How to identify Shale with Pyrite

To correctly identify Shale with Pyrite (Shale (sedimentary rock) with Pyrite (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 is typically dark gray to black, but can be various shades of gray, green, or red. Pyrite inclusions are brassy yellow, metallic.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Shale matrix is dull to earthy. Pyrite inclusions exhibit a metallic luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Shale is fine-grained, smooth to the touch, and often fissile (splits into thin layers). Pyrite can be crystalline (euhedral cubes, octahedra, pyritohedra), framboidal, or massive.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite commonly forms cubic, octahedral, or pyritohedral crystals, or framboids. Disseminated grains are often anhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Shale exhibits fissility (a type of parting along bedding planes), not true mineral cleavage. Pyrite has indistinct cleavage on {100} and {111}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Formed in low-energy, anoxic depositional environments such as deep marine basins, stagnant lakes, lagoons, and swamps, where fine sediment and organic matter accumulate and oxygen is depleted, allowing for sulfate reduction and pyrite precipitation.

Key Facts

Hardness

Shale: 2.5-4 (Mohs). Pyrite: 6-6.5 (Mohs).

Specific Gravity

Shale: 2.0-2.8 g/cm^3. Pyrite: 4.95-5.03 g/cm^3.

Crystal System

Shale: Not applicable (amorphous to cryptocrystalline aggregate). Pyrite: Isometric (cubic).

Color

Shale: Dark gray, black, green, red. Pyrite: Pale brass-yellow.

Luster

Shale: Dull, earthy. Pyrite: Metallic.

Transparency

Shale: Opaque. Pyrite: Opaque.

Fracture

Shale: Splintery, conchoidal (rare). Pyrite: Uneven to conchoidal.

Cleavage

Shale: Fissility (parting along bedding planes). Pyrite: Indistinct on {100} and {111}.

Composition

Shale: Predominantly clay minerals (e.g., illite, kaolinite, smectite), quartz, feldspar, micas, organic matter. Pyrite: Iron disulfide (FeS2).

Physical characteristics

• Crystal Habit: Shale: Massive, laminated. Pyrite: Cubes, octahedra, pyritohedra, framboids, massive, granular, radiating.
• Cleavage Type: Shale: Fissility (parting). Pyrite: Indistinct.
• Fracture Type: Shale: Splintery, irregular. Pyrite: Uneven to conchoidal.
• Tenacity: Shale: Brittle. Pyrite: Brittle.
• Luster Type: Shale: Dull, earthy. Pyrite: Metallic.

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