mineral
Limonite / Goethite Ironstone
FeO(OH)·nH2O (Hydrous iron oxide)
Hardness: 4.0-5.5 on Mohs scale. Color: Yellowish-brown, reddish-brown, dark brown to blackish. Luster: Earthy, dull, to sub-metallic. Crystal System: Amorphous or fine-grained microcrystalline aggregates. Cleavage: None. Specific Gravity: 2.7 - 4.3.
- Hardness
- 4
- Color
- Yellowish-brown, reddish-brown, dark brown to blackish
- Luster
- Earthy, dull, to sub-metallic
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Physical properties
Hardness: 4.0-5.5 on Mohs scale. Color: Yellowish-brown, reddish-brown, dark brown to blackish. Luster: Earthy, dull, to sub-metallic. Crystal System: Amorphous or fine-grained microcrystalline aggregates. Cleavage: None. Specific Gravity: 2.7 - 4.3.
Formation & geological history
Formed primarily as a secondary mineral through the weathering and oxidation of iron-rich primary minerals such as pyrite, marcasite, and siderite in near-surface aqueous environments. Common in oxidized zones of ore deposits (gossans) and sedimentary ironstones.
Uses & applications
Historically used as an important iron ore and a natural pigment (yellow ochre, raw umber). Currently used as a mineral specimen for educational purposes and local iron production in some regions.
Geological facts
Limonite isn't a true single mineral, but rather a field name for mixtures of hydrated iron oxide minerals, mainly goethite and lepidocrocite. It often forms pseudomorphs after other minerals like pyrite, replacing the original mineral while preserving its original shape.
Field identification & locations
Identified in the field by its characteristic rusty reddish-brown to yellowish earthy coating, high density relative to typical sedimentary rocks, and yellowish-brown streak on an unglazed porcelain plate. Commonly found in weathered iron-bearing rock outcrops, stream beds, and bog iron deposits.
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