Tourmalinated Quartz, Schorl in Quartz

Description

Tourmaline in Quartz is a striking mineral aggregate characterized by black, often needle-like or prismatic crystals of schorl (black tourmaline) embedded within a translucent to opaque quartz matrix. The contrast between the dark tourmaline and the lighter quartz creates a distinctive appearance. The tourmaline inclusions can range from sparse, delicate needles to dense, radiating clusters. The quartz itself can be clear, milky, or smoky.

How to Identify

Color

Quartz matrix is typically colorless, white, or smoky gray. Schorl inclusions are opaque black.

Luster

Quartz is vitreous (glassy). Schorl is vitreous to resinous.

Texture

Smooth to slightly rough, depending on the polish and the prominence of tourmaline crystals.

Crystal Form

Quartz typically forms hexagonal prisms with pyramidal terminations, though in massive form it can be anhedral. Schorl forms prismatic, often striated crystals, which can be acicular (needle-like) or columnar within the quartz.

Cleavage

Quartz has no true cleavage, exhibiting conchoidal fracture. Schorl has very poor to indistinct cleavage parallel to the c-axis.

Geological Environment

Found in granitic pegmatites, hydrothermal veins, and regionally metamorphosed rocks. It is often associated with other pegmatitic minerals like feldspar and mica.

Key Facts

• Hardness: Quartz: 7 (Mohs); Schorl: 7-7.5 (Mohs). The aggregate will scratch steel.
• Specific Gravity: Quartz: 2.65 g/cm³; Schorl: 3.06-3.26 g/cm³. The aggregate will have an intermediate specific gravity.
• Crystal System: Quartz: Trigonal; Schorl: Trigonal.
• Color: Colorless, white, or smoky quartz with opaque black schorl inclusions.
• Luster: Vitreous (glassy).
• Transparency: Quartz: Transparent to translucent; Schorl: Opaque.
• Fracture: Conchoidal (for quartz); Uneven to conchoidal (for schorl).
• Cleavage: None (for quartz); Very poor to indistinct (for schorl).
• Composition: Quartz: SiO₂ (Silicon Dioxide); Schorl: NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄ (Sodium Iron Aluminum Borosilicate Hydroxide).

Quick Check

• Color: Clear, white, or smoky quartz with black inclusions.
• Luster: Vitreous (glassy).
• Streak: White (for quartz), black (for schorl, though typically not powdered for streak test).

Physical Characteristics

• Crystal Habit: Quartz: Massive, granular, or prismatic crystals. Schorl: Acicular, prismatic, columnar, or radiating aggregates within the quartz.
• Cleavage Type: Quartz: None; Schorl: Very poor to indistinct on {1011} and {1120}.
• Fracture Type: Quartz: Conchoidal; Schorl: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy).

Formation

Tourmaline in Quartz forms when schorl (black tourmaline) crystals grow within a quartz matrix. This typically occurs in pegmatites, hydrothermal veins, or metamorphic rocks. The tourmaline crystals are often acicular (needle-like) or prismatic and can be randomly oriented or aligned. The formation process involves the crystallization of both minerals from silica-rich, boron-bearing fluids at elevated temperatures and pressures. The presence of boron is crucial for tourmaline formation.

Usage

Primarily used as an ornamental stone, for carvings, and in jewelry (cabochons, beads). It is also collected by mineral enthusiasts. Historically, tourmaline has been used for its pyroelectric and piezoelectric properties, though schorl in quartz is less commonly used for these industrial applications due to the mixed composition.

Age Distribution

Precambrian to Cenozoic, depending on the host rock and geological setting.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicates (Quartz: Tectosilicate; Tourmaline: Cyclosilicate)

Group

Quartz Group (for Quartz); Tourmaline Group (for Schorl)

Crystal System

Trigonal

Chemical Formula

SiO₂ (Quartz) + NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄ (Schorl)

Composition

Silicon dioxide with complex sodium iron aluminum borosilicate hydroxide inclusions.

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