sedimentary
Ironstone Concretion
Siderite or Limonite Concretion (FeCO3 or FeO(OH)·nH2O)
Hardness: 4.0-5.5 (Mohs scale); Color: Rusty brown, orange, or dark gray; Luster: Dull or earthy; Structure: Nodular, often with concentric layers; Specific Gravity: 3.0-4.0.
- Hardness
- 4
- Color
- Rusty brown, orange, or dark gray
- Luster
- Dull or earthy
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Physical properties
Hardness: 4.0-5.5 (Mohs scale); Color: Rusty brown, orange, or dark gray; Luster: Dull or earthy; Structure: Nodular, often with concentric layers; Specific Gravity: 3.0-4.0.
Formation & geological history
Formed by the precipitation of mineral cement (iron oxides or carbonates) around a nucleus, such as a fossil or organic fragment, within sedimentary layers like shale or sandstone. Common in Cretaceous and Paleozoic deposits.
Uses & applications
Used historically as low-grade iron ore; occasionally used in landscaping or as curiosities for fossil and mineral collectors.
Geological facts
Mazon Creek in Illinois is famous for concretions that split open to reveal exceptionally preserved soft-bodied fossils. Some concretions are mistaken for meteorites or dinosaur eggs due to their round shapes.
Field identification & locations
Identify in the field by their rounded or disc-like shape, high weight relative to size (high density), and reddish-brown weathering. Often found in riverbeds or eroding shale cliffs.
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