mineral

Limonite (Iron Ore)

Limonite (unstructured hydrated iron(III) oxide-hydroxides, often FeO(OH)·nH2O)

Hardness: 4.0 - 5.5 (Mohs); Color: Yellowish-brown to dark brown; Luster: Dull, earthy; Crystal Structure: Amorphous (no internal structure); Streak: Yellowish-brown; Specific Gravity: 2.7 - 4.3.

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

Identified April 3, 2026More mineral →

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Physical properties

Hardness: 4.0 - 5.5 (Mohs); Color: Yellowish-brown to dark brown; Luster: Dull, earthy; Crystal Structure: Amorphous (no internal structure); Streak: Yellowish-brown; Specific Gravity: 2.7 - 4.3.

Formation & geological history

Formed by the oxidation and hydration of iron-rich sulfide minerals (like pyrite) or the weathering of iron-rich silicates. It typically forms in the oxidation zones of ore deposits or as sedimentary deposits in bogs.

Uses & applications

A historic source of iron ore; currently used as a yellow-brown pigment (ochre) in paints, enamels, and concrete coloring.

Geological facts

Limonite is not a true mineral but a mixture of similar hydrated iron oxide minerals. It has been used by humans as a pigment since prehistoric times, appearing in cave paintings.

Field identification & locations

Identify in the field by its distinct mustard-yellow to brown streak on a porcelain plate and its frequently 'rusty' or earthy appearance. It often occurs as a coating or mass associated with other iron minerals.

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