sedimentary
Limonite Concretion (Ironstone)
Limonite (FeO(OH)·nH2O) with Goethite and Hematite
Hardness: 4-5.5 (variable); Color: Ochre-yellow to rusty brown and dark reddish-black; Luster: Dull to earthy; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3
- Hardness
- 4-5
- Color
- Ochre-yellow to rusty brown and dark reddish-black
- Luster
- Dull to earthy
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Physical properties
Hardness: 4-5.5 (variable); Color: Ochre-yellow to rusty brown and dark reddish-black; Luster: Dull to earthy; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3
Formation & geological history
Formed through the chemical precipitation of iron-rich minerals around a nucleus (like a leaf, shell, or pebble) in sedimentary environments. Common in bog iron deposits, oxidized zones of ore deposits, and groundwater-saturated sandstones. Timeframe spans from Paleozoic to modern stages.
Uses & applications
Historically used as a low-grade iron ore and as a source of ochre pigment for paints. Today, specimens are primarily collected for geological education or as decorative curiosities.
Geological facts
Limonite is not a singular mineral but a mixture of hydrated iron oxides. It was a primary source of iron for early civilizations and the 'yellow ochre' pigment used in prehistoric cave paintings.
Field identification & locations
Identify by its distinctive rusty-orange exterior and high density compared to surrounding rocks. Often has a concentric 'onion-skin' layering if broken. Found globally in iron-rich sedimentary basins and riverbeds.
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