sedimentary
Ironstone Concretion / Hematite Nodules
Ferruginous Sedimentary Rock (primarily Hematite Fe2O3 and Goethite FeO(OH))
Hardness: 5-6 on Mohs scale; Color: Reddish-brown to yellowish-ochre; Luster: Earthy to metallic; Structure: Often nodular or cylindrical; Cleavage: None; Specific Gravity: 4.5-5.3.
- Hardness
- 5-6 on Mohs scale
- Color
- Reddish-brown to yellowish-ochre
- Luster
- Earthy to metallic
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Physical properties
Hardness: 5-6 on Mohs scale; Color: Reddish-brown to yellowish-ochre; Luster: Earthy to metallic; Structure: Often nodular or cylindrical; Cleavage: None; Specific Gravity: 4.5-5.3.
Formation & geological history
Formed by the precipitation of iron-rich minerals from groundwater within the pore spaces of sedimentary rocks like sandstone or shale. These often form around a nucleus (like a fossil or organic matter) during the diagenesis process, frequently in the Cretaceous or Paleogene periods.
Uses & applications
Primary source of iron ore for industrial steel production. Historically used as earth pigments (ochre). Collected by amateur geologists for their unique rounded or cylindrical shapes.
Geological facts
Small, spherical hematite concretions found on Mars, nicknamed 'blueberries,' helped scientists prove the past presence of liquid water on the red planet. Terrestrial versions are often mistaken for meteorites or fossils due to their weight and odd shapes.
Field identification & locations
Identify in the field by their high density (feel heavy for their size) and reddish-brown streak when rubbed against unglazed porcelain. Commonly found in eroded sedimentary basins and badlands.
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