sedimentary
Limonite Concretion
Limonite (Hydrated iron(III) oxide-hydroxides, FeO(OH)·nH2O)
Hardness: 4-5.5; Color: Dark brown to yellowish-brown; Luster: Dull or earthy; Crystal Structure: Amorphous; Streak: Yellow-brown; Specific Gravity: 2.7-4.3.
- Hardness
- 4-5
- Color
- Dark brown to yellowish-brown
- Luster
- Dull or earthy
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Physical properties
Hardness: 4-5.5; Color: Dark brown to yellowish-brown; Luster: Dull or earthy; Crystal Structure: Amorphous; Streak: Yellow-brown; Specific Gravity: 2.7-4.3.
Formation & geological history
Formed by the chemical precipitation of iron-rich fluids within sedimentary rock layers, often as groundwater moves through sand or clay environments. They can form at various geological ages and often grow concentrically around a nucleus.
Uses & applications
Historically used as a low-grade iron ore and as a pigment for ochre. Today, they are primarily of interest to collectors and hobbyists.
Geological facts
Limonite is not a true mineral but a mixture of similar hydrated iron oxide minerals. Concretions like this are often mistaken for meteorites or fossils due to their unusual shapes.
Field identification & locations
Identify in the field by its characteristic yellow-brown streak, heavy weight for its size, and common occurrence in sandstone or shale beds. Often found in streambeds where the harder concretion has eroded out of softer rock.
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