sedimentary
Limonite Concretion
Limonite (hydrated iron(III) oxide-hydroxide, FeO(OH)nH2O)
Hardness: 4-5.5 (Mohs scale); Color: Dark brown to yellow-brown; Luster: Dull or earthy; Crystal structure: Amorphous; Mohs hardness is slightly variable based on water content.
- Hardness
- 4-5
- Color
- Dark brown to yellow-brown
- Luster
- Dull or earthy
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Physical properties
Hardness: 4-5.5 (Mohs scale); Color: Dark brown to yellow-brown; Luster: Dull or earthy; Crystal structure: Amorphous; Mohs hardness is slightly variable based on water content.
Formation & geological history
Formed by the precipitation of iron oxide minerals from groundwater within sedimentary layers. Often forms around a nucleus like a leaf, shell, or pebble during the diagenesis process.
Uses & applications
Historically used as an iron ore; currently used as a natural earth pigment (ochre) and as decorative specimens for collectors.
Geological facts
Limonite is not a single mineral but a mixture of related hydrated iron oxide minerals. It often forms the 'rust' on weathered iron-bearing rocks.
Field identification & locations
Look for rounded or nodular shapes that are heavy for their size due to iron content. Found frequently in sandstone and shale formations. Collectors look for unique shapes known as 'Moqui marbles' or 'bog iron'.
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