sedimentary
Limonite Concretion
FeO(OH)·nH2O (Hydrated Iron Oxide)
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
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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.
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