sedimentary
Ironstone Concretion
Siderite or Hematite-rich Concretion (FeCO3 / Fe2O3)
Hardness: 4.0-5.5 (Mohs), Color: Dark brown to reddish-brown, Luster: Dull or sub-metallic, Crystal structure: Microcrystalline, Cleavage: None/Conchoidal fracture, Specific gravity: 3.5-4.5.
- Hardness
- 4
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Physical properties
Hardness: 4.0-5.5 (Mohs), Color: Dark brown to reddish-brown, Luster: Dull or sub-metallic, Crystal structure: Microcrystalline, Cleavage: None/Conchoidal fracture, Specific gravity: 3.5-4.5.
Formation & geological history
Formed by the precipitation of mineral cement (iron oxides or carbonates) within the spaces between sediment particles, often around a nucleus like a leaf, shell, or bone. Common in shale or sandstone layers.
Uses & applications
Primarily of interest to collectors or as decorative items. Historically, some ironstone nodules were used as a minor iron ore.
Geological facts
These are often mistaken for meteorites or fossils due to their unusual shapes and high density. They are known as 'Moqui Marbles' if they are spherical and found in the American Southwest.
Field identification & locations
Identify by its heavy weight relative to size and smooth, weathered surface with possible shrinkage cracks. Common in desert environments and sedimentary basins.
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