sedimentary
Limonite or Ironstone Nodule
FeO(OH)·nH2O (Hydrated iron(III) oxide-hydroxide)
Hardness: 4.0-5.5; Color: Dark brown to earthy yellow; Luster: Dull, earthy; Crystal Structure: Amorphous or cryptocrystalline; Specific Gravity: 2.7-4.3
- Hardness
- 4
- Color
- Dark brown to earthy yellow
- Luster
- Dull, earthy
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Physical properties
Hardness: 4.0-5.5; Color: Dark brown to earthy yellow; Luster: Dull, earthy; Crystal Structure: Amorphous or cryptocrystalline; Specific Gravity: 2.7-4.3
Formation & geological history
Formed by the oxidation of iron-rich mineral deposits or through the precipitation of iron from groundwater in sedimentary environments like marshes or streambeds.
Uses & applications
Historically used as an iron ore; currently used as a natural earth pigment (ochre) and as low-value geological specimens for teaching.
Geological facts
Limonite is not a true mineral but a mineraloid. It was one of the first pigments used by early humans in cave paintings, such as those at Lascaux.
Field identification & locations
Identified by its yellow-brown streak on a porcelain plate and its earthy, non-metallic appearance. Common in boggy areas and oxidized zones of ore deposits.
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