sedimentary
Sandstone Concretion
Ferruginous Sandstone Concretion (SiO2 + FeO(OH))
Hardness: 6-7 (quartz component); Color: Reddish-brown, tan, ochre; Luster: Dull, earthy; Crystal structure: Microcrystalline/Amorphous cement; Cleavage: None; Specific Gravity: 2.6-3.2
- Hardness
- 6-7 (quartz component)
- Color
- Reddish-brown, tan, ochre
- Luster
- Dull, earthy
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Physical properties
Hardness: 6-7 (quartz component); Color: Reddish-brown, tan, ochre; Luster: Dull, earthy; Crystal structure: Microcrystalline/Amorphous cement; Cleavage: None; Specific Gravity: 2.6-3.2
Formation & geological history
Formed by the precipitation of mineral cement (often iron oxide or calcite) within the spaces between sediment grains. This typically occurs in sedimentary basins during diagenesis, often around a nucleus like a leaf, shell, or pebble.
Uses & applications
Primarily used for geological study, decorative garden stones, and as keepsakes for hobbyist collectors. Some large concretions are tourist attractions.
Geological facts
Concretions are often mistaken for fossils, eggs, or meteorites due to their unusual symmetrical or nodular shapes. The famous 'Moqui Marbles' in Utah are a specific type of iron oxide concretion.
Field identification & locations
Identify in the field by looking for rounded, nodular masses that are harder and more weather-resistant than the surrounding host rock. Commonly found in eroded sandstone cliffs or stream beds.
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