igneous
Limonite Stained Basalt
Hydrated Iron Oxide on Mafic Volcanic Rock (FeO(OH)·nH2O)
Hardness: 5-5.5 (limonite) / 6 (basalt); Color: Rusty orange to yellow-brown on charcoal black/grey; Luster: Dull or earthy; Structure: Microcrystalline; Specific Gravity: Approx 3.0
- Hardness
- 5-5
- Color
- Rusty orange to yellow-brown on charcoal black/grey
- Luster
- Dull or earthy
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Physical properties
Hardness: 5-5.5 (limonite) / 6 (basalt); Color: Rusty orange to yellow-brown on charcoal black/grey; Luster: Dull or earthy; Structure: Microcrystalline; Specific Gravity: Approx 3.0
Formation & geological history
Formed from the surface oxidation and hydration of iron-rich mineral crystals (like olivine or pyroxene) within igneous rock. This process typically occurs in the Cenozoic era due to weathering in moist environments.
Uses & applications
Primarily used as industrial iron ore in large deposits; the base basalt rock is used for construction aggregate and road base.
Geological facts
Limonite isn't a single mineral but a mixture of hydrated iron oxide minerals. It was historically used as a pigment (yellow ochre) in ancient cave paintings and is a common indicator of underlying mineralization.
Field identification & locations
Identify by its distinctive rusty streak and presence on dark, fine-grained volcanic rock. Common in volcanic fields and weathered outcrops worldwide.
