mineral

Pyrite in Limestone

Iron Sulfide (FeS2) in Calcium Carbonate (CaCO3)

Hardness: 6-6.5 (Pyrite), 3 (Limestone); Color: Brass-yellow metallic flakes in a gray/tan dull matrix; Luster: Metallic (crystals) to dull (matrix); Crystal Structure: Isometric (cubic/pyritohedral); Cleavage: Indistinct; Specific Gravity: 4.9-5.2 (Pyrite).

Hardness: 6-6
Color: Brass-yellow metallic flakes in a gray/tan dull matrix
Luster: Metallic (crystals) to dull (matrix)

Identified April 3, 2026More mineral →

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Physical properties

Formation & geological history

Formed in sedimentary environments via the precipitation of sulfides in anoxic, organic-rich marine muds during diagenesis. The limestone matrix was formed by the accumulation of shell, coral, and algal debris over millions of years.

Uses & applications

A source of sulfur and iron (though rarely used for iron today); primarily used as a collector specimen, for educational kits, and in historical 'strike-a-light' fire starting.

Geological facts

Commonly known as 'Fool's Gold' because its metallic luster and brassy color frequently trick novice prospectors. Pyrite can actually contain trace amounts of real gold within its structure.

Field identification & locations

Identify by looking for metallic, brassy flakes that do not scratch easily with a copper penny but will scratch glass. Found worldwide in sedimentary basins, notably in the Midwestern United States and Europe.

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