sedimentary
Limonite on Sandstone
Iron Oxyhydroxide (FeO(OH)·nH2O) on Quartz Sandstone
Hardness: 4-5.5 (limonite) and 7 (sandstone); Color: Yellow-ochre to brownish-yellow streaks; Luster: Dull, earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Grained texture.
- Hardness
- 4-5
- Color
- Yellow-ochre to brownish-yellow streaks
- Luster
- Dull, earthy
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Physical properties
Hardness: 4-5.5 (limonite) and 7 (sandstone); Color: Yellow-ochre to brownish-yellow streaks; Luster: Dull, earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Grained texture.
Formation & geological history
Formed via the chemical weathering of iron-rich minerals such as pyrite or magnetite within sedimentary layers. The limonite precipitates as a secondary mineral coating sandstone grains or filling pore spaces.
Uses & applications
Historically used as a yellow ochre pigment for paints; also an ore of iron. Sandstone is widely used in construction and landscaping.
Geological facts
Limonite is not a single mineral but a mixture of hydrated iron oxide minerals, primarily goethite. It was the source of some of the earliest pigments used by humans for cave paintings.
Field identification & locations
Identify by the characteristic ochre-yellow streak it leaves on unglazed porcelain. Commonly found in oxidized zones of mineral deposits and in sedimentary basins where groundwater flow is prevalent.
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