Mineral (Sedimentary context)
Limonite (Hydrated Iron Oxide / Bog Iron)
FeO(OH)·nH2O
Hardness: 4.0-5.5 (Mohs scale); Color: Yellowish-brown to dark brown; Luster: Dull/Earthy; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3.
- Hardness
- 4
- Color
- Yellowish-brown to dark brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 4.0-5.5 (Mohs scale); Color: Yellowish-brown to dark brown; Luster: Dull/Earthy; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3.
Formation & geological history
Formed by the chemical weathering of iron-rich minerals (like pyrite or magnetite) or through biological activity in swampy, oxygen-poor environments. It is a secondary mineral that often forms as a coating or mass in sedimentary layers.
Uses & applications
Historically used as an lower-grade iron ore and as a source of yellow ochre pigment. Currently used primarily in pigments and occasionally for low-value mineral specimens.
Geological facts
Limonite is not a 'true' mineral but a mineraloid mixture of various hydrated iron oxides. It was a primary source of iron for early civilizations due to its relative ease of mining in bogs.
Field identification & locations
Identify by its distinctive yellowish-brown streak on a porcelain plate and earthy texture. Commonly found in gossans (weathered ore deposits) and sedimentary basins. Look for 'rust-like' coatings on other rocks.
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