mineral
Limonite (Pseudomorph after Pyrite/Marcasite)
Hydrated Iron Oxide (FeO(OH)·nH2O)
Hardness: 4-5.5 (Mohs); Color: yellowish-brown to deep ochre; Luster: dull, earthy, or sub-metallic; Crystal structure: Amorphous or microcrystalline; Cleavage: none; Specific gravity: 2.7-4.3.
- Hardness
- 4-5
- Color
- yellowish-brown to deep ochre
- Luster
- dull, earthy, or sub-metallic
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Physical properties
Hardness: 4-5.5 (Mohs); Color: yellowish-brown to deep ochre; Luster: dull, earthy, or sub-metallic; Crystal structure: Amorphous or microcrystalline; Cleavage: none; Specific gravity: 2.7-4.3.
Formation & geological history
Formed through the oxidation and hydration of iron-rich sulfide minerals (like pyrite) in the weathering zone of ore deposits. This specimen appears to be a 'pseudomorph,' where limonite replaced a precursor mineral's crystal shape, likely during the Holocene to Pleistocene epochs.
Uses & applications
Historically used as a major iron ore and as a natural pigment (ochre) for paints. In modern times, it is primarily a minor ore and a specimen for mineral collectors.
Geological facts
Limonite is not a true mineral but a mixture of related hydrated iron oxides. It was the source of 'yellow ochre' used in prehistoric cave paintings, such as those in Lascaux, France.
Field identification & locations
Identify in the field by its characteristic yellow-brown streak (produced by scratching it against unglazed porcelain), its heavy weight for its size, and its earthy smell when damp. Common in bog iron deposits and oxidized caps of sulfide mines.
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