Black Tourmaline in Mica Schist

Metamorphic Rock with Mineral Inclusion

Schorl (Tourmaline Group) in Mica Schist

Also known as: Schorl in Mica Schist

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Description

Black Tourmaline in Mica Schist refers to the occurrence of the mineral schorl (black tourmaline) as distinct crystals or aggregates within a mica schist matrix. The schist is a medium- to high-grade metamorphic rock characterized by a strong foliation (schistosity) due to the parallel alignment of platy mica minerals. The black tourmaline crystals, often prismatic and euhedral to subhedral, stand out against the silvery or dark background of the mica schist, providing a striking textural and mineralogical contrast.

How to Identify

Color: The schist matrix is typically silvery-gray, dark gray, or brownish due to muscovite and/or biotite. The tourmaline crystals are distinctly black.
Luster: Mica minerals exhibit a pearly to vitreous luster. Schorl has a vitreous to resinous luster.
Texture: Foliated (schistose) with visible platy mica minerals. Tourmaline crystals are often prismatic and may be porphyroblastic (larger than the surrounding matrix minerals).
Crystal Form: Schorl typically forms elongated, prismatic crystals with characteristic triangular cross-sections and vertical striations on prism faces. Mica forms platy, tabular crystals.
Cleavage: Mica minerals exhibit perfect basal cleavage (one direction). Schorl has poor to indistinct cleavage, but often shows conchoidal to uneven fracture.
Geological Environment: Regional metamorphic terrains, particularly in areas affected by medium to high-grade metamorphism of pelitic or psammitic protoliths, often associated with granitic intrusions or pegmatites that can be sources of boron.

Key Facts

Hardness: Schorl: 7-7.5 (Mohs); Mica: 2-3 (Mohs)
Specific Gravity: Schorl: 3.0-3.2; Mica: 2.7-3.1
Crystal System: Schorl: Trigonal; Mica: Monoclinic
Color: Schorl: Black; Mica: Silvery, gray, brown, black
Luster: Schorl: Vitreous to resinous; Mica: Pearly to vitreous
Transparency: Schorl: Opaque to translucent; Mica: Transparent to translucent
Fracture: Schorl: Conchoidal to uneven; Mica: Uneven
Cleavage: Schorl: Poor to indistinct; Mica: Perfect basal (one direction)
Composition: Schorl: NaFe3Al6(BO3)3Si6O18(OH)4 (Sodium Iron Aluminum Boro-silicate Hydroxide); Mica (e.g., Muscovite): KAl2(AlSi3O10)(OH)2 (Potassium Aluminum Silicate Hydroxide)

Quick Check

Color: Black crystals in a silvery/dark gray foliated matrix
Luster: Vitreous to resinous (tourmaline), pearly to vitreous (mica)
Streak: White (for schorl, though rarely tested in situ)

Physical Characteristics

Crystal Habit: Schorl: Prismatic, often with triangular cross-section and vertical striations. Mica: Platy, tabular.
Cleavage Type: Schorl: Poor/indistinct; Mica: Perfect basal (001)
Fracture Type: Schorl: Conchoidal to uneven; Mica: Uneven
Tenacity: Schorl: Brittle; Mica: Flexible and elastic
Luster Type: Schorl: Vitreous to resinous; Mica: Pearly to vitreous

Formation

This rock forms during regional metamorphism of argillaceous (clay-rich) or psammitic (sand-rich) sedimentary rocks, or felsic igneous rocks. The presence of boron, often introduced by metasomatic fluids during metamorphism, is crucial for tourmaline formation. Schorl crystallizes within the schistosity planes or as porphyroblasts during the growth of mica minerals (muscovite, biotite) under medium to high-grade metamorphic conditions (e.g., greenschist to amphibolite facies).

Usage

Primarily of interest to mineral collectors, geologists for studying metamorphic processes, and occasionally used in lapidary arts for its aesthetic contrast. Schorl itself has minor industrial uses as a source of boron, and historically as a piezoelectric material, though not typically when embedded in schist.

Age Distribution

Precambrian to Cenozoic, depending on the age of the metamorphic event and protolith.

Where to Find

Brazil

Minas Gerais and other regions are famous for various tourmaline occurrences, including schorl in metamorphic rocks.

United States

Maine, California, and other Appalachian regions have metamorphic rocks with tourmaline.

Africa

Various countries, including Namibia and Madagascar, are known for tourmaline deposits in metamorphic and pegmatitic environments.

Europe

Alps (e.g., Switzerland, Austria), and other metamorphic belts.

Finding Tips

Look for Metamorphic Terrains

Focus your search in regions known for regional metamorphism, especially those with pelitic (shale-derived) or psammitic (sandstone-derived) schists.

Identify Schistosity

Recognize the characteristic foliation (schistosity) of the host rock, which is indicative of metamorphic processes.

Search for Porphyroblasts

Look for larger, often black, prismatic crystals that stand out from the finer-grained mica matrix. These are likely schorl porphyroblasts.

Check for Boron-Rich Environments

While not always obvious, the presence of nearby granitic intrusions or pegmatites can indicate a source of boron, which is essential for tourmaline formation.

Similar Rocks

Garnet Mica Schist

Garnet-bearing Mica Schist

Also known as: Garnet Schist

Staurolite Mica Schist

Staurolite-bearing Mica Schist

Also known as: Staurolite Schist

Kyanite Mica Schist

Kyanite-bearing Mica Schist

Also known as: Kyanite Schist

Scientific Classification

Mineral Class: Silicates
Group: Tourmaline Group (Schorl) and Mica Group (Muscovite, Biotite)
Crystal System: Trigonal (Schorl), Monoclinic (Mica)
Chemical Formula: Schorl: NaFe3Al6(BO3)3Si6O18(OH)4; Mica (e.g., Muscovite): KAl2(AlSi3O10)(OH)2
Composition: Complex boro-silicate (Schorl) and phyllosilicate (Mica) minerals, with varying amounts of iron, magnesium, aluminum, sodium, potassium, and hydroxyl.

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