sedimentary
Limonite (Ironstone) Concretion
FeO(OH)·nH2O (Hydrous ferric oxide)
Hardness: 4-5.5 (Mohs scale); Color: Yellow-brown to dark brown/black; Luster: Dull to sub-metallic; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
- Hardness
- 4-5
- Color
- Yellow-brown to dark brown/black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 4-5.5 (Mohs scale); Color: Yellow-brown to dark brown/black; Luster: Dull to sub-metallic; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
Formation & geological history
Formed through the oxidation and hydration of iron-rich minerals in sedimentary environments. Common in bog iron deposits or as cement in sandstones. Found globally, especially in oxidized zones of iron ore deposits.
Uses & applications
Historically used as a low-grade iron ore. Currently used as an ocher pigment in paints and as a geological specimen for education and mineral collecting.
Geological facts
Limonite is not a true mineral but a mineraloid mixture of various hydrous iron oxides. It gives many desert sands and soils their characteristic rusty yellow-brown color. The specimen displays a typical 'desert varnish' or weathered crust appearance.
Field identification & locations
Identify by its brownish-yellow streak, lack of magnetism (unlike magnetite), and density. Common in the coordinates provided (Visakhapatnam, India region), which features khondalite and charnockite belts known for iron-rich weathering products, laterites, and hematite/limonite deposits.
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