mineral

Marcasite Concretion (Propeller/Sand Dollar shape)

Marcasite (FeS2)

Hardness: 6-6.5 Mohs. Color: Tin-white to pale bronze-yellow, often weathering to dark brown/grey. Luster: Metallic. Crystal structure: Orthorhombic. Cleavage: Distinct. Specific Gravity: 4.88.

Hardness: 6-6
Luster: Metallic

Identified May 23, 2026More mineral →

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Physical properties

Hardness: 6-6.5 Mohs. Color: Tin-white to pale bronze-yellow, often weathering to dark brown/grey. Luster: Metallic. Crystal structure: Orthorhombic. Cleavage: Distinct. Specific Gravity: 4.88.

Formation & geological history

Formed in an oxygen-depleted marine sedimentary environment by the precipitation of iron disulfide. These specific wheel-shaped concretions are often found in Cretaceous or Jurassic chalk or clay deposits.

Uses & applications

Primarily a collector's specimen. In the past, marcasite was used as a minor source of sulfur for making sulfuric acid, and it is sometimes used in vintage-style jewelry (though most 'marcasite' jewelry is actually pyrite).

Geological facts

Marcasite is chemically identical to pyrite but has a different crystal structure. It is unstable and prone to 'pyrite decay,' where it reacts with moisture in the air to form sulfuric acid, potentially destroying the specimen and its storage container over time.

Field identification & locations

Identify in the field by its distinct radial, botryoidal, or wheel-like growth patterns and metallic luster on fresh surfaces. Commonly found in limestone, chalk, and coal beds across Europe and North America.

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