sedimentary
Ironstone Concretion (Limonite/Goethite)
Goethite/Limonite concretion (FeO(OH)·nH2O)
Hardness: 4-5.5 (Mohs scale); Color: Yellowish-brown to dark brown or black; Luster: Dull to sub-metallic; Structure: Often amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 3.3-4.3.
- Hardness
- 4-5
- Color
- Yellowish-brown to dark brown or black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 4-5.5 (Mohs scale); Color: Yellowish-brown to dark brown or black; Luster: Dull to sub-metallic; Structure: Often amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 3.3-4.3.
Formation & geological history
Formed by the precipitation of iron oxide minerals from groundwater within sedimentary rock layers, often around a biological nucleus like a seed or small fossil. Common in sandstone and mudstone formations.
Uses & applications
Historically used as iron ore. Today, used as a pigment (ochre), soil conditioner, or as a decorative 'curiosity' for collectors.
Geological facts
These are often mistaken for meteorites or small fossils because of their unusual, organic-like shapes. Some varieties are known as 'Moqui Marbles' or 'shaman stones'.
Field identification & locations
Identify in the field by its heavy weight relative to size and its distinct yellowish-brown streak if rubbed on unglazed porcelain. Commonly found in desert environments or eroded sandstone beds.
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