Where is Iron-rich Concretion found?
Goethite/Limonite (iron oxyhydroxides)
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Open the appIron-rich Concretion occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Iron-rich Concretion
Midwestern United States
Abundant in Cretaceous and Tertiary sedimentary formations, particularly in states like Kansas, Nebraska, and North Dakota, often associated with marine shales and sandstones.
Eastern United States
Found in various sedimentary units, including the Appalachian Basin, often as bog iron deposits in wetlands or concretions in weathered bedrock.
Europe
Widespread in sedimentary basins across Europe, including the Jurassic ironstones of England and the bog iron deposits of Scandinavia and Germany.
Australia
Common in weathered profiles and sedimentary rocks, particularly in regions with extensive lateritic soils.
Globally
Iron-rich concretions are a ubiquitous feature in sedimentary environments worldwide where iron-bearing solutions interact with porous media.
Collecting tips
- Look in Sedimentary Outcrops. Examine road cuts, stream banks, and quarries exposing sandstones, shales, and siltstones. Concretions often weather out of softer host rock and accumulate at the base of slopes.
- Check for Rusty Stains. Areas with reddish-brown or yellowish-brown staining on rocks or soil can indicate the presence of iron oxides and potentially concretions.
- Explore Wetlands and Bogs. Bog iron concretions can be found in swampy or marshy areas where iron precipitates from groundwater.
- Examine Weathered Zones. Concretions are often more resistant to weathering than the surrounding rock, making them stand out on weathered surfaces.
- Use a Magnet (with caution). While goethite/limonite are not strongly magnetic, some concretions may contain minor amounts of magnetite or maghemite, which are magnetic. This is not a definitive test but can be a secondary indicator.
Quick Check
- Color:
- Yellowish-brown to dark brown/black
- Luster:
- Dull, earthy, submetallic
- Streak:
- Yellowish-brown to brownish-yellow (for goethite/limonite)
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