How to identify Shale with Calcite and Marcasite

To correctly identify Shale with Calcite and Marcasite (Shale with Calcite (CaCO3) and Marcasite (FeS2)), 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

   Shale matrix typically dark gray to black (due to organic matter), but can be lighter gray, green, or reddish. Calcite is usually white, gray, or yellowish. Marcasite is brassy yellow to silvery-white, often tarnishing to a dull bronze or iridescent blue/green.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Shale matrix is dull to earthy. Calcite can be vitreous to dull. Marcasite has a metallic luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fine-grained, clastic, fissile (splits into thin layers). Marcasite may form radiating orthorhombic crystals, concretions, or nodules within the fine-grained matrix. Calcite may be microcrystalline, granular, or fossiliferous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Shale minerals are typically too fine to see individual crystals without magnification. Calcite may show rhombohedral cleavage or be massive/granular. Marcasite forms orthorhombic crystals, often twinned, radiating, or in concretions (e.g., 'cockscomb' marcasite).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Shale exhibits fissility (rock cleavage) parallel to bedding. Calcite has perfect rhombohedral cleavage in three directions. Marcasite has indistinct cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Low-energy, anoxic sedimentary environments such as deep marine basins, stagnant lake bottoms, or swamps, where fine clastic sedimentation, carbonate precipitation, and sulfur-reducing bacteria activity co-occurred.

Key Facts

Hardness

Shale: 2.5-4 (Mohs). Calcite: 3 (Mohs). Marcasite: 6-6.5 (Mohs).

Specific Gravity

Shale: 2.0-2.8. Calcite: 2.71. Marcasite: 4.8-4.9.

Crystal System

Shale: Not applicable (aggregate of fine minerals). Calcite: Trigonal. Marcasite: Orthorhombic.

Color

Variable for shale (dark gray, black, green, red); white, gray, yellow for calcite; brassy yellow, silvery-white for marcasite.

Luster

Dull to earthy for shale; vitreous to dull for calcite; metallic for marcasite.

Transparency

Opaque for shale and marcasite; transparent to translucent for calcite.

Fracture

Shale: Splintery to conchoidal. Calcite: Conchoidal. Marcasite: Uneven to conchoidal.

Cleavage

Shale: Fissility (rock cleavage). Calcite: Perfect rhombohedral (3 directions). Marcasite: Indistinct.

Composition

Shale: Hydrous aluminum silicates (clay minerals), quartz, feldspar. Calcite: Calcium carbonate (CaCO3). Marcasite: Iron disulfide (FeS2).

Physical characteristics

• Crystal Habit: Shale: Platy, massive. Calcite: Rhombohedral, granular, massive, fibrous, stalactitic. Marcasite: Orthorhombic, often radiating, tabular, pyramidal, cockscomb aggregates, concretions.
• Cleavage Type: Shale: Fissility (splits along bedding planes). Calcite: Perfect rhombohedral {1011}. Marcasite: Indistinct {101}.
• Fracture Type: Shale: Splintery to conchoidal. Calcite: Conchoidal. Marcasite: Uneven to conchoidal.
• Tenacity: Shale: Brittle. Calcite: Brittle. Marcasite: Brittle.
• Luster Type: Shale: Dull, earthy. Calcite: Vitreous, pearly, dull. Marcasite: Metallic.

Explore Shale with Calcite and Marcasite