How to identify Tourmaline Granite

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

   Overall light-colored (white, gray, pink, red) with distinct black, prismatic crystals of tourmaline. The color of the granite matrix depends on the feldspar composition.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Granite minerals exhibit vitreous (quartz), pearly to vitreous (feldspar), and submetallic to vitreous (biotite) lusters. Tourmaline (schorl) has a vitreous to resinous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained) to very coarse-grained. Individual mineral grains are visible to the naked eye. Tourmaline crystals are typically prismatic and often well-formed.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz: anhedral to subhedral, irregular grains. Feldspar: subhedral to euhedral, blocky crystals. Mica: flaky, platy crystals. Tourmaline (schorl): euhedral to subhedral, elongated prismatic crystals with characteristic triangular cross-sections and vertical striations.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Feldspar exhibits two distinct cleavages at or near 90 degrees. Mica has perfect basal cleavage. Quartz has no cleavage. Tourmaline has poor to indistinct cleavage, often appearing to have none.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Intrusive igneous bodies (plutons, batholiths, stocks) within continental crust, often associated with orogenic belts, subduction zones, and areas of crustal thickening. Tourmaline-rich granites are frequently found in the margins of granitic intrusions, in aplite-pegmatite dikes, or in areas affected by boron-rich hydrothermal fluids.

Key Facts

Hardness

Granite (overall): 6-7 on Mohs scale. Schorl (tourmaline): 7-7.5 on Mohs scale. Quartz: 7. Feldspar: 6-6.5. Mica: 2-4.

Specific Gravity

Granite (overall): 2.6-2.7 g/cm³. Schorl (tourmaline): 3.0-3.2 g/cm³.

Crystal System

Granite (rock): Polycrystalline aggregate. Schorl (mineral): Trigonal.

Color

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

Luster

Schorl: Vitreous to resinous. Quartz: Vitreous. Feldspar: Pearly to vitreous. Mica: Vitreous to silky.

Transparency

Schorl: Opaque to translucent in thin splinters. Quartz: Transparent to translucent. Feldspar: Translucent to opaque. Mica: Transparent to translucent.

Fracture

Granite (overall): Irregular to conchoidal. Schorl: Uneven to conchoidal. Quartz: Conchoidal. Feldspar: Uneven to conchoidal.

Cleavage

Schorl: Poor to indistinct (rhombohedral). Quartz: None. Feldspar: Two directions at ~90°. Mica: Perfect basal.

Composition

Granite: Predominantly quartz (20-60%), alkali feldspar (35-90% of total feldspar), plagioclase feldspar (10-65% of total feldspar), and micas (biotite, muscovite). Schorl: Complex borosilicate of aluminum, iron, and other elements, specifically Na(Fe²⁺)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄.

Physical characteristics

• Crystal Habit: Granite: Granular, interlocking crystals. Schorl: Prismatic, often elongated, with characteristic vertical striations and a pseudo-hexagonal or triangular cross-section.
• Cleavage Type: Schorl: Poor/indistinct. Feldspar: Good in two directions. Mica: Perfect in one direction.
• Fracture Type: Schorl: Uneven to conchoidal. Quartz: Conchoidal. Feldspar: Uneven to conchoidal.
• Tenacity: Schorl: Brittle. Quartz: Brittle. Feldspar: Brittle. Mica: Flexible and elastic.
• Luster Type: Schorl: Vitreous to resinous.

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