How to identify Shale with Pyrite

To correctly identify Shale with Pyrite (Shale with Pyrite), 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, sometimes brownish or greenish. Pyrite inclusions are brassy yellow, metallic.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Shale matrix is dull to earthy. Pyrite has a distinct metallic luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fine-grained, clastic, smooth to slightly gritty feel. Exhibits fissility (splits into thin layers). Pyrite crystals can be visible as distinct grains or nodules.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite commonly forms euhedral to subhedral crystals, often cubes, octahedra, or pyritohedra. It can also occur as framboids (spherical aggregates of microcrystals) or anhedral masses.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Shale exhibits fissility, which is a parting along bedding planes, not true mineral cleavage. Pyrite has indistinct cleavage, typically parting along crystal faces.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Low-energy, anoxic sedimentary environments such as deep marine basins, stagnant lakes, restricted lagoons, and swamps. Often associated with organic-rich sediments.

Key Facts

Hardness

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

Specific Gravity

Shale: 2.0-2.8; Pyrite: 4.95-5.10

Crystal System

Shale: Amorphous/Cryptocrystalline (clay minerals); Pyrite: Isometric

Color

Dark gray to black (shale), brassy yellow (pyrite)

Luster

Dull to earthy (shale), metallic (pyrite)

Transparency

Opaque

Fracture

Shale: Splintery to conchoidal; Pyrite: Uneven to conchoidal

Cleavage

Shale: Fissility (parting along bedding planes); Pyrite: Indistinct to poor {001}

Composition

Shale: Predominantly clay minerals (kaolinite, illite, smectite), quartz, feldspar, organic matter; Pyrite: Iron disulfide (FeS2)

Physical characteristics

• Crystal Habit: Shale: Platy, microcrystalline aggregates; Pyrite: Cubes, octahedra, pyritohedra, framboids, massive, granular.
• Cleavage Type: Shale: Fissility (parting along bedding planes); Pyrite: Indistinct to poor {001}.
• Fracture Type: Shale: Splintery to conchoidal; Pyrite: Uneven to conchoidal.
• Tenacity: Shale: Brittle; Pyrite: Brittle.
• Luster Type: Shale: Dull, earthy; Pyrite: Metallic.

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