How to identify Pyrite in host rock

To correctly identify Pyrite in host rock (Pyrite (FeS2) in a fine-grained metamorphic or sedimentary rock), 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 golden. Host rock: Varies widely, often dark gray, black, brown, or greenish depending on the rock type (e.g., shale, slate, phyllite).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pyrite: Metallic, bright. Host rock: Dull, earthy, or sometimes silky (e.g., phyllite) or vitreous (e.g., fine-grained quartzite).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Pyrite: Crystalline, often euhedral (well-formed crystals) or anhedral (irregular grains). Host rock: Fine-grained, clastic (sedimentary) or foliated/non-foliated (metamorphic).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite: Commonly cubes, pyritohedrons (12-sided pentagonal dodecahedra), octahedra, or combinations thereof. Also massive, granular, or radiating aggregates. Host rock: No inherent crystal form, but may show bedding or foliation.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Pyrite: Indistinct to poor on {001}. Host rock: Varies; sedimentary rocks may have parting along bedding planes, metamorphic rocks may exhibit slaty cleavage or schistosity.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Sedimentary environments (e.g., anoxic shales, coal seams, black shales, limestones), hydrothermal veins, and various metamorphic settings (e.g., slates, phyllites, schists, contact metamorphic aureoles).

Key Facts

Hardness

6-6.5 (Mohs scale for pyrite)

Specific Gravity

4.95-5.10 (for pyrite)

Crystal System

Isometric (for pyrite)

Color

Brassy yellow, golden, sometimes tarnished to iridescent (pyrite)

Luster

Metallic (pyrite)

Transparency

Opaque (pyrite)

Fracture

Conchoidal to uneven (pyrite)

Cleavage

Indistinct to poor on {001} (pyrite)

Composition

Iron disulfide (FeS2) for pyrite; host rock composition varies (e.g., silicates, carbonates, carbonaceous matter)

Physical characteristics

• Crystal Habit: Cubic, pyritohedral, octahedral, massive, granular, radiating, stalactitic (for pyrite)
• Cleavage Type: Indistinct to poor (for pyrite)
• Fracture Type: Conchoidal to uneven (for pyrite)
• Tenacity: Brittle (for pyrite)
• Luster Type: Metallic (for pyrite)

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