sedimentary
Limonite Concretion
Hydrated iron(III) oxide-hydroxide (FeO(OH)·nH2O)
Hardness: 4-5.5 on Mohs scale; Color: Yellowish-brown to dark brown; Luster: Dull/earthy; Crystal Structure: Amorphous (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 on Mohs scale; Color: Yellowish-brown to dark brown; Luster: Dull/earthy; Crystal Structure: Amorphous (microcrystalline); Cleavage: None; Specific Gravity: 2.7–4.3
Formation & geological history
Formed through the chemical precipitation of iron-rich minerals in sedimentary environments, often triggered by groundwater moving through sandstone or clay beds. Can range from Recent to Paleozoic in age.
Uses & applications
Historically used as an iron ore; currently used as a pigment (yellow ochre) in paints and dyes, and frequently kept as a geological 'oddity' or specimen by collectors.
Geological facts
Limonite is not a true mineral but a 'mineraloid' composed of various hydrous iron oxides like goethite. Concretions like this one often form around a nucleus, such as a fossil or a pebble, growing outward in concentric layers.
Field identification & locations
Identify in the field by its distinct rusty yellow-brown color, earthy texture, and relatively high density compared to surrounding rocks. Commonly found in weathered sedimentary basins, stream beds, and iron-bearing soils.
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