How to identify Pyrite in matrix

To correctly identify Pyrite in matrix (Iron sulfide (FeS2) in 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: Brassy yellow to golden. Matrix: Highly variable, depending on the host rock (e.g., gray for shale, white for quartz, dark for basalt).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pyrite: Metallic, bright. Matrix: Variable, can be dull, vitreous, earthy, etc.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Pyrite: Crystalline, often euhedral to subhedral crystals. Matrix: Granular, foliated, massive, or clastic, depending on the host rock type.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite: Commonly cubic, octahedral, or pyritohedral crystals, often striated. Can also be massive, granular, or radiating. Matrix: Reflects the texture and structure of the host rock.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Pyrite: Indistinct to poor on {001}. Matrix: Variable, depending on the host rock (e.g., perfect in mica, absent in quartz).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Pyrite in matrix is found in a vast array of geological settings: sedimentary rocks (shales, limestones, coal seams) formed under anoxic conditions; hydrothermal veins associated with igneous intrusions; metamorphic rocks (slates, schists); and disseminated in various igneous rocks. The specific environment dictates the nature of the matrix.

Key Facts

Hardness

6-6.5 on Mohs scale (pyrite)

Specific Gravity

4.95-5.10 g/cm³ (pyrite)

Crystal System

Isometric (pyrite)

Color

Brassy yellow, golden (pyrite); variable (matrix)

Luster

Metallic (pyrite); variable (matrix)

Transparency

Opaque (pyrite); variable (matrix)

Fracture

Conchoidal to uneven (pyrite)

Cleavage

Indistinct to poor on {001} (pyrite)

Composition

Iron disulfide (FeS2) (pyrite); variable silicates, carbonates, clays, etc. (matrix)

Physical characteristics

• Crystal Habit: Cubic, octahedral, pyritohedral, massive, granular, radiating (pyrite); reflects host rock texture (matrix)
• Cleavage Type: Indistinct (pyrite); variable (matrix)
• Fracture Type: Conchoidal to uneven (pyrite); variable (matrix)
• Tenacity: Brittle (pyrite); variable (matrix)
• Luster Type: Metallic (pyrite); variable (matrix)

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