sedimentary
Limonite Concretion
Limonite (Hydrated iron(III) oxide-hydroxide) FeO(OH)·nH2O
Hardness: 4-5.5; Color: Dark brown to reddish-brown; Luster: Dull or earthy; Structure: Amorphous/cryptocrystalline; Cleavage: None; SG: 2.7-4.3
- Hardness
- 4-5
- Color
- Dark brown to reddish-brown
- Luster
- Dull or earthy
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Physical properties
Hardness: 4-5.5; Color: Dark brown to reddish-brown; Luster: Dull or earthy; Structure: Amorphous/cryptocrystalline; Cleavage: None; SG: 2.7-4.3
Formation & geological history
Formed by the chemical precipitation of iron-rich minerals in sedimentary environments, often through the weathering of iron sulfides like pyrite or siderite in boggy or groundwater-saturated soils.
Uses & applications
Historically used as an iron ore (yellow ochre pigment). Today, mainly a collector's item or geological curiosity.
Geological facts
Limonite is not a single mineral but a mixture of hydrated iron oxide minerals, mostly goethite. Formations like this are often called 'ironstone' and can sometimes form hollow 'rattle stones' if a sandy core shrinks inside.
Field identification & locations
Identify in the field by its high density relative to other sedimentary rocks and its distinctive yellowish-brown streak when rubbed on a porcelain plate. Found commonly in sedimentary basins and bog environments.
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