mineral
Limonite (Iron Ore)
Hydrated ferric oxide (FeO(OH)·nH2O)
Hardness: 4.0-5.5 (Mohs); Color: Yellow-brown to dark brown; Luster: Dull to earthy; Crystal structure: Amorphous; Cleavage: None; Specific gravity: 2.7-4.3
- Hardness
- 4
- Color
- Yellow-brown to dark brown
- Luster
- Dull to earthy
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Physical properties
Hardness: 4.0-5.5 (Mohs); Color: Yellow-brown to dark brown; Luster: Dull to earthy; Crystal structure: Amorphous; Cleavage: None; Specific gravity: 2.7-4.3
Formation & geological history
Formed as a secondary mineral through the oxidation and hydration of primary iron minerals like pyrite and magnetite. Found in gossans (weathered ore caps), bog deposits, and sedimentary layers.
Uses & applications
Historically used as a major iron ore and as a pigment for ochre paints. Today it's used as a chemical catalyst and occasionally in specimen collections.
Geological facts
Limonite is 'Fool's gold' that has essentially rusted. It is one of the pigments found in the famous prehistoric Lascaux cave paintings in France.
Field identification & locations
Identify by its yellowish-brown streak on a porcelain plate and lack of magnetism compared to magnetite. Found worldwide in oxidised zones of ore deposits.
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