How to identify Graphitic Quartz

To correctly identify Graphitic Quartz (SiO2 with C), 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

   Typically dark gray to black due to graphite, often with translucent to opaque white or gray quartz. The graphite can appear as silvery-black flakes.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) for quartz, contrasting with metallic to sub-metallic or dull/earthy for graphite.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be massive, granular, or foliated (schistose) if the graphite is aligned. Graphite often occurs as fine flakes or lamellae within the quartz matrix.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz typically forms anhedral to subhedral grains, sometimes euhedral crystals in vugs. Graphite forms hexagonal plates or flakes, often anhedral and disseminated.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz has no true cleavage but exhibits conchoidal fracture. Graphite has perfect basal cleavage (one direction), which can be observed as flaky partings.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in regionally metamorphosed terrains, particularly in areas derived from carbonaceous shales, limestones, or sandstones. Also occurs in contact metamorphic aureoles and hydrothermal veins associated with carbon-rich host rocks.

Key Facts

Hardness

Quartz: 7 (Mohs); Graphite: 1-2 (Mohs). The overall hardness of the rock will depend on the proportion and texture of each mineral.

Specific Gravity

Quartz: 2.65 g/cm³; Graphite: 2.09-2.23 g/cm³. The aggregate specific gravity will be intermediate.

Crystal System

Quartz: Trigonal; Graphite: Hexagonal

Color

Dark gray to black, often with white or translucent gray areas

Luster

Vitreous to greasy for quartz, metallic to dull for graphite

Transparency

Quartz: Transparent to translucent; Graphite: Opaque

Fracture

Quartz: Conchoidal; Graphite: Uneven to flaky

Cleavage

Quartz: None; Graphite: Perfect basal (one direction)

Composition

Silicon dioxide (SiO2) and elemental carbon (C)

Physical characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular; Graphite: Flaky, platy, massive, disseminated
• Cleavage Type: Quartz: None; Graphite: Perfect basal (0001)
• Fracture Type: Quartz: Conchoidal; Graphite: Uneven, flaky
• Tenacity: Quartz: Brittle; Graphite: Sectile, flexible (thin flakes)
• Luster Type: Quartz: Vitreous; Graphite: Metallic to sub-metallic, dull, earthy

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