sedimentary
Limonite (Ironstone Concretion)
Limonite (Hydrated iron(III) oxide-hydroxide, mixtures of FeO(OH)·nH2O)
Hardness: 4.0-5.5 (variable); Color: Yellowish-brown to dark brown; Luster: Dull or earthy; Crystal Structure: Amorphous/microcrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
- Hardness
- 4
- Color
- Yellowish-brown to dark brown
- Luster
- Dull or earthy
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Physical properties
Hardness: 4.0-5.5 (variable); Color: Yellowish-brown to dark brown; Luster: Dull or earthy; Crystal Structure: Amorphous/microcrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
Formation & geological history
Formed through the chemical weathering of iron-rich minerals (like pyrite or magnetite) or by precipitation in bogs. This specimen likely represents an ironstone concretion formed within sedimentary layers via localized chemical precipitation.
Uses & applications
A major ore of iron historically; currently used as a yellow ochre pigment for paints and in the production of low-cost iron-rich fillers.
Geological facts
Limonite is not a 'true mineral' but a 'mineraloid' as it is a mixture of several iron oxide minerals. It has been used as a pigment (ochre) by humans since the Cave Art period roughly 30,000 years ago.
Field identification & locations
Identify in the field by its distinct yellow-brown 'rusty' streak when rubbed on unglazed porcelain and its lack of magnetism. Often found in weathered outcrops of sedimentary rocks and near iron-sulfide deposits.
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