sedimentary
Ironstone Nodule (Ironstone Concretion)
Siderite-rich Mudstone (FeCO3) or Limonite (FeO(OH)·nH2O)
Hardness: 3.5-4.5 on Mohs scale; Color: Rusty brown, reddish-orange, or dark grey; Luster: Dull/Earthy; Crystal Structure: Usually microcrystalline/amorphous within a matrix; Cleavage: None (conchoidal or irregular fracture); Specific Gravity: 3.0-3.9 (noticeably heavy for its size).
- Hardness
- 3
- Color
- Rusty brown, reddish-orange, or dark grey
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3.5-4.5 on Mohs scale; Color: Rusty brown, reddish-orange, or dark grey; Luster: Dull/Earthy; Crystal Structure: Usually microcrystalline/amorphous within a matrix; Cleavage: None (conchoidal or irregular fracture); Specific Gravity: 3.0-3.9 (noticeably heavy for its size).
Formation & geological history
Formed by the precipitation of iron-rich minerals around a nucleus (like a leaf, shell, or pebble) in sedimentary basins, marshes, or marine environments. They often accumulate in shale or sandstone layers and can date from the Paleozoic to the Cenozoic eras.
Uses & applications
Historically used as a low-grade iron ore in early smelting (e.g., the Wealden iron industry). Today, they are mainly of interest to collectors, occasionally used in landscaping, or searched for fossils contained inside.
Geological facts
Many 'Mazon Creek' style fossils are found inside similar ironstone nodules. They are often mistaken for meteorites or 'dinosaur eggs' due to their rounded shape and heavy weight.
Field identification & locations
Identify in the field by its distinct orange-brown weathering rind and high density compared to surrounding rocks. Often found in stream beds or eroded cliffsides. Collectors can tap them with a hammer to see if they 'split' along a bedding plane to reveal a fossil.
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