sedimentary
Limonite / Ironstone Concretion
Hydrated Iron Oxide (FeO(OH)%C2%B7nH2O)
Hardness: 4 to 5.5 on Mohs scale. Color: Yellowish-brown, dark brown, or blackish. Luster: Earthy, dull, or sub-metallic. Texture: Rough, granular, or cemented silt/sand matrix.
- Hardness
- 4 to 5
- Color
- Yellowish-brown, dark brown, or blackish
- Luster
- Earthy, dull, or sub-metallic
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Physical properties
Hardness: 4 to 5.5 on Mohs scale. Color: Yellowish-brown, dark brown, or blackish. Luster: Earthy, dull, or sub-metallic. Texture: Rough, granular, or cemented silt/sand matrix.
Formation & geological history
Formed through the chemical precipitation of iron oxide minerals (such as goethite and hematite) from iron-rich groundwater, coating and cementing surrounding sediment particles.
Uses & applications
Historically used as an ore of iron and as a natural earth pigment (yellow ocher/sienna). Generally used as aggregate or collected as minor geological curiosity.
Geological facts
Limonite is not a true single mineral, but rather a mixture of hydrated iron oxide minerals. It often forms crusts, concretions, or iron-cemented sandstone fragments.
Field identification & locations
Identified by its distinctive yellowish-brown to dark rusty brown color and streak, high density compared to normal sandstone, and rough, weathered surface. Commonly found in sedimentary basins and weathered rock outcrops.
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