sedimentary
Ironstone Nodules (Iron Concretion)
Siderite or Hematite Concretion
Hardness: 5-6.5 (Mohs scale), Color: Dark brown to reddish-black, Luster: Dull or sub-metallic, Crystal Structure: Amorphous or microcrystalline, Cleavage: None, Specific Gravity: 3.5-5.0 (feels heavy for its size)
- Hardness
- 5-6
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Physical properties
Hardness: 5-6.5 (Mohs scale), Color: Dark brown to reddish-black, Luster: Dull or sub-metallic, Crystal Structure: Amorphous or microcrystalline, Cleavage: None, Specific Gravity: 3.5-5.0 (feels heavy for its size)
Formation & geological history
Formed by the precipitation of mineral cement within the spaces between sediment grains. These typically form in sedimentary host rock layers like sandstone or shale as mineral-rich groundwater deposits iron oxides around a nucleus.
Uses & applications
Historically used as a low-grade iron ore. In modern times, they are primarily collected as geological curiosities or used as landscape accents.
Geological facts
These are often mistaken for meteorites due to their weight and dark, weathered exterior. Some specific types found in the Western US are known as "Moqui Marbles."
Field identification & locations
Identify in the field by their rounded or irregular globular shapes, heavy weight (high iron content), and dark streak if rubbed on unglazed porcelain. Commonly found in desert environments or eroding out of shale cliffs.
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