mineral

Limonite (Ironstone)

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

Hardness: 4-5.5 on Mohs scale. Color: Yellowish-brown to dark brown or black. Luster: Dull, earthy to submetallic. Crystal structure: Amorphous to microcrystalline. Cleavage: None. Streak: Yellowish-brown.

Hardness: 4-5
Color: Yellowish-brown to dark brown or black
Luster: Dull, earthy to submetallic

Identified April 5, 2026More mineral →

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Physical properties

Formation & geological history

Formed as a secondary mineral through the oxidation and hydration of iron-rich minerals such as pyrite, magnetite, or biotite. It is common in the oxidation zones of ore deposits and as bog iron in swampy environments.

Uses & applications

Historically used as a major ore of iron. Today, it is primarily used as a pigment (ochre or sienna) for paints and dyes. It is also of interest for geological specimen collecting.

Geological facts

Limonite is not a single mineral but a mixture of hydrated iron oxide minerals, primarily goethite. It has been used as a pigment since prehistoric times, notably in cave paintings like those at Lascaux.

Field identification & locations

Identify in the field by its characteristic 'rust' color and yellowish-brown streak. It is often found as a coating or mass in sedimentary environments. Look for heavy, earthy nodules in iron-rich soil or near mine tailings.

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