How to identify Black Tourmaline in Mica Schist

To correctly identify Black Tourmaline in Mica Schist (Schorl (Tourmaline Group) in Mica Schist), 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

   The schist matrix is typically silvery-gray, dark gray, or brownish due to muscovite and/or biotite. The tourmaline crystals are distinctly black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Mica minerals exhibit a pearly to vitreous luster. Schorl has a vitreous to resinous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Foliated (schistose) with visible platy mica minerals. Tourmaline crystals are often prismatic and may be porphyroblastic (larger than the surrounding matrix minerals).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Schorl typically forms elongated, prismatic crystals with characteristic triangular cross-sections and vertical striations on prism faces. Mica forms platy, tabular crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Mica minerals exhibit perfect basal cleavage (one direction). Schorl has poor to indistinct cleavage, but often shows conchoidal to uneven fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)

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

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