sedimentary
Limonite Concretion
Limonite (FeO(OH)·nH2O) Pseudomorph
Hardness: 4-5.5 (Mohs), Color: Dark brown to blackish-brown, Luster: Dull or earthy, Structure: Amorphous or cuboidal (if replacing pyrite), Opaque, Specific Gravity: 2.7-4.3
- Hardness
- 4-5
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Physical properties
Hardness: 4-5.5 (Mohs), Color: Dark brown to blackish-brown, Luster: Dull or earthy, Structure: Amorphous or cuboidal (if replacing pyrite), Opaque, Specific Gravity: 2.7-4.3
Formation & geological history
Formed via the oxidation of iron-rich mineral precursors (like pyrite) in sedimentary rock environments where water and oxygen are present to chemically weather the original mineral.
Uses & applications
Primarily of interest to mineral collectors and as educational specimens; historically used as a low-grade iron ore.
Geological facts
These are often called 'Propeller Stones' or 'Devil's Dice' when they retain the cubic shape of the original pyrite they replaced. A concretion forms by the precipitation of mineral cement between particles.
Field identification & locations
Identify by its heavy weight relative to size, dark brown streak, and often unusual geometric or rounded shapes found in clay or shale beds. Common in many sedimentary regions worldwide.
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