Feldspar with Tourmaline

Igneous (typically pegmatitic)

Potassium Feldspar (Orthoclase/Microcline) with Schorl (Black Tourmaline)

Also known as: Potassium Feldspar with Schorl, Graphic Granite (if intergrowth is present)

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Description

This describes an aggregate of two distinct minerals: Potassium Feldspar (either Orthoclase or Microcline) and Schorl (black tourmaline). Potassium feldspar typically forms large, blocky, often euhedral to subhedral crystals, ranging in color from white, pink, salmon-orange, to reddish-brown. Schorl appears as black, prismatic, often striated crystals, frequently embedded within or growing alongside the feldspar. The texture can vary from individual, well-formed crystals of both minerals to intergrowths, such as graphic granite where quartz and feldspar are intergrown, with tourmaline present as an accessory.

How to Identify

Color: Potassium Feldspar: White, pink, salmon-orange, reddish-brown. Schorl: Opaque black.
Luster: Potassium Feldspar: Vitreous to pearly. Schorl: Vitreous to resinous.
Texture: Coarse-grained, often with large, distinct crystals of both minerals. Can exhibit graphic intergrowths.
Crystal Form: Potassium Feldspar: Prismatic, tabular, or blocky crystals, often twinned (Carlsbad twinning common). Schorl: Prismatic crystals, typically with a triangular cross-section and vertical striations.
Cleavage: Potassium Feldspar: Two distinct cleavages at or near 90 degrees. Schorl: Poor to indistinct cleavage, often exhibiting conchoidal fracture.
Geological Environment: Granitic pegmatites, often found in the margins of granitic intrusions or within metamorphic terrains.

Key Facts

Hardness: Potassium Feldspar: 6-6.5 on Mohs scale. Schorl: 7-7.5 on Mohs scale.
Specific Gravity: Potassium Feldspar: 2.55-2.63. Schorl: 3.06-3.26.
Crystal System: Potassium Feldspar: Monoclinic (Orthoclase) or Triclinic (Microcline). Schorl: Trigonal.
Color: Potassium Feldspar: White, pink, salmon-orange, reddish-brown. Schorl: Opaque black.
Luster: Potassium Feldspar: Vitreous to pearly. Schorl: Vitreous to resinous.
Transparency: Potassium Feldspar: Translucent to opaque. Schorl: Opaque.
Fracture: Potassium Feldspar: Uneven to conchoidal. Schorl: Conchoidal to uneven.
Cleavage: Potassium Feldspar: Two perfect cleavages at 90 degrees (Orthoclase) or near 90 degrees (Microcline). Schorl: Poor to indistinct.
Composition: Potassium Feldspar: KAlSi3O8. Schorl: Na(Fe2+3)Al6(BO3)3Si6O18(OH)4.

Quick Check

Color: Pink, white, or salmon (feldspar) with black (tourmaline)
Luster: Vitreous to pearly (feldspar) and vitreous to resinous (tourmaline)
Streak: White (feldspar) and colorless (tourmaline)

Physical Characteristics

Crystal Habit: Potassium Feldspar: Prismatic, tabular, blocky, often twinned. Schorl: Prismatic, columnar, acicular, often with vertical striations and a characteristic triangular cross-section.
Cleavage Type: Potassium Feldspar: Perfect in two directions. Schorl: Poor/indistinct.
Fracture Type: Potassium Feldspar: Uneven to conchoidal. Schorl: Conchoidal to uneven.
Tenacity: Potassium Feldspar: Brittle. Schorl: Brittle.
Luster Type: Potassium Feldspar: Vitreous to pearly. Schorl: Vitreous to resinous.

Formation

This association typically forms in granitic pegmatites, which are coarse-grained igneous rocks that crystallize from residual magmatic fluids enriched in incompatible elements and volatiles (like water, fluorine, and boron). The slow cooling of these fluids allows for the growth of large crystals. Potassium feldspar (Orthoclase or Microcline) is a primary constituent, while schorl (black tourmaline) crystallizes from the boron-rich volatile phase.

Usage

While not a distinct rock type with specific industrial uses beyond its individual components, specimens are highly valued by mineral collectors and for educational purposes. Feldspar is used in ceramics, glass, and as a filler. Tourmaline, particularly schorl, has minor uses in pressure gauges (due to its piezoelectric properties) and as a gemstone (though black schorl is less valued than other tourmaline varieties).

Age Distribution

Precambrian to Cenozoic, common in orogenic belts and stable cratons.

Where to Find

Brazil

Minas Gerais is famous for its pegmatites, yielding large crystals of feldspar and tourmaline.

United States

Maine (e.g., Mount Mica, Newry), California (e.g., Pala District), and North Carolina are known for pegmatite occurrences with tourmaline and feldspar.

Madagascar

Numerous pegmatite fields produce significant quantities of both minerals.

Afghanistan/Pakistan

Himalayan pegmatites are renowned for their mineral diversity, including large tourmaline and feldspar crystals.

Russia

Ural Mountains and other regions have pegmatite deposits.

Finding Tips

Look for Pegmatites

Focus your search on areas known for granitic pegmatites. These are often found near larger granite bodies or in metamorphic terranes.

Identify Key Minerals

Learn to recognize the distinct characteristics of potassium feldspar (color, cleavage, blocky habit) and schorl (black color, prismatic habit, striations, triangular cross-section).

Check for Intergrowths

Sometimes, tourmaline can be intimately intergrown with feldspar, or even with quartz in graphic granite textures. Look for these characteristic patterns.

Safety Precautions

When collecting in the field, always wear appropriate safety gear, including eye protection and gloves. Be aware of your surroundings and potential hazards in mining or quarry environments. No specific chemical hazards are associated with handling these minerals, but dust from crushing or cutting should be avoided by wearing a mask due to potential silica content.

Similar Rocks

Granite

Also known as: Granitic rock

Pegmatite

Also known as: Granitic Pegmatite

Aplite

Also known as: Aplitic rock

Scientific Classification

Mineral Class: Potassium Feldspar: Tectosilicate. Schorl: Cyclosilicate (Borosilicate).
Group: Potassium Feldspar: Feldspar Group. Schorl: Tourmaline Group.
Crystal System: Potassium Feldspar: Monoclinic (Orthoclase) or Triclinic (Microcline). Schorl: Trigonal.
Chemical Formula: Potassium Feldspar: KAlSi3O8. Schorl: Na(Fe2+3)Al6(BO3)3Si6O18(OH)4.
Composition: Potassium Feldspar: Potassium aluminum silicate. Schorl: Sodium iron aluminum borosilicate hydroxide.

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