sedimentary
Iron Nodule
Iron-manganese concretion (Fe-Mn hydroxide)
Hardness: 5-6 (variable), Color: Dark brown to black with rust-colored exterior, Luster: Dull to sub-metallic, Crystal Structure: Amorphous or microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 3.3-4.3
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Physical properties
Hardness: 5-6 (variable), Color: Dark brown to black with rust-colored exterior, Luster: Dull to sub-metallic, Crystal Structure: Amorphous or microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 3.3-4.3
Formation & geological history
Formed by the precipitation of iron and manganese oxides around a nucleus in sedimentary environments, often associated with soil horizons or boggy conditions. The age can range from Recent to millions of years old.
Uses & applications
Mostly of interest to collectors or as a minor iron ore; historically used for pigments (ochre).
Geological facts
These nodules are often mistaken for meteorites because of their dark, heavy appearance and sometimes magnetic properties (if magnetite is present). They often form 'blueberries' on ancient seafloors.
Field identification & locations
Identify by the dark, metallic interior and rusty-brown 'rind'. Found in desert pavement areas or within sandstone beds. Use a streak test: iron oxides will usually leave a dark brown/red streak.
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