sedimentary
Limonite (Ironstone Concretion)
Limonite: FeO(OH)·nH2O
Hardness: 4 to 5.5 Mohs; Color: Yellowish-brown to dark brown; Luster: Dull, earthy; Crystal Structure: Amorphous (often pseudomorphous); Specific Gravity: 2.7 to 4.3
- Hardness
- 4 to 5
- Color
- Yellowish-brown to dark brown
- Luster
- Dull, earthy
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Physical properties
Hardness: 4 to 5.5 Mohs; Color: Yellowish-brown to dark brown; Luster: Dull, earthy; Crystal Structure: Amorphous (often pseudomorphous); Specific Gravity: 2.7 to 4.3
Formation & geological history
Formed primary by the chemical weathering of iron-bearing minerals or by precipitation from water in marshes or marine environments. This specific specimen appears to be an ironstone concretion featuring Liesegang rings.
Uses & applications
Historically used as an iron ore (yellow ochre). Today, it is used as a pigment in paints and as a geological specimen for education.
Geological facts
Limonite is not a true mineral but a mixture of hydrated iron oxide minerals, primarily goethite. It was the source of 'yellow ochre' used by early humans in cave paintings.
Field identification & locations
Identified by its distinctive yellowish-brown streak, earthy texture, and lack of magnetism compared to magnetite. Found frequently in sedimentary rock layers or as a coating on other rocks.
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