sedimentary
Limonite/Ironstone
Limonite (Hydrous ferric oxides), FeO(OH)·nH2O
Hardness: 4-5.5 (Mohs scale), Color: Yellowish-brown to reddish-brown, Luster: Dull to earthy, Crystal Structure: Amorphous/microcrystalline, Cleavage: None, Specific Gravity: 2.7-4.3
- Hardness
- 4-5
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Physical properties
Hardness: 4-5.5 (Mohs scale), Color: Yellowish-brown to reddish-brown, Luster: Dull to earthy, Crystal Structure: Amorphous/microcrystalline, Cleavage: None, Specific Gravity: 2.7-4.3
Formation & geological history
Formed by the oxidation (rusting) of iron-rich minerals such as pyrite or magnetite. It is secondary in origin, typically found in the weathered zones of iron ore deposits or as a bog iron precipitate in swampy conditions.
Uses & applications
Historically used as a major source of iron ore; currently used primarily as a natural earth pigment (yellow ochre) in paints, sealers, and concrete coloring.
Geological facts
Limonite was the primary source of iron for many early civilizations before modern mining of hematite. It is not a true mineral but a mixture of hydrous iron oxides like goethite.
Field identification & locations
Identify in the field by its distinct yellow-brown streak (if rubbed on porcelain) and its earthy, rusted appearance. It is commonly found in sedimentary layer caps or as bog iron in wet environments.
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