sedimentary
Sandstone Concretion
Ferruginous Sandstone Concretion (Primarily SiO2 with Iron Oxide cement)
Hardness: 6-7 (Mohs scale), Color: Reddish-brown to tan, Luster: Dull/Earthy, Crystal Structure: Granular/Uncrystallized matrix, Cleavage: None, Specific Gravity: 2.0–2.6.
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Reddish-brown to tan, Luster: Dull/Earthy, Crystal Structure: Granular/Uncrystallized matrix, Cleavage: None, Specific Gravity: 2.0–2.6.
Formation & geological history
Formed by the precipitation of mineral cement (often iron oxide or calcite) within the spaces between sediment grains. This process typically occurs early in the burial history of the sediment, before it is fully lithified into rock.
Uses & applications
Primary use is for educational purposes, geological study, and decorative collecting. In some contexts, iron-rich concretions can be an indicator of iron ore deposits.
Geological facts
Many people mistake these for fossils or meteorites due to their strange, lumpy shapes. They are often called 'Moqui Marbles' or 'Loess Puppies' depending on their specific mineralogy and location.
Field identification & locations
Identify by looking for a shape that is distinct from the surrounding rock layering, often appearing more rounded or lumpy. Commonly found in desert environments or eroding out of sandy cliffs.
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