sedimentary
Limonite Concretion (Ironstone)
Limonite (Hydrated Iron(III) Oxide-Hydroxide, FeO(OH)·nH2O)
Hardness: 4-5.5 (Mohs scale); Color: Dark brown to reddish-brown with yellowish-orange weathering; Luster: Dull, earthy, to sub-metallic; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3
- Hardness
- 4-5
- Color
- Dark brown to reddish-brown with yellowish-orange weathering
- Luster
- Dull, earthy, to sub-metallic
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Physical properties
Hardness: 4-5.5 (Mohs scale); Color: Dark brown to reddish-brown with yellowish-orange weathering; Luster: Dull, earthy, to sub-metallic; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3
Formation & geological history
Formed through the chemical weathering of iron-rich mineral crystals or the precipitation of iron oxides from groundwater in sedimentary environments. Common in bog or swamp deposits.
Uses & applications
Historically used as a low-grade iron ore (pig iron); used as a natural earth pigment (yellow ochre) in paints and cosmetics.
Geological facts
Limonite is not a true mineral but a mixture of hydrated iron oxide minerals. It was often called 'bog iron' during the colonial era and was the primary source of iron for early blacksmiths.
Field identification & locations
Identify by its high density compared to common rocks, earthy yellowish streak, and 'onion-skin' weathering appearance where layers peel off. Common in sandstone beds and stream deposits.
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