Black Tourmaline

Description

Schorl is the most common member of the tourmaline group, characterized by its opaque black color. It is an iron-rich borosilicate mineral with a complex and variable chemical composition. It typically forms elongated, prismatic crystals with characteristic vertical striations on its crystal faces. Schorl is known for its pyroelectric and piezoelectric properties, meaning it can generate an electrical charge when heated or subjected to mechanical stress, respectively. It is a robust and chemically resistant mineral, often found in association with quartz, feldspar, and mica.

How to Identify

Color

Opaque black, sometimes with a very dark blue or brown tint.

Luster

Vitreous to resinous, sometimes sub-metallic on fresh fracture surfaces.

Texture

Typically forms elongated, prismatic crystals. Can also be massive or granular.

Crystal Form

Hexagonal (trigonal) system, often forming long, slender to stout prismatic crystals with characteristic vertical striations parallel to the c-axis. Cross-sections are typically triangular with rounded edges.

Cleavage

Poor to indistinct, exhibiting a conchoidal to uneven fracture rather than true cleavage.

Geological Environment

Granitic pegmatites, granites, aplites, high-temperature hydrothermal veins, and metamorphic rocks like schists and 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 very dark blue or brown tint
• Luster: Vitreous to resinous
• Transparency: Opaque
• Fracture: Conchoidal to uneven
• Cleavage: Poor to indistinct (no true cleavage)
• 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, massive, granular. Crystals are typically elongated with vertical striations.
• Cleavage Type: None to very poor, parting may be observed along rhombohedral planes.
• Fracture Type: Conchoidal to uneven, brittle.
• Tenacity: Brittle
• Luster Type: Vitreous to resinous, sometimes sub-metallic.

Formation

Schorl forms primarily in igneous and metamorphic environments. It is a common accessory mineral in granitic pegmatites, granites, and aplites. It also forms in high-temperature hydrothermal veins and in metamorphic rocks such as schists and gneisses, particularly those derived from boron-rich sediments. Its formation is often associated with the presence of boron-rich fluids.

Usage

Historically, schorl has been used as a gemstone, though its opaque nature limits its use compared to other tourmaline varieties. It is sometimes used in jewelry as cabochons or beads. Industrially, tourmaline minerals, including schorl, have been investigated for their pyroelectric and piezoelectric properties, though these applications are not widespread for schorl itself. It is also collected by mineral enthusiasts.

Age Distribution

Found in rocks of various ages, from Precambrian to Cenozoic, depending on the geological setting.

Where to Find

Finding Tips

Similar Rocks

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. The iron (Fe) content is dominant, giving it its characteristic black color.

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