How to identify Schorl in Quartz

To correctly identify Schorl in Quartz (Schorl (Na(Fe2+)_3Al_6(BO_3)_3Si_6O_18(OH)_4) and Quartz (SiO_2)), 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

   Schorl is typically opaque black. Quartz can be colorless, white (milky quartz), or smoky gray. The combination presents black needles or prisms within a lighter quartz matrix.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Schorl has a vitreous to resinous luster. Quartz has a vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The texture is typically crystalline, with distinct prismatic schorl crystals embedded in a granular to massive quartz matrix. Schorl crystals often show vertical striations.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Schorl forms elongated, prismatic crystals, often with a triangular cross-section and vertical striations. Quartz forms hexagonal prisms with pyramidal terminations, though it can also be massive or anhedral when intergrown.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Schorl has poor to indistinct cleavage, often appearing as conchoidal to uneven fracture. Quartz has no cleavage, exhibiting conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Pegmatites, granitic intrusions, hydrothermal veins, and high-grade metamorphic rocks.

Key Facts

Hardness

Schorl: 7-7.5 on Mohs scale; Quartz: 7 on Mohs scale

Specific Gravity

Schorl: 3.0-3.2; Quartz: 2.65

Crystal System

Schorl: Trigonal; Quartz: Trigonal

Color

Black (schorl); Colorless, white, smoky (quartz)

Luster

Vitreous to resinous (schorl); Vitreous (quartz)

Transparency

Opaque (schorl); Transparent to translucent (quartz)

Fracture

Conchoidal to uneven (schorl); Conchoidal (quartz)

Cleavage

Poor to indistinct (schorl); None (quartz)

Composition

Schorl: Sodium Iron Aluminum Borosilicate Hydroxide; Quartz: Silicon Dioxide

Physical characteristics

• Crystal Habit: Schorl: Prismatic, acicular, columnar, often with vertical striations. Quartz: Prismatic, massive, granular.
• Cleavage Type: Schorl: Poor to indistinct basal cleavage; Quartz: None.
• Fracture Type: Conchoidal to uneven (both minerals).
• Tenacity: Brittle (both minerals).
• Luster Type: Vitreous to resinous (schorl); Vitreous (quartz).

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