Where is Iron Oxide Concretion found?
Iron Oxide (e.g., Goethite, Hematite)
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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
Utah, USA
Famous for 'Moqui Marbles' (Navajo Sandstone), which are small, spherical iron oxide concretions, primarily hematite, found in the Navajo Sandstone formation.
Kansas, USA
Large, spherical concretions, sometimes called 'cannonball concretions,' are found in the Cretaceous Pierre Shale and other sedimentary units.
North Dakota, USA
Similar to Kansas, large concretions are found in the Cretaceous Cannonball Member of the Fort Union Formation.
England, UK
Ironstone concretions are common in Jurassic and Cretaceous sedimentary sequences, such as the Lias Group and Wealden Group.
Australia
Extensive occurrences in various sedimentary basins, often associated with lateritic weathering profiles.
Mars
The Mars Exploration Rover Opportunity discovered abundant small, spherical hematite concretions, dubbed 'Martian blueberries,' providing evidence of past water activity.
Collecting tips
- Look in Sedimentary Rocks. Focus your search in areas with exposed sedimentary rock formations, particularly sandstones, shales, and siltstones. Riverbeds, road cuts, and eroded hillsides are good places to start.
- Identify Distinct Shapes. Concretions often stand out from the surrounding rock due to their distinct spherical, ovoid, or irregular shapes. They may weather out of the host rock and accumulate on the surface.
- Check for Iron Staining. The presence of reddish-brown or yellowish-brown staining on the host rock or surrounding soil can indicate the presence of iron oxides and potentially concretions.
- Test Hardness and Density. Iron oxide concretions are generally harder and denser than the surrounding sedimentary rock, making them feel heavier for their size.
- Observe Internal Structure. If a concretion is broken, examine its internal structure for concentric layers, radial patterns, or a massive core, which are characteristic features.
Quick Check
- Color:
- Reddish-brown, dark brown, black, yellowish-brown
- Luster:
- Dull to earthy, sometimes submetallic
- Streak:
- Reddish-brown (hematite) or yellowish-brown (goethite/limonite)
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