sedimentary
Limonite (Iron Ore)
Hydrated ferric oxide (FeO(OH)·nH2O)
Hardness: 4.0-5.5; Color: Yellowish-brown to dark brown; Luster: Dull to earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
- Hardness
- 4
- Color
- Yellowish-brown to dark brown
- Luster
- Dull to earthy
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Physical properties
Hardness: 4.0-5.5; Color: Yellowish-brown to dark brown; Luster: Dull to earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
Formation & geological history
Formed as a secondary mineral through the oxidation and hydration of other iron-bearing minerals like pyrite, magnetite, and siderite in the weathering zone. It often forms in bogs (bog iron) or as gossans over ore deposits.
Uses & applications
Major use as a low-grade iron ore; also used as a pigment (yellow ochre) in paints and dyes, and historically as a flux in glassmaking.
Geological facts
Limonite is not a true mineral but a mixture of similar hydrated iron oxide minerals, mostly goethite. It has been used by humans as a pigment since prehistoric times, appearing in cave paintings.
Field identification & locations
Identify by its distinctive yellow-brown streak, earthy appearance, and lack of magnetism compared to magnetite. Commonly found in boggy environments, weathered outcrops, and as a coating on other rocks. Collectors look for botryoidal (grape-like) forms.
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