mineral

Limonite (Iron Ore)

FeO(OH)·nH2O (Hydrous ferric oxide)

Hardness: 4-5.5 on Mohs scale; Color: yellowish-brown to dark brown; Luster: dull to earthy; Crystal Structure: Amorphous (often pseudomorphic after other minerals); Cleavage: None; Specific Gravity: 2.7–4.3

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

Identified March 16, 2026More mineral →

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Physical properties

Formation & geological history

Formed by the oxidation and hydration of iron-rich sulfide minerals (like pyrite) in the weathering zones of ore deposits. It can form as sedimentary bog iron or as gossans (iron hats) capping metallic veins.

Uses & applications

A historic source of iron ore (though less important today than hematite/magnetite). Used as an earthy pigment (yellow ochre) in paints, dyes, and cosmetics. Occasionally used as a low-cost specimen for educational geological collections.

Geological facts

Limonite is not a true mineral species but a mixture of related hydrated iron oxides, primarily goethite. It was used as a pigment in cave paintings over 15,000 years ago and was a primary source of iron during the early Iron Age.

Field identification & locations

Identify in the field by its distinctive 'yellow-brown' streak on a porcelain plate and its rusty appearance. Commonly found in oxidizing environments, mine tailings, and stream beds. Collectors should look for 'pseudomorphs' where limonite has replaced the original shape of a pyrite cube.

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