How to identify Pyrite in matrix

To correctly identify Pyrite in matrix (Pyrite (FeS2) in a fine-grained host 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: Pale brass-yellow to golden-yellow, often with a metallic sheen. Matrix: Varies widely depending on the host rock, commonly dark gray, black (shale, slate), greenish (chlorite-rich rocks), or reddish-brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pyrite: Distinctive metallic luster. Matrix: Can be dull, earthy, vitreous, or silky depending on the host rock minerals.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Pyrite: Crystalline, often euhedral (cubes, octahedra, pyritohedra), granular, or massive. Matrix: Fine-grained, often fissile (shale, slate), compact, or schistose.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite: Commonly forms cubes, octahedra, and pyritohedra, often with striations on cube faces. Also found as massive, granular, or radiating aggregates. Matrix: Typically massive or foliated, reflecting the fine-grained nature of the host rock.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Pyrite: Indistinct to poor cleavage on {001}. Matrix: Varies; shales and slates exhibit good cleavage (fissility), while other fine-grained rocks may have none or poor cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Sedimentary rocks (especially black shales, mudstones), metamorphic rocks (slates, phyllites, schists), hydrothermal veins, and disseminated in igneous rocks. The presence of a fine-grained matrix often points towards sedimentary or low-grade metamorphic origins, or fine-grained volcanic rocks.

Key Facts

Hardness

6-6.5 on Mohs scale (pyrite)

Specific Gravity

4.95-5.10 g/cm³ (pyrite)

Crystal System

Isometric (pyrite)

Color

Pale brass-yellow to golden-yellow (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; matrix composition varies widely (e.g., silicates, carbonates, organic matter)

Physical characteristics

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

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