How to identify Pyrite in matrix

To correctly identify Pyrite in matrix (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 itself is a pale brass-yellow to golden-yellow. The matrix color will vary widely (e.g., white for quartz, gray/black for shale, pink/red for granite).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Pyrite exhibits a distinctive metallic luster. The matrix luster will vary (e.g., vitreous for quartz, dull for shale).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Pyrite crystals are typically euhedral to subhedral, often forming cubes, octahedra, or pyritohedra. The texture of the matrix will depend on its rock type (e.g., granular for igneous, layered for sedimentary).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Pyrite commonly forms cubes, octahedra, and pyritohedra, often with striations on crystal faces. It can also occur as massive, granular, or radiating aggregates. The matrix will show its characteristic crystal forms or grain structures.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Pyrite has very poor to indistinct cleavage, typically exhibiting a conchoidal to uneven fracture. The cleavage of the matrix minerals will vary.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Pyrite in matrix can be found in a vast array of environments: hydrothermal veins, sedimentary rocks (especially black shales, coal seams), metamorphic rocks, and as an accessory mineral in some igneous rocks. The specific matrix rock type provides clues to the exact geological environment.

Key Facts

Hardness

6-6.5 on Mohs scale (pyrite); matrix hardness varies.

Specific Gravity

4.95-5.10 (pyrite); matrix specific gravity varies.

Crystal System

Isometric (pyrite); matrix crystal system varies.

Color

Pale brass-yellow to golden-yellow (pyrite); matrix color varies.

Luster

Metallic (pyrite); matrix luster varies.

Transparency

Opaque (pyrite); matrix transparency varies.

Fracture

Conchoidal to uneven (pyrite); matrix fracture varies.

Cleavage

Very poor/indistinct (pyrite); matrix cleavage varies.

Composition

Iron disulfide (FeS2) for pyrite; matrix composition varies widely.

Physical characteristics

• Crystal Habit: Cubic, octahedral, pyritohedral, massive, granular, radiating (pyrite); matrix habit varies.
• Cleavage Type: Very poor/indistinct on {100} (pyrite); matrix cleavage varies.
• Fracture Type: Conchoidal to uneven (pyrite); matrix fracture varies.
• Tenacity: Brittle (pyrite); matrix tenacity varies.
• Luster Type: Metallic (pyrite); matrix luster varies.

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