How to identify Tourmaline Granite

To correctly identify Tourmaline Granite (Granite (igneous rock) with Schorl (mineral)), 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

   Typically light-colored (white, gray, pink, red) with distinct black, prismatic tourmaline crystals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Granite matrix is vitreous to dull; tourmaline crystals are vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained), equigranular to porphyritic. Tourmaline crystals are often well-formed and conspicuous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Granite minerals (quartz, feldspar, mica) are anhedral to subhedral. Tourmaline (schorl) typically forms euhedral to subhedral prismatic crystals, often with characteristic triangular cross-sections and striations parallel to the c-axis.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Feldspars exhibit good cleavage (two directions at ~90 degrees); micas have perfect basal cleavage. Quartz has no cleavage. Tourmaline has indistinct to poor cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Intrusive igneous bodies (batholiths, stocks, dikes), often associated with continental collision zones, subduction zones, and areas of crustal thickening. Commonly found in the margins of granitic intrusions or in associated pegmatites and hydrothermal veins.

Key Facts

Hardness

Granite: 6-7 (Mohs); Schorl (Tourmaline): 7-7.5 (Mohs)

Specific Gravity

Granite: 2.6-2.7 g/cm³; Schorl (Tourmaline): 3.0-3.2 g/cm³

Crystal System

Granite (polycrystalline aggregate); Schorl (Trigonal)

Color

Granite matrix: white, gray, pink, red; Schorl: typically black, sometimes dark blue or brown.

Luster

Granite: vitreous to dull; Schorl: vitreous.

Transparency

Granite: opaque; Schorl: opaque to translucent (thin edges).

Fracture

Granite: uneven to conchoidal; Schorl: conchoidal to uneven.

Cleavage

Granite: Feldspars have good cleavage, micas perfect basal cleavage, quartz no cleavage; Schorl: indistinct to poor.

Composition

Granite: primarily quartz (SiO₂), feldspar (KAlSi₃O₈, NaAlSi₃O₈, CaAl₂Si₂O₈), mica (K(Mg,Fe)₃AlSi₃O₁₀(OH)₂, KAl₂(AlSi₃O₁₀)(OH)₂); Schorl: Na(Fe₃)₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH).

Physical characteristics

• Crystal Habit: Granite: granular aggregate; Schorl: prismatic, often columnar, acicular, or massive, with characteristic triangular cross-section and vertical striations.
• Cleavage Type: Granite: Feldspars (2 directions at ~90°), Micas (1 perfect basal); Schorl: Indistinct to poor.
• Fracture Type: Granite: Uneven to conchoidal; Schorl: Conchoidal to uneven.
• Tenacity: Granite: Brittle; Schorl: Brittle.
• Luster Type: Granite: Vitreous to dull; Schorl: Vitreous.

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