mineral
Limonite (Iron Stone Concretion)
Limonite (Hydrous Iron Oxide) - FeO(OH)·nH2O
Hardness: 4-5.5 (Mohs scale), Color: Yellow-brown to blackish-brown, Luster: Dull or earthy, Crystal Structure: Amorphous (no visible crystals), Cleavage: None, Specific Gravity: 2.7-4.3.
- Hardness
- 4-5
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Physical properties
Hardness: 4-5.5 (Mohs scale), Color: Yellow-brown to blackish-brown, Luster: Dull or earthy, Crystal Structure: Amorphous (no visible crystals), Cleavage: None, Specific Gravity: 2.7-4.3.
Formation & geological history
Formed as a secondary material through the oxidation (weathering) of iron-rich mineral ores like pyrite and magnetite. Often found in bog iron deposits or as sedimentary cement. Its age varies by sedimentary basin but is often found in Mesozoic strata.
Uses & applications
Historically used as a low-grade iron ore and as a pigment for ochre paints. Today, it is primarily of interest for geological studies and minor construction fill.
Geological facts
Limonite is the primary ingredient in yellow ochre, used by prehistoric humans as one of the first pigments for cave paintings. Unlike many minerals, it does not have a specific chemical formula as it is a mixture of various iron oxides.
Field identification & locations
Identify by its yellowish-brown streak on a porcelain plate and lack of magnetism (unlike magnetite). It often forms rounded 'concretions' or nodular shapes in soils and stream beds. Common in iron-rich sedimentary environments worldwide.
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