sedimentary
Ironstone (likely Limonite or Goethite nodule)
Hydrothermally altered Ferric Hydroxide (FeO(OH)·nH2O)
Hardness: 4-5.5 (Mohs), Color: Dark brown to black with rusty patches, Luster: Dull to sub-metallic, Structure: Amorphous/microcrystalline, Specific Gravity: 3.3-4.3
- Hardness
- 4-5
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Physical properties
Hardness: 4-5.5 (Mohs), Color: Dark brown to black with rusty patches, Luster: Dull to sub-metallic, Structure: Amorphous/microcrystalline, Specific Gravity: 3.3-4.3
Formation & geological history
Formed through the precipitation of iron minerals from ground water or the weathering of iron-rich sulfide minerals. Often found as nodules or concretions in sedimentary basins.
Uses & applications
Primarily used as an ore of iron in historical industry; currently used for soil pigment (ochre) or as a minor decorative garden stone.
Geological facts
These nodules are often mistaken for meteorites because of their high density and dark, weathered 'fusion crust' appearance, which is actually just iron oxidation.
Field identification & locations
Identify in the field by its high weight-to-size ratio and its tendency to leave a yellow-brown streak if rubbed on unglazed porcelain. Commonly found in creek beds and clay deposits.
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