sedimentary
Limonite (Ironstone Concretion)
Limonite (Hydrous ferric oxide mixture, FeO(OH)·nH2O)
Hardness: 4-5.5 (Mohs); Color: Yellow-brown to dark brown; Luster: Dull/Earthy; Crystal Structure: Amorphous; Streak: Yellowish-brown; Specific Gravity: 2.7-4.3
- Hardness
- 4-5
- Color
- Yellow-brown to dark brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 4-5.5 (Mohs); Color: Yellow-brown to dark brown; Luster: Dull/Earthy; Crystal Structure: Amorphous; Streak: Yellowish-brown; Specific Gravity: 2.7-4.3
Formation & geological history
Formed through the chemical weathering of iron-rich minerals such as pyrite, magnetite, and hematite. It often precipitates from bog water or forms as secondary enrichment in oxidized zones of ore deposits.
Uses & applications
Historically used as a low-grade iron ore and as a source of yellow ochre pigment. Currently used in soil conditioning and as a minor collector's specimen.
Geological facts
Limonite is not a single mineral but a mixture of hydrous iron oxides, primarily Goethite. It was the primary source of 'bog iron' used during the early Industrial Revolution.
Field identification & locations
Identify in the field by its distinctive yellowish-brown streak and earthy appearance. Commonly found in gossans (iron hats), stream beds, and sedimentary layers where iron has leached and re-precipitated.
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