mineral

Limonite (Ironstone Concretion)

Hydrated iron(III) oxide-hydroxide (FeO(OH)·nH2O)

Hardness: 4-5.5 (Mohs scale); Color: Yellowish-brown to dark brown; Luster: Dull, earthy to sub-metallic; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7–4.3

Hardness: 4-5
Color: Yellowish-brown to dark brown
Luster: Dull, earthy to sub-metallic

Identified March 14, 2026More mineral →

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Physical properties

Formation & geological history

Formed through the chemical weathering of iron-rich mineral silicates or precipitation in marshy/sedimentary environments (bog iron). These concretions often form in sedimentary layers where iron oxides cement around a nucleus.

Uses & applications

Historically used as a low-grade iron ore. In modern times, it is used as a pigment (yellow ochre) and occasionally by collectors for unique botryoidal or concretionary shapes.

Geological facts

Limonite is not a single mineral but a mixture of hydrated iron oxides, primarily goethite. It was one of the earliest pigments used by prehistoric humans in cave paintings.

Field identification & locations

Identify in the field by its distinct yellow-brown streak, yellowish coloring, and tendency to form rounded, lumpy masses (concretions) or earthy crusts. Common in sedimentary basins and weathered iron deposits.

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