sedimentary
Ironstone Concretion
Siderite or Hematite-bonded Concretion
Hardness: 5-6 (Mohs); Color: Rusty brown, reddish-orange, or dusky tan; Luster: Dull, earthy; Crystal structure: Amorphous or fine-grained crystalline; Cleavage: None; Specific gravity: 3.5-4.5 (dense due to iron content).
- Hardness
- 5-6 (Mohs)
- Color
- Rusty brown, reddish-orange, or dusky tan
- Luster
- Dull, earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Rusty brown, reddish-orange, or dusky tan; Luster: Dull, earthy; Crystal structure: Amorphous or fine-grained crystalline; Cleavage: None; Specific gravity: 3.5-4.5 (dense due to iron content).
Formation & geological history
Formed through the precipitation of mineral cement (often iron oxides or carbonates) within the spaces between sediment grains. They grow centrifugally around a nucleus, such as a fossil or pebble, in sedimentary basins during diagenesis.
Uses & applications
Primarily of interest to geological collectors or used as ornamental garden stones. Historically, some iron-rich concretions were smelted as a low-grade iron ore. They are often mistaken for meteorites or fossils.
Geological facts
Commonly nicknamed 'Moqui Marbles' (in Utah) or 'ironstone nodules.' They often feature a hard outer shell with a softer, sandy core. Some concretions are millions of years old, appearing from the Jurassic to the Cretaceous periods.
Field identification & locations
Identify in the field by their spherical or discoidal shape, heavy weight relative to size, and dark, rusty weathered surface. They are frequently found in sandstone formations or weathered out on desert floors and riverbeds.
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