sedimentary

Limonite Concretion

FeO(OH)·nH2O (Hydrated Iron Oxide)

AI-generated identification · may be incorrect

Hardness: 4-5.5; Color: Dark brown to yellow-brown; Luster: Dull, earthy; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3

Hardness: 4-5
Color: Dark brown to yellow-brown
Luster: Dull, earthy

Identified July 5, 2026More sedimentary →

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Physical properties

Hardness: 4-5.5; Color: Dark brown to yellow-brown; Luster: Dull, earthy; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3

Formation & geological history

Formed by the precipitation of iron-rich minerals from groundwater within sedimentary rock layers, often as nodules or concretions during the diagenesis process.

Uses & applications

Historically used as a low-grade iron ore and as a natural earth pigment (brown ochre). It is also collected as a geological curiosity.

Geological facts

Limonite is not a true mineral but a mixture of hydrated iron oxide minerals, mostly goethite. It is responsible for the rust-brown color in many soils and rock faces.

Field identification & locations

Identified by its yellow-brown streak, earthy appearance, and relatively high density for a sedimentary rock. Often found in iron-rich shale or sandstone beds.