sedimentary
Limonite (Iron-rich Siltstone / Sandstone base)
Hydrous ferric oxide mixture (FeO(OH)·nH2O)
Hardness: 4-5.5 (Mohs); Color: yellowish-brown to deep rust; Luster: earthy to submetallic; Crystal structure: amorphous; Cleavage: none; Specific Gravity: 2.7–4.3.
- Hardness
- 4-5
- Color
- yellowish-brown to deep rust
- Luster
- earthy to submetallic
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Physical properties
Hardness: 4-5.5 (Mohs); Color: yellowish-brown to deep rust; Luster: earthy to submetallic; Crystal structure: amorphous; Cleavage: none; Specific Gravity: 2.7–4.3.
Formation & geological history
Formed as a secondary material through the oxidation and hydration of iron-rich mineral ores (like pyrite or magnetite) or as a precipitate in swamps (bog iron) and shallow marine environments. Often found in weathered zones of hydrothermal deposits.
Uses & applications
Historically used as a minor iron ore and as a source of yellow and brown ochre pigments for paints. Currently of interest primarily to collectors and as an indicator of mineralized zones.
Geological facts
Limonite is not a true mineral but a 'mineraloid'—a mixture of similar hydrated iron oxide minerals like goethite. It is responsible for the yellowish-brown coloring of many soils and weathered rock faces.
Field identification & locations
Identify in the field by its characteristic yellowish-brown streak (when rubbed on unglazed porcelain) and its typical earthy, rusty appearance. Often found in sedimentary beds or as coatings on other rocks.
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