sedimentary
Ironstone Concretion (Limonite/Goethite)
Iron oxide concretion (contains FeO(OH)·nH2O)
Hardness: 4-5.5 on Mohs scale; Color: Rusty brown, ochre, or dark tan; Luster: Dull to earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7-4.3.
- Hardness
- 4-5
- Color
- Rusty brown, ochre, or dark tan
- Luster
- Dull to earthy
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Physical properties
Hardness: 4-5.5 on Mohs scale; Color: Rusty brown, ochre, or dark tan; Luster: Dull to earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7-4.3.
Formation & geological history
Formed by the precipitation of iron oxide minerals within the spaces between sediment grains, often around a nucleus like a leaf, shell, or fossil. These typically form in sedimentary basins or soil horizons where groundwater is rich in dissolved iron.
Uses & applications
Historically used as a low-grade iron ore. Today, used primarily for decorative landscaping, educational specimens, or occasionally as a pigment (ochre).
Geological facts
These are often mistaken for meteorites or 'fossilized poop' (coprolites) due to their strange, organic shapes. They can range from marble-sized 'Moqui marbles' to several feet in diameter.
Field identification & locations
Identify in the field by its heavy feel relative to its size, rusty streak when rubbed on unglazed porcelain, and lack of internal crystalline structure. Common in sandstone or shale formations.
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