sedimentary
Ferruginous Sandstone (Ironstone)
Arenite with Limonite/Goethite cement
Hardness: 6-7 (individual quartz grains) but overall rock can be friable (3-5). Color: Rusty brown, orange, or deep ochre. Luster: Dull to earthy. Crystal Structure: Granular/clastic textures. Cleavage: None. Specific Gravity: 2.0-3.0.
- Color
- Rusty brown, orange, or deep ochre
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 (individual quartz grains) but overall rock can be friable (3-5). Color: Rusty brown, orange, or deep ochre. Luster: Dull to earthy. Crystal Structure: Granular/clastic textures. Cleavage: None. Specific Gravity: 2.0-3.0.
Formation & geological history
Formed through the lithification of sand-sized grains (mostly quartz) cemented together by iron oxides like limonite or hematite. Typically found in riverbeds, coastal areas, or ancient sedimentary basins where iron-rich water percolated through sand deposits.
Uses & applications
Primarily used in construction as fill material or aggregate. Higher-quality ironstone was historically used for smelting iron. Fine-colored specimens are sometimes used as landscape stones or for primitive pigment (ochre).
Geological facts
The distinct rusty color is caused by atmospheric oxidation of iron minerals, very similar to the process of steel rusting. This type of rock is often responsible for the 'red rock' scenery found in national parks across the American Southwest.
Field identification & locations
Identify in the field by its gritty sandpaper-like texture and its tendency to leave a yellowish or reddish streak on a ceramic plate. Common in areas with high groundwater levels or near bog iron deposits. Collectors look for unique banding patterns (Liesegang rings).
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