sedimentary
Ironstone Concretion
Siderite or Ferruginous Concretion (FeCO3 or Iron Oxides)
Hardness: 3.5-4.5 (Mohs); Color: Rusty brown, reddish, or dark grey; Luster: Dull/Earthy; Crystal Structure: Trigonal (microcrystalline); Specific Gravity: 3.0-4.0
- Hardness
- 3
- Color
- Rusty brown, reddish, or dark grey
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3.5-4.5 (Mohs); Color: Rusty brown, reddish, or dark grey; Luster: Dull/Earthy; Crystal Structure: Trigonal (microcrystalline); Specific Gravity: 3.0-4.0
Formation & geological history
Formed through the precipitation of mineral cement within the spaces between sediment grains (usually sand or mud). They often grow around a nucleus such as a leaf, bone, or shell during diagenesis in sedimentary basins.
Uses & applications
Historically used as a low-grade iron ore; nowadays primarily used for landscaping, educational specimens, or cut open by collectors to reveal potential fossils inside.
Geological facts
Concretions are often mistaken for meteorites, eggs, or cannonballs due to their rounded shapes. The famous Mazon Creek fossils in Illinois are found inside similar ironstone concretions.
Field identification & locations
Identify in the field by their rounded or discoidal shape, heavy weight for their size, and distinct rusty exterior compared to surrounding rock. Common in shale or sandstone cliffs and riverbeds.
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