sedimentary

Limonite Concretion (Ironstone)

Limonite (Hydrated Iron(III) Oxide-Hydroxide, FeO(OH)·nH2O)

AI-generated identification · may be incorrect

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

Identified May 21, 2026More sedimentary →

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Physical properties

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.