sedimentary
Ironstone Concretion
Goethite-rich Siltstone / Siderite (FeCO3)
Hardness: 4.0-5.5 (Mohs), Color: Rusty brown, yellowish-orange to dark grey, Luster: Dull/earthy/metallic, Crystal Structure: Amorphous or Cryptocrystalline, cleavage: none
- Hardness
- 4
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Physical properties
Hardness: 4.0-5.5 (Mohs), Color: Rusty brown, yellowish-orange to dark grey, Luster: Dull/earthy/metallic, Crystal Structure: Amorphous or Cryptocrystalline, cleavage: none
Formation & geological history
Formed by the precipitation of mineral cement within the spaces between sediment grains. This often occurs in claystones or siltstones when iron-rich fluids react with organic matter or groundwater during diagenesis.
Uses & applications
Historically used as a low-grade iron ore; commonly collected as geological curiosities due to their unique shapes (sometimes resembling fossils).
Geological facts
Ironstones can sometimes form 'Septarian nodules' with internal cracks filled with calcite. They are often mistaken for meteorites or dinosaur eggs by amateur collectors due to their weight and shape.
Field identification & locations
Identify in the field by its high density relative to surrounding rocks and its rusty, oxidized outer crust. Commonly found in eroded creek beds or shale outcrops. Use a magnet to check for high iron content.
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