How to identify Pyrite Concretion

To correctly identify Pyrite Concretion (Pyrite (FeS2) in a concretionary form), 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

   Brassy yellow to golden metallic, often tarnishing to a duller bronze or iridescent hue upon exposure to air and moisture.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Distinctive metallic luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically smooth to slightly rough on the exterior, often showing radial or concentric growth patterns on fractured surfaces. 'Pyrite suns' have a characteristic radiating crystalline texture.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   While the overall concretion is massive or discoidal, individual pyrite crystals within the concretion are typically euhedral to subhedral, often cubic, pyritohedral, or octahedral, sometimes radiating outwards from a central point.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Poor to indistinct on individual crystals; the concretion itself does not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Most commonly found in fine-grained sedimentary rocks such as shales, mudstones, and siltstones, particularly those rich in organic matter and deposited in anoxic marine or lacustrine environments. Can also occur in coal seams.

Key Facts

Hardness

6 to 6.5 on the Mohs scale

Specific Gravity

4.95 to 5.03 g/cm³

Crystal System

Isometric (cubic)

Color

Pale brass-yellow, golden metallic

Luster

Metallic

Transparency

Opaque

Fracture

Conchoidal to uneven

Cleavage

Poor to indistinct {100}

Composition

Iron disulfide (FeS2)

Physical characteristics

• Crystal Habit: Concretionary, massive, discoidal, radiating, granular. Individual crystals are typically cubic, pyritohedral, or octahedral.
• Cleavage Type: Poor to indistinct, typically not observed in concretions.
• Fracture Type: Conchoidal to uneven, brittle.
• Tenacity: Brittle
• Luster Type: Metallic

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