Where is Iron Oxide Concretion found?
Goethite/Hematite (Iron Oxide)
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Open the appIron Oxide 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 Oxide Concretion
Midwestern United States
Abundant in Cretaceous sandstones (e.g., Dakota Formation) and shales, often weathering out as 'cannonball' concretions.
Utah, USA (Navajo Sandstone)
Famous for 'Moqui Marbles' or 'Navajo Sandstone Concretions', which are hematite-cemented sandstone concretions.
England (Jurassic Coast)
Found in various Jurassic sedimentary formations, often containing fossil nuclei.
Australia (Pilbara Region)
While primarily known for massive iron ore deposits, iron oxide concretions are also present in weathered profiles and sedimentary units.
Bog Iron Deposits Worldwide
Goethite-rich concretions form in wetlands and bogs where iron precipitates from groundwater.
Collecting tips
- Look for Weathered Outcrops. Concretions are often harder and more resistant to weathering than their host rock, causing them to stand out on eroded surfaces or accumulate at the base of slopes.
- Examine Sedimentary Layers. Search within layers of sandstone, shale, and siltstone, particularly those with evidence of past water flow or redox changes.
- Check Stream Beds and Beaches. Water erosion can concentrate concretions in these environments, as they are often denser than other sediments.
- Observe Color and Shape Anomalies. Their distinct dark colors and rounded or irregular shapes make them noticeable against lighter host rocks.
- Use a Magnet (for Hematite-rich types). While not strongly magnetic, some hematite-rich concretions can be weakly attracted to a strong magnet, especially if they contain minor magnetite. Goethite is generally non-magnetic.
Quick Check
- Color:
- Dark reddish-brown to black
- Luster:
- Dull to earthy
- Streak:
- Reddish-brown (for hematite-rich) or yellowish-brown (for goethite-rich)
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