mineral

Limonite (Iron Ore Congcretion)

Limonite (a mixture of hydrated iron(III) oxide-hydroxides, FeO(OH)·nH2O)

Hardness: 4-5.5 (variable); Color: Dark brown to yellowish-brown; Luster: Dull, earthy; Crystal structure: Amorphous; Cleavage: None; Specific gravity: 2.7-4.3

Hardness: 4-5
Color: Dark brown to yellowish-brown
Luster: Dull, earthy

Identified April 10, 2026More mineral →

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Physical properties

Hardness: 4-5.5 (variable); Color: Dark brown to yellowish-brown; Luster: Dull, earthy; Crystal structure: Amorphous; Cleavage: None; Specific gravity: 2.7-4.3

Formation & geological history

Formed as a secondary material through the oxidation or chemical weathering of iron-rich minerals. Often found as nodules or concretions in sedimentary layers or bog deposits.

Uses & applications

Major historical source of iron ore (bog iron); used today as a natural earth pigment (yellow ochre) and in soil conditioning.

Geological facts

Limonite is not a 'true' mineral but a mineraloid consisting of varying proportions of goethite and other iron oxides. It has been used as a pigment by humans for over 30,000 years in cave paintings.

Field identification & locations

Identify by its yellowish-brown streak on a porcelain plate and its high weight relative to its size. Commonly found in swamps (bog iron) or as 'rust' coatings on other rocks.

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