How to identify Pyrite in Host Rock

To correctly identify Pyrite in Host Rock (Pyrite (FeS2) in a dark gray host rock (likely shale or slate)), 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

   Pyrite: Brassy yellow to pale gold. Host rock: Dark gray to black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pyrite: Metallic, bright. Host rock: Dull to earthy (shale), dull to sub-vitreous (slate).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Pyrite: Crystalline, often euhedral to subhedral crystals. Host rock: Fine-grained, smooth to slightly gritty (shale), very fine-grained, smooth, and often foliated (slate).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite: Commonly cubic, pyritohedral (12-sided with pentagonal faces), or octahedral. Also massive, granular, or radiating. Host rock: Massive, platy (shale), or foliated (slate).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Pyrite: Indistinct to poor cleavage, conchoidal to uneven fracture. Host rock: Shale exhibits fissility (splits along bedding planes). Slate exhibits excellent slaty cleavage (splits into thin, flat sheets independent of bedding).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Pyrite in shale/slate typically forms in anoxic sedimentary environments (e.g., deep marine basins, stagnant lakes) or during low-grade regional metamorphism of such sediments. It is often associated with organic-rich sediments.

Key Facts

Hardness

6-6.5 on Mohs scale (pyrite); 2.5-4 (shale/slate)

Specific Gravity

4.95-5.03 (pyrite); 2.6-2.8 (shale/slate)

Crystal System

Isometric (pyrite); Not applicable for host rock, but constituent minerals are typically monoclinic (clays) or triclinic (quartz, feldspar)

Color

Brassy yellow to pale gold (pyrite); Dark gray to black (host rock)

Luster

Metallic (pyrite); Dull to earthy (shale), dull to sub-vitreous (slate)

Transparency

Opaque (pyrite); Opaque (host rock)

Fracture

Conchoidal to uneven (pyrite); Splintery to irregular (shale), platy (slate)

Cleavage

Indistinct to poor (pyrite); Fissility (shale), excellent slaty cleavage (slate)

Composition

Iron disulfide (FeS2) for pyrite. Host rock primarily composed of clay minerals (e.g., illite, kaolinite, smectite), quartz, and feldspar, with significant organic matter and minor accessory minerals.

Physical characteristics

• Crystal Habit: Pyrite: Cubes, pyritohedrons, octahedrons, massive, granular, radiating. Host rock: Fine-grained, massive, platy (shale), foliated (slate).
• Cleavage Type: Pyrite: None to very poor. Host rock: Fissility (shale), perfect slaty cleavage (slate).
• Fracture Type: Pyrite: Conchoidal to uneven. Host rock: Irregular to splintery (shale), platy (slate).
• Tenacity: Pyrite: Brittle. Host rock: Brittle.
• Luster Type: Pyrite: Metallic. Host rock: Dull to earthy (shale), dull to sub-vitreous (slate).

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