sedimentary
Limonite pseudo-concretion (Ironstone)
Limonite (FeO(OH)·nH2O) / Goethite
Hardness: 4.0–5.5; Color: Dark brown to metallic yellowish-brown; Luster: Submetallic, dull, or earthy; Cleavage: None; Opaque with a characteristic yellowish-brown streak.
- Hardness
- 4
- Color
- Dark brown to metallic yellowish-brown
- Luster
- Submetallic, dull, or earthy
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Physical properties
Hardness: 4.0–5.5; Color: Dark brown to metallic yellowish-brown; Luster: Submetallic, dull, or earthy; Cleavage: None; Opaque with a characteristic yellowish-brown streak.
Formation & geological history
Forms through the chemical weathering of iron-rich minerals in sedimentary rocks or as a precipitate in oxygenated water. Many are Cretaceous or younger, formed in boggy or swampy environments.
Uses & applications
Historically used as a low-grade iron ore. Today, it is primarily used as a pigment (brown ochre) and as a curiosity for amateur geologists due to its unique textures.
Geological facts
Limonite is not a specific mineral but a mixture of hydrated iron oxide minerals. It often forms 'pseudomorphs', where it replaces the internal structure of fossils or other minerals while keeping their shape.
Field identification & locations
Look for its high density relative to its size and its distinctive yellow-brown streak when rubbed on unglazed porcelain. Commonly found in creek beds or loose soil in iron-rich sedimentary basins.
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