sedimentary
Ironstone Concretion / Limonite Specimen
Goethite and Limonite (Hydrous Iron Oxides)
Hardness: 4.0-5.5 (variable); Color: Yellowish-brown to dark brown/black; Luster: Dull, earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
- Hardness
- 4
- Color
- Yellowish-brown to dark brown/black
- Luster
- Dull, earthy
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Physical properties
Hardness: 4.0-5.5 (variable); Color: Yellowish-brown to dark brown/black; Luster: Dull, earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
Formation & geological history
Formed through the precipitation of iron oxide minerals within sedimentary layers, often around a nucleus like a leaf, shell, or pebble. They occur in boggy environments, lake beds, or through the weathering of iron-rich rocks over thousands to millions of years.
Uses & applications
Historically used as a low-grade iron ore. Today, primarily used for geological education, landscaping, and as curiosities for collectors.
Geological facts
Limonite is not a true mineral but a mixture of various hydrous iron oxides. It was a primary source of 'ochre' pigment used in prehistoric cave paintings due to its distinct yellow-brown streak.
Field identification & locations
Identify in the field by its high density relative to other sedimentary rocks and its characteristic rusty yellow-brown streak when scratched. Common in boggy soil and sandstone formations.
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