sedimentary
Limonite (Limonitic Sandstone)
Hydrated Iron(III) Oxide-Hydroxide ($FeO(OH)·nH_2O$)
Hardness: 4-5.5 (variable); Color: Yellowish-brown to dark brown; Luster: Dull/Earthy; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3
- Hardness
- 4-5
- Color
- Yellowish-brown to dark brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 4-5.5 (variable); Color: Yellowish-brown to dark brown; Luster: Dull/Earthy; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3
Formation & geological history
Formed through the oxidation and hydration of iron-rich minerals (like pyrite or magnetite) in sedimentary environments or as a secondary precipitate in boggy or hydrothermal settings.
Uses & applications
Historically used as a low-grade iron ore and as a pigment for ochre. Today, it is primarily of interest for geological studies and amateur mineral collecting.
Geological facts
Limonite is not a true mineral but a mixture of hydrated iron oxide minerals. It was a primary source of iron for many ancient civilizations and is the source of the pigment 'yellow ochre'.
Field identification & locations
Identify by its characteristic 'rust' color and yellow-brown streak when rubbed on a porcelain plate. It is common in weathered sedimentary outcrops and near iron deposits worldwide.
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