mineral

Limonite / Ironstone Nodule

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

Hardness: 4.0-5.5; Color: Yellowish-brown to dark brown/black; Luster: Dull, earthy to sub-metallic; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3

Hardness: 4
Color: Yellowish-brown to dark brown/black
Luster: Dull, earthy to sub-metallic

Identified April 12, 2026More mineral →

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Physical properties

Hardness: 4.0-5.5; Color: Yellowish-brown to dark brown/black; Luster: Dull, earthy to sub-metallic; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3

Formation & geological history

Formed via the weathering and oxidation of iron-rich mineral deposits (like pyrite or magnetite) or through precipitation in sedimentary environments such as bogs and lake beds.

Uses & applications

Historically used as an important iron ore; currently used as a pigment (yellow ochre) in paints and dyes, and occasionally as a minor ore of iron.

Geological facts

Limonite is not a true mineral but a mixture of similar hydrated iron oxide minerals. It is responsible for the yellowish-brown coloring in many soils and sedimentary rocks.

Field identification & locations

Identify by its characteristic yellowish-brown streak on a porcelain plate and high density for its size. It is often found in sedimentary layers or as 'gossan' capping metallic ore veins.

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