sedimentary

Limonite (Ironstone)

Limonite (FeO(OH)·nH2O)

Hardness: 4-5.5 (variable); Color: Yellowish-brown to rusty brown; Luster: Dull or earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Opaque.

Hardness: 4-5
Color: Yellowish-brown to rusty brown
Luster: Dull or earthy

Identified July 3, 2026More sedimentary →

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Physical properties

Hardness: 4-5.5 (variable); Color: Yellowish-brown to rusty brown; Luster: Dull or earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Opaque.

Formation & geological history

Formed by the weathering of other iron-rich minerals such as hematite, magnetite, and pyrite. It often occurs in gossans, bog iron deposits, and as a cementing agent in sedimentary rocks.

Uses & applications

Historically used as a common ore of iron; used as a pigment (yellow ochre) in paints; industrial filler; occasionally a collector's specimen.

Geological facts

Limonite is not a true mineral species but a mixture of hydrated iron oxide minerals. It has been used as a pigment by humans for over tens of thousands of years, as evidenced by cave paintings.

Field identification & locations

Identify by its characteristic yellowish-brown streak on a porcelain plate and its earthy appearance. Often found in oxidization zones of ore deposits and boggy environments.