sedimentary

Siderite Concretion (Nodule)

Iron Carbonate (FeCO3) Concretion

AI-generated identification · may be incorrect

Hardness: 3.5-4.5 (Mohs scale), Color: Brown to reddish-brown (due to iron oxidation), Luster: Dull or earthy, Structure: Typically rounded or disc-shaped with concentric layering, Specific Gravity: 3.8

Hardness: 3

Identified April 3, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of mineral cement within the spaces between sediment grains (like sand or mud), often around a nucleus such as a leaf or bone. Most common in Coal Measures from the Carboniferous period.

Uses & applications

Historically used as a low-grade iron ore. Today, they are primarily of interest to paleontologists and amateur fossil collectors as they often contain well-preserved fossils inside.

Geological facts

Many of the world's best fossils from the Carboniferous period, including the famous Mazon Creek flora and fauna, were found inside siderite concretions that protected them from erosion.

Field identification & locations

Identify by its heavy weight (relative to size) and typical 'ovoid' or pancake shape. Found in shale beds and near coal seams. Collectors often split them open using a hammer to look for fossilized remains.