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Schorl is the most common member of the tourmaline group, characterized by its opaque black color. It is a complex borosilicate mineral with a trigonal crystal system, typically forming elongated, prismatic crystals with characteristic striations parallel to the c-axis. Its robust nature and common occurrence make it a significant accessory mineral in various igneous and metamorphic rocks.
How to Identify
- Color
- Opaque black to brownish-black.
- Luster
- Vitreous to resinous.
- Texture
- Typically forms prismatic crystals, often with distinct vertical striations on crystal faces.
- Crystal Form
- Elongated, often columnar or acicular crystals with a characteristic triangular cross-section (though often appearing hexagonal due to truncation). Terminations can be complex, often hemimorphic.
- Cleavage
- Poor to indistinct, typically exhibiting conchoidal to uneven fracture rather than true cleavage.
- Geological Environment
- Granitic pegmatites, high-temperature hydrothermal veins, greisens, and metamorphic rocks (schists, gneisses).
Key Facts
- Hardness: 7-7.5 (Mohs scale)
- Specific Gravity: 3.06-3.26 g/cm3
- Crystal System: Trigonal
- Color: Opaque black, sometimes with a brownish or bluish tint
- Luster: Vitreous to resinous
- Transparency: Opaque
- Fracture: Conchoidal to uneven
- Cleavage: Poor to indistinct (basal)
- Composition: Complex borosilicate of aluminum, iron, and other elements
Quick Check
- Color: Opaque black
- Luster: Vitreous to resinous
- Streak: White to colorless
Physical Characteristics
- Crystal Habit: Prismatic, columnar, acicular, often with vertical striations; massive, granular.
- Cleavage Type: Poor to indistinct on {1011} and {0001}.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous to resinous.
Formation
Schorl forms primarily in granitic pegmatites, high-temperature hydrothermal veins, and metamorphic rocks such as schists and gneisses. It crystallizes from boron-rich fluids during the final stages of magmatic differentiation or during regional and contact metamorphism.
Usage
Historically, schorl has been used as a gemstone (though less valued than other tourmaline varieties due to its opacity), in pressure gauges due to its piezoelectric properties, and as a source of boron. Today, its primary use is as a collector's mineral and in some metaphysical practices. It is also a significant indicator mineral for certain geological environments.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, often associated with late-stage magmatic and metamorphic processes.
Where to Find
Brazil
Minas Gerais is a world-renowned source for large, well-formed schorl crystals, often found in pegmatite deposits.
Pakistan
Northern Areas, particularly Gilgit-Baltistan, produce excellent quality schorl crystals, sometimes associated with other tourmaline varieties.
United States
Maine (e.g., Mount Mica, Newry) and California (e.g., Pala District) are known for schorl, often in pegmatite occurrences.
Afghanistan
Nuristan and Kunar provinces yield significant schorl specimens, often from pegmatite bodies.
Namibia
Erongo Region is famous for its schorl specimens, sometimes found in association with quartz and feldspar.
Finding Tips
Look for Pegmatites
Schorl is most commonly found in granitic pegmatites. These are coarse-grained igneous rocks, often found as dikes or sills cutting through other igneous or metamorphic rocks. Look for areas with exposed pegmatite bodies.
Identify Associated Minerals
Schorl often occurs with quartz, feldspar (orthoclase, albite), muscovite, lepidolite, and sometimes beryl or garnet. The presence of these minerals can indicate a favorable environment for schorl.
Check for Striations
The characteristic vertical striations on the prismatic faces of tourmaline crystals are a key identification feature. Even in broken fragments, these striations can often be observed.
Test Hardness
With a Mohs hardness of 7-7.5, schorl will scratch glass and steel. This helps differentiate it from softer black minerals like biotite.
Observe Crystal Habit
Schorl typically forms elongated, columnar, or acicular crystals. Look for these distinct crystal shapes in outcrops or in weathered material.
Similar Rocks
Hornblende
Hornblende
Also known as: Amphibole
Augite
Augite
Also known as: Pyroxene
Biotite
Biotite
Also known as: Black Mica
Scientific Classification
- Mineral Class
- Silicate (Cyclosilicate)
- Group
- Tourmaline Group
- Crystal System
- Trigonal
- Chemical Formula
- NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
- Composition
- Sodium iron aluminum borosilicate hydroxide
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