sedimentary

Pyritic Mudstone

Argillaceous rock (FeS2)

Hardness: 3-5 (matrix) to 6 (pyrite); Color: Dark grey/black with gold metallic flecks; Luster: Dull/earthy for matrix, metallic for inclusions; Structure: Fine-grained, laminated; Cleavage: None.

Hardness: 3-5 (matrix) to 6 (pyrite)
Color: Dark grey/black with gold metallic flecks
Luster: Dull/earthy for matrix, metallic for inclusions

Identified June 4, 2026More sedimentary →

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Physical properties

Hardness: 3-5 (matrix) to 6 (pyrite); Color: Dark grey/black with gold metallic flecks; Luster: Dull/earthy for matrix, metallic for inclusions; Structure: Fine-grained, laminated; Cleavage: None.

Formation & geological history

Formed in low-oxygen marine or lacustrine environments where organic matter accumulates and reacts with iron and sulfate to form iron sulfide (pyrite) within fine-grained silt and clay sediments over millions of years.

Uses & applications

Primarily used for geological study; pyrite was historically mined for sulfur and sulfuric acid. Mudstones can be used in the production of bricks and cement.

Geological facts

The metallic gold-colored inclusions are pyrite, famously known as 'fools gold'. Finding it in dark mudstone often indicates an environment that was rich in organic material and anaerobic bacteria during deposition.

Field identification & locations

Identify by its dark, fine-grained texture and characteristic metallic yellow/gold flecks that do not tarnish easily. Commonly found in coastal cliffs, riverbeds, and coal-bearing sedimentary basins.