How to identify Graphitic Quartz

To correctly identify Graphitic Quartz (SiO2 (Quartz) with C (Graphite)), 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, often with a metallic luster from the graphite. The quartz itself may be clear, white, or gray, but the graphite imparts the dominant dark coloration.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) for quartz, submetallic to metallic for graphite. The overall luster can be a combination, often appearing dull to submetallic due to the disseminated graphite.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be massive, granular, or schistose/gneissic if part of a metamorphic rock. 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 hexagonal prisms with pyramidal terminations, though in massive forms it is anhedral. Graphite forms hexagonal plates or flakes, often anhedral when 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). The aggregate will show the fracture of quartz and the cleavage of graphite.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in medium to high-grade metamorphic rocks (schists, gneisses, marbles) derived from carbonaceous sediments. Also occurs in some hydrothermal veins and occasionally in igneous rocks.

Key Facts

Hardness

Quartz: 7 (Mohs); Graphite: 1-2 (Mohs). The overall hardness will be dominated by quartz, but graphite inclusions will be soft.

Specific Gravity

Quartz: 2.65 g/cm³; Graphite: 2.09-2.23 g/cm³. The aggregate's specific gravity will be between these values, depending on the proportion of each mineral.

Crystal System

Quartz: Trigonal; Graphite: Hexagonal

Color

Dark gray to black, often with a metallic sheen

Luster

Vitreous (quartz) to submetallic/metallic (graphite)

Transparency

Opaque (due to graphite) to translucent (quartz)

Fracture

Conchoidal (quartz), but the presence of graphite can make it more irregular.

Cleavage

None for quartz; Perfect basal cleavage for graphite (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 to flaky
• Tenacity: Quartz: Brittle; Graphite: Sectile
• Luster Type: Vitreous (quartz), Submetallic to metallic (graphite)

Explore Graphitic Quartz