Schorl

Description

Schorl is the most common member of the tourmaline group, characterized by its opaque black color and iron-rich composition. It typically forms elongated, prismatic crystals with characteristic triangular cross-sections and vertical striations on its crystal faces. It is a complex borosilicate mineral, often found in association with quartz, feldspar, and mica in granitic environments. Its robust nature and resistance to weathering make it a common accessory mineral in sedimentary rocks derived from tourmaline-bearing source rocks.

How to Identify

Color

Opaque black to very dark brown. Rarely, it can appear dark blue or green in thin sections.

Luster

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

Texture

Typically forms euhedral to subhedral prismatic crystals. Can also be massive or acicular.

Crystal Form

Elongated, prismatic crystals with a characteristic rounded triangular cross-section. Often exhibits prominent vertical striations parallel to the c-axis. Terminations can be complex, often hemimorphic.

Cleavage

Poor to indistinct basal cleavage {0001}, and poor prismatic cleavage {1120}. Fracture is more common than cleavage.

Geological Environment

Granitic pegmatites, high-temperature hydrothermal veins, greisens, and metamorphic rocks (schists, gneisses). Also found as an accessory mineral in some granites and aplites.

Key Facts

• Hardness: 7-7.5 (Mohs scale)
• Specific Gravity: 3.18-3.22 g/cm
• Crystal System: Trigonal
• Color: Opaque black, sometimes very dark brown or bluish-black
• Luster: Vitreous to resinous, sometimes sub-metallic
• Transparency: Opaque
• Fracture: Conchoidal to uneven
• Cleavage: Poor to indistinct basal {0001} and prismatic {1120}
• Composition: Sodium Iron Aluminum Borosilicate Hydroxide

Quick Check

• Color: Opaque black
• Luster: Vitreous to resinous
• Streak: White to colorless

Physical Characteristics

• Crystal Habit: Prismatic, columnar, acicular, massive. Crystals often show a characteristic rounded triangular cross-section and vertical striations.
• Cleavage Type: Poor to indistinct. Fracture is more prevalent.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to resinous, sometimes sub-metallic.

Formation

Schorl forms primarily in granitic pegmatites, high-temperature hydrothermal veins, and metamorphic rocks such as schists and gneisses. It crystallizes from boron-rich, silica-saturated fluids, often in the presence of iron. The formation temperature typically ranges from 300C to 700C, and pressures can vary widely depending on the geological setting.

Usage

Historically, schorl was used as a mourning stone and for its pyroelectric and piezoelectric properties. Today, it is primarily a collector's mineral, and due to its abundance, it is not typically used as a gemstone, though well-formed crystals are highly prized. It has minor industrial applications in pressure gauges and as a source of boron in some specialized ceramics, though other boron minerals are more commonly used.

Age Distribution

Precambrian to Cenozoic, commonly associated with granitic intrusions and metamorphic events.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate (Cyclosilicate)

Group

Tourmaline Group

Crystal System

Trigonal

Chemical Formula

Na(Fe2+3)Al6(BO3)3Si6O18(OH)4

Composition

Sodium Iron Aluminum Borosilicate Hydroxide. The iron (Fe2+) is the dominant cation in the Y-site, giving it its characteristic black color.

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