Graphitic Sandstone

Description

Sandstone with graphite is a clastic sedimentary rock primarily composed of sand-sized mineral grains (typically quartz, feldspar, and rock fragments) cemented together, with the notable inclusion of the mineral graphite. The graphite imparts a characteristic dark gray to black color to the rock, often giving it a somewhat metallic sheen, especially on freshly broken surfaces or where graphite flakes are concentrated. The texture is typically arenaceous (sandy), and individual sand grains may be visible. The graphite can be finely disseminated throughout the matrix, forming thin laminae, or occasionally occurring as larger flakes or aggregates. The rock's overall appearance and properties are a combination of the sandstone's characteristics and the graphite's presence.

How to Identify

Color

Dark gray to black, often with a subtle metallic luster due to graphite. The color can vary depending on the proportion of graphite and the color of the primary sand grains.

Luster

Dull to sub-metallic or metallic, particularly where graphite flakes are visible. The overall rock luster will be influenced by the dominant mineral (quartz, feldspar) and the graphite.

Texture

Clastic, arenaceous (sandy). Individual sand grains are typically visible and palpable. The graphite may appear as fine, greasy-feeling particles or thin, platy flakes within the sandy matrix.

Crystal Form

Graphite within the sandstone typically occurs as anhedral (lacking well-formed crystal faces) flakes, scales, or fine disseminations. It rarely forms macroscopic euhedral crystals in this context.

Cleavage

Graphite itself exhibits perfect basal cleavage (one direction). In the sandstone, this cleavage may be observed on individual flakes, but the overall rock will not show a pervasive cleavage unless it has undergone significant metamorphism to become a graphitic slate or schist. The sandstone matrix typically lacks cleavage.

Geological Environment

Formed in sedimentary basins where organic matter was deposited alongside sand. Subsequent burial and low-grade metamorphism (regional or contact) or deep diagenesis converted the organic matter to graphite. Common in areas with ancient sedimentary sequences that have experienced some degree of metamorphism, such as fold-and-thrust belts or cratonic margins.

Key Facts

• Hardness: Sandstone matrix: 6-7 (quartz). Graphite: 1-2 (Mohs scale). The overall hardness of the rock will be dominated by the quartz grains, but graphite flakes will be very soft.
• Specific Gravity: Sandstone matrix: 2.6-2.7. Graphite: 2.09-2.23. The overall specific gravity will be slightly lower than pure quartz sandstone due to the lighter graphite.
• Crystal System: Graphite: Hexagonal. Sandstone minerals (e.g., quartz): Trigonal.
• Color: Dark gray to black.
• Luster: Dull to sub-metallic or metallic.
• Transparency: Opaque (graphite). Translucent to opaque (sandstone grains).
• Fracture: Uneven to conchoidal (sandstone). Graphite: flaky, micaceous.
• Cleavage: Graphite: Perfect basal cleavage (one direction). Sandstone: None (rock), individual mineral grains may have cleavage (e.g., feldspar).
• Composition: Primarily quartz (SiO2) and other detrital minerals (feldspar, rock fragments) with a significant component of graphite (C).

Quick Check

• Color: Dark gray to black
• Luster: Dull to sub-metallic/metallic (due to graphite)
• Streak: Black (from graphite)

Physical Characteristics

• Crystal Habit: Graphite: Flaky, scaly, massive, earthy. Sandstone: Granular, clastic.
• Cleavage Type: Graphite: Perfect basal {0001}. Sandstone: Not applicable as a rock, individual mineral grains may have cleavage.
• Fracture Type: Sandstone: Uneven to conchoidal. Graphite: Flaky.
• Tenacity: Graphite: Flexible, inelastic. Sandstone: Brittle.
• Luster Type: Graphite: Metallic to sub-metallic. Sandstone: Vitreous to dull.

Formation

Sandstone with graphite forms when sand-sized detrital grains (typically quartz) are deposited along with organic matter in a sedimentary basin. Subsequent burial and diagenesis compact and lithify the sand into sandstone. If this sandstone then undergoes low-grade metamorphism (e.g., regional metamorphism or contact metamorphism near an igneous intrusion) or deep burial with sufficient temperature and pressure, the organic matter (kerogen, coal fragments) is transformed into graphite. The graphite can occur as disseminated flakes, laminae, or even small lenses within the sandstone matrix or cementing material. The presence of graphite indicates a reducing environment during the original deposition of the organic matter.

Usage

Graphitic sandstone itself has limited direct industrial uses beyond general construction aggregate if the graphite content is low and doesn't compromise structural integrity. However, the presence of graphite can be an indicator for larger graphite deposits in the region. In some cases, if the graphite content is sufficiently high and concentrated, it might be considered a low-grade graphite ore, though extraction from a sandstone matrix would be challenging and uneconomical compared to other graphite deposit types. Historically, some dark sandstones were used for building materials where their color was desired.

Age Distribution

Can occur in sedimentary sequences of various ages, from Precambrian to Cenozoic, wherever organic matter was present and subsequently metamorphosed or deeply buried.

Where to Find

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Scientific Classification

Mineral Class

Native Element (Graphite). Siliciclastic Sedimentary Rock (Sandstone).

Group

Native Elements (Graphite). Clastic Sedimentary Rocks (Sandstone).

Crystal System

Hexagonal (Graphite). Trigonal (Quartz).

Chemical Formula

C (Graphite). SiO2 (Quartz, primary component of sandstone).

Composition

Carbon (Graphite). Silicon dioxide (Quartz), with varying amounts of other silicates and rock fragments in the sandstone matrix.

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