How to identify Tourmalinated Quartz

To correctly identify Tourmalinated Quartz (Quartz (SiO2) with Schorl (Na(Fe2+)_3Al_6(BO_3)_3Si_6O_18(OH)_4)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Quartz is typically colorless, white, or smoky gray. Schorl is opaque black. The combination results in clear to smoky quartz with black needle-like inclusions.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Quartz exhibits a vitreous (glassy) luster. Schorl has a vitreous to resinous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to conchoidal fracture in quartz; tourmaline inclusions are typically smooth and prismatic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz forms hexagonal prisms with pyramidal terminations. Schorl forms elongated, prismatic crystals, often with characteristic striations parallel to the c-axis, embedded within the quartz.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz has no true cleavage, exhibiting conchoidal fracture. Schorl has indistinct cleavage parallel to the prism faces.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in pegmatites, granitic intrusions, and hydrothermal veins, often associated with other pegmatitic minerals like feldspar, mica, and other tourmaline varieties.

Key Facts

Hardness

Quartz: 7 on Mohs scale. Schorl: 7-7.5 on Mohs scale. The aggregate will scratch glass.

Specific Gravity

Quartz: 2.65 g/cm³. Schorl: 3.06-3.26 g/cm³. The aggregate will have an intermediate specific gravity depending on the proportion of each mineral.

Crystal System

Quartz: Trigonal. Schorl: Trigonal.

Color

Colorless, white, or smoky gray quartz with opaque black schorl inclusions.

Luster

Vitreous (glassy).

Transparency

Quartz is transparent to translucent. Schorl is opaque.

Fracture

Conchoidal in quartz. Uneven to conchoidal in schorl.

Cleavage

Quartz: None. Schorl: Indistinct.

Composition

Silicon dioxide (SiO2) for quartz, and a complex borosilicate of sodium, iron, and aluminum (Na(Fe2+)_3Al_6(BO_3)_3Si_6O_18(OH)_4) for schorl.

Physical characteristics

• Crystal Habit: Quartz: Prismatic, often terminated. Schorl: Acicular, prismatic, often embedded.
• Cleavage Type: Quartz: Absent. Schorl: Indistinct on {1011} and {1120}.
• Fracture Type: Conchoidal in quartz. Uneven to conchoidal in schorl.
• Tenacity: Brittle.
• Luster Type: Vitreous.

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