sedimentary
Limonite/Goethite Concretion
Hydrated Iron(III) Oxide-hydroxide (FeO(OH)·nH2O)
Hardness: 4-5.5 (variable); Color: Yellowish-brown to dark brown; Luster: Dull, earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
- Hardness
- 4-5
- Color
- Yellowish-brown to dark brown
- Luster
- Dull, earthy
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Physical properties
Hardness: 4-5.5 (variable); Color: Yellowish-brown to dark brown; Luster: Dull, earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
Formation & geological history
Formed through the oxidation and hydration of iron-rich mineral deposits in sedimentary environments. Often found as nodules or concretions in sandstone, shale, or limestone that have weathered over millions of years.
Uses & applications
Historically used as a low-grade iron ore and as a pigment for ochre paint. Occasionally used as a decorative stone or for lapidary tumbling if solid enough.
Geological facts
Limonite is not a true mineral but a 'mineraloid' or a mixture of various hydrated iron oxides. It has been used as a pigment (brown ochre) since prehistoric times for cave paintings.
Field identification & locations
Identified in the field by its distinct mustard-yellow to reddish-brown streak and pitted, 'rusted' appearance. It is commonly found in wetlands, bogs, and as a weathering product of hydrothermal veins.
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