mineral

Limonite (Iron Ore)

Limonite; FeO(OH)·nH2O

Hardness: 4 to 5.5 on the Mohs scale; Color: yellowish-brown to dark brown or black; Luster: dull to earthy; Crystal structure: Amorphous (does not form visible crystals); Specific gravity: 2.7 to 4.3.

Hardness: 4 to 5
Color: yellowish-brown to dark brown or black
Luster: dull to earthy

Identified March 26, 2026More mineral →

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Physical properties

Formation & geological history

Forms as a secondary mineral through the weathering and oxidation of iron-rich mineral deposits (like pyrite or magnetite) in sedimentary environments. Common in bog iron deposits and oxidized zones of ore veins.

Uses & applications

Historically used as a major iron ore. Today, it is primarily used as a pigment in paints (ochre and sienna) and as an iron supplement in fertilizers. It is also a source for collectors of ore samples.

Geological facts

Limonite is not a singular mineral but a mixture of hydrated iron oxide minerals. Before synthetic pigments, it was one of the most important sources of 'ochre' for early human cave paintings.

Field identification & locations

Identify in the field by its distinct yellow-brown streak (test by rubbing on unglazed porcelain) and its lack of magnetism compared to magnetite. Found worldwide in stream beds, bogs, and oxidized rock outcrops.