type tourmaline (copper-bearing Elbaite).
Sri Lanka
Historically a source of various gem tourmalines, often found in alluvial deposits.
Finding Tips
Pegmatite Environments
Focus on areas with granitic pegmatites, especially those with signs of hydrothermal alteration. Tourmaline often forms in pockets within these pegmatites.
Metamorphic Terrains
Look for tourmaline in schists, gneisses, and quartzites, particularly where boron-rich fluids may have been present during metamorphism.
Alluvial Deposits
Due to its hardness and resistance to weathering, tourmaline can be found in placer deposits downstream from primary sources. Look in stream beds and gravels.
Distinctive Crystal Habit
Learn to recognize the characteristic prismatic crystals with vertical striations and often a rounded triangular cross-section. This is a key identifier in the field.
Color Variation
Be aware of the vast color range. Black tourmaline (Schorl) is the most common and can indicate the presence of other tourmaline varieties nearby.
Similar Rocks
Hornblende
Hornblende Group
Also known as: Amphibole
Biotite
Biotite Group
Also known as: Black Mica
Epidote
Epidote
Also known as: Pistacite
Actinolite
Actinolite
Also known as: Nephrite (a variety)
Scientific Classification
- Mineral Class
- Silicates
- Group
- Cyclosilicates (specifically, the Tourmaline Group)
- Crystal System
- Trigonal
- Chemical Formula
- General formula: XY3Z6(T6O18)(BO3)3V3W, where X = Na, Ca, K, vacancy; Y = Li, Mg, Fe2+, Fe3+, Mn2+, Al, Cr3+, V3+; Z = Al, Fe3+, Cr3+, V3+, Mg; T = Si, Al, B; B = B; V = OH, O; W = OH, F, O. Specific formulas vary greatly by species.
- Composition
- Complex borosilicate with varying amounts of sodium, calcium, lithium, magnesium, iron, manganese, and aluminum. The presence of trace elements, particularly iron, manganese, chromium, vanadium, and copper, is responsible for the wide range of colors.
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