How to identify Schorl

To correctly identify Schorl (Na(Fe2+3)Al6(BO3)3Si6O18(OH)4), 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

   Opaque black to very dark brown. Rarely, it can appear dark blue or green in thin sections.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to resinous, sometimes sub-metallic on fresh fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically forms euhedral to subhedral prismatic crystals. Can also be massive or acicular.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Elongated, prismatic crystals with a characteristic rounded triangular cross-section. Often exhibits prominent vertical striations parallel to the c-axis. Terminations can be complex, often hemimorphic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Poor to indistinct basal cleavage {0001}, and poor prismatic cleavage {1120}. Fracture is more common than cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Granitic pegmatites, high-temperature hydrothermal veins, greisens, and metamorphic rocks (schists, gneisses). Also found as an accessory mineral in some granites and aplites.

Key Facts

Hardness

7-7.5 (Mohs scale)

Specific Gravity

3.18-3.22 g/cm

Crystal System

Trigonal

Color

Opaque black, sometimes very dark brown or bluish-black

Luster

Vitreous to resinous, sometimes sub-metallic

Transparency

Opaque

Fracture

Conchoidal to uneven

Cleavage

Poor to indistinct basal {0001} and prismatic {1120}

Composition

Sodium Iron Aluminum Borosilicate Hydroxide

Physical characteristics

• Crystal Habit: Prismatic, columnar, acicular, massive. Crystals often show a characteristic rounded triangular cross-section and vertical striations.
• Cleavage Type: Poor to indistinct. Fracture is more prevalent.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to resinous, sometimes sub-metallic.

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