Tourmaline Granite

Igneous Rock (Plutonic)

Granite (igneous rock) with Schorl (black tourmaline)

Also known as: Schorl Granite

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Description

Granite with tourmaline is a coarse-grained, felsic intrusive igneous rock characterized by its essential minerals: quartz, feldspar (orthoclase and/or plagioclase), and mica (biotite and/or muscovite), with the distinctive addition of black tourmaline (schorl). The tourmaline typically occurs as euhedral to subhedral prismatic crystals, often with characteristic striations parallel to the c-axis, and can range in size from a few millimeters to several centimeters. The overall color of the granite is typically light-colored (pink, red, gray, white) due to the dominant feldspar and quartz, with the black tourmaline providing a striking contrast.

How to Identify

Color: 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.
Luster: Granite minerals exhibit vitreous (quartz), pearly to vitreous (feldspar), and submetallic to vitreous (biotite) lusters. Tourmaline (schorl) has a vitreous to resinous luster.
Texture: 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.
Crystal Form: 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.
Cleavage: 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.
Geological Environment: 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.

Quick Check

Color: Light-colored matrix (white, gray, pink) with distinct black, prismatic crystals.
Luster: Vitreous to resinous for tourmaline; vitreous for quartz; pearly to vitreous for feldspar.
Streak: White (for quartz and feldspar); colorless (for tourmaline, though it's too hard to streak easily).

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.

Formation

Granite with tourmaline forms from the slow crystallization of felsic magma deep within the Earth's crust. The presence of tourmaline, specifically schorl, indicates a boron-rich magmatic source, often associated with late-stage magmatic differentiation, metasomatism, or interaction with boron-rich fluids, particularly in pegmatitic phases or hydrothermal alteration zones within granitic intrusions.

Usage

Primarily used as a dimension stone for construction (countertops, flooring, building facades), monuments, and decorative aggregates. The presence of prominent tourmaline crystals can enhance its aesthetic appeal for ornamental purposes. Historically, granite has been used for tools and grinding stones.

Age Distribution

Found in geological formations ranging from Precambrian to Cenozoic, typically associated with orogenic events and continental crustal thickening.

Where to Find

Cornwall, England

Famous for its tourmaline-rich granites and associated tin-tungsten mineralization, particularly the Carnmenellis and Bodmin Moor granites.

Brazil

Various states, including Minas Gerais, are renowned for pegmatites and granites containing abundant tourmaline, often of gem quality, but also schorl.

United States

Localities in Maine (e.g., Mount Mica), California (e.g., Pala District), and Colorado are known for tourmaline-bearing granites and pegmatites.

Africa

Countries like Namibia and Madagascar have significant occurrences of tourmaline-bearing granites and pegmatites.

Australia

Western Australia and other regions host granitic intrusions with tourmaline, often associated with mineral deposits.

Finding Tips

Look for Plutonic Environments

Focus on areas known for large intrusive igneous bodies, such as batholiths or stocks, especially in ancient mountain belts or continental shields.

Identify Boron-Rich Zones

Tourmaline is a boron-silicate. Look for granites associated with pegmatites, aplites, or areas showing signs of hydrothermal alteration, as these environments are often enriched in boron.

Recognize Schorl's Characteristics

Schorl (black tourmaline) is easily identifiable by its black color, prismatic habit, often with vertical striations, and triangular cross-section. It stands out against the lighter-colored granite matrix.

Examine Outcrops and Road Cuts

Freshly exposed rock surfaces in quarries, road cuts, and stream beds often provide the best views of mineral textures and compositions.

Use a Hand Lens

A hand lens (10x magnification) will help in identifying the individual mineral grains, their crystal forms, and the characteristic striations on tourmaline.

Similar Rocks

Standard Granite

Granite

Also known as: Granite

Pegmatite

Also known as: Granitic Pegmatite

Granodiorite

Also known as: Quartz Diorite

Scientific Classification

Mineral Class: Granite (rock): Igneous rock. Schorl (mineral): Cyclosilicate (ring silicate).
Group: Granite (rock): Felsic intrusive igneous rock. Schorl (mineral): Tourmaline Group.
Crystal System: Granite (rock): Polycrystalline. Schorl (mineral): Trigonal.
Chemical Formula: Granite (rock): Variable, primarily SiO₂, Al₂O₃, K₂O, Na₂O. Schorl (mineral): Na(Fe²⁺)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
Composition: Granite: Quartz, alkali feldspar, plagioclase, micas, and accessory minerals including schorl. Schorl: Sodium iron aluminum borosilicate hydroxide.

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