Mineral
Limonite (Iron Ore)
Limonite (an FeO(OH)·nH2O mineraloid, not a true mineral due to variable composition)
AI-generated identification · may be incorrectHardness: 4-5.5 on Mohs scale; Color: Brown, yellowish-brown, reddish-brown, dark brown; Luster: Dull, earthy, sometimes submetallic; Crystal Structure: Amorphous or cryptocrystalline, rarely forms distinct crystals (usually botryoidal, stalactitic, or massive); Cleavage: None; Specific Gravity: 2.7-4.3
- Hardness
- 4-5
- Color
- Brown, yellowish-brown, reddish-brown, dark brown
- Luster
- Dull, earthy, sometimes submetallic
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Physical properties
Hardness: 4-5.5 on Mohs scale; Color: Brown, yellowish-brown, reddish-brown, dark brown; Luster: Dull, earthy, sometimes submetallic; Crystal Structure: Amorphous or cryptocrystalline, rarely forms distinct crystals (usually botryoidal, stalactitic, or massive); Cleavage: None; Specific Gravity: 2.7-4.3
Formation & geological history
Limonite is a common secondary mineral formed from the hydration of iron oxides and hydroxides. It often results from the weathering and oxidation of iron-bearing sulfide minerals like pyrite or magnetite, and also from other iron-rich rocks. It can form in various environments, including swamps, bogs, and as a component of gossans (oxidized cap of an ore deposit). Its formation is ongoing in surface environments.
Uses & applications
Historically, Limonite was a significant iron ore, especially for early iron smelting. Today, it is less important as a primary iron ore due to lower iron content compared to hematite or magnetite, but it can still be used. It is also used as a pigment (ochre) and as a moderate abrasive. Some botryoidal or stalactitic forms are collected as mineral specimens.
Geological facts
Limonite is not a true mineral in the strictest sense because its composition is variable and it lacks a consistent crystal structure; rather, it is a mineraloid. It is often a mixture of hydrous iron oxides like goethite and lepidocrocite. The characteristic 'rust' color of many weathered rocks and soils comes from limonite.
Field identification & locations
In the field, Limonite can be identified by its distinctive brown to yellowish-brown streak (even if the specimen itself is dark brown), its earthy luster, and its relatively low specific gravity for an iron-rich material. It often forms as coatings or crusts on other minerals or rocks. It is commonly found in weathered iron ore deposits, bogs, and as concretions in sedimentary rocks. Collectors can look for well-formed botryoidal or stalactitic specimens.
