sedimentary
Limonite (Ironstone Concretion)
Limonite (Hydrous Ferric Oxides mixed with Goethite)
Hardness: 4-5.5 (Mohs), Color: Dark brown to yellowish-orange, Luster: Dull to earthy, Crystal Structure: Amorphous/Microcrystalline, Specific Gravity: 2.7-4.3. Often has an 'orange' interior with a dark 'iron' skin.
- Hardness
- 4-5
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Physical properties
Hardness: 4-5.5 (Mohs), Color: Dark brown to yellowish-orange, Luster: Dull to earthy, Crystal Structure: Amorphous/Microcrystalline, Specific Gravity: 2.7-4.3. Often has an 'orange' interior with a dark 'iron' skin.
Formation & geological history
Formed by the precipitation of iron oxides from groundwater, often concentrated around a nucleus in sedimentary layers. These are frequently found in sandstone or clay-rich environments where oxidation occurs.
Uses & applications
Historically used as a low-grade iron ore (bog iron) and as a pigment for yellow or brown ochre. Often collected by amateur geologists for its unique 'rattle stone' or layered appearance.
Geological facts
Limonite is not a single mineral mineralogically, but a mixture of hydrated iron oxide minerals. It was one of the earliest pigments used by humans, found in cave paintings dating back thousands of years.
Field identification & locations
Identify in the field by its characteristic yellowish-brown streak (when rubbed on unglazed porcelain) and its often rusted appearance. It is commonly found in stream beds or eroded embankments as hard, rounded nodules.
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