sedimentary
Concretion
Sedimentary Concretion (e.g., Siderite or Ironstone)
Hardness: 3.5-5.5 (Mohs), Color: Brown to rust-red, Luster: Dull or earthy, Crystal structure: Usually microcrystalline or amorphous within a rounded mass, Cleavage: None/Conchoidal fracture, Specific gravity: 3.0-4.0
- Hardness
- 3
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Physical properties
Hardness: 3.5-5.5 (Mohs), Color: Brown to rust-red, Luster: Dull or earthy, Crystal structure: Usually microcrystalline or amorphous within a rounded mass, Cleavage: None/Conchoidal fracture, Specific gravity: 3.0-4.0
Formation & geological history
Formed by the precipitation of mineral cement in the spaces between particles of sediment, typically around a nucleus like a leaf, shell, or bone. Common in sedimentary rock layers like shale and sandstone.
Uses & applications
Primarily used for geological study and as decorative pieces for collectors; sometimes processed for iron ore if siderite-rich.
Geological facts
Concretions are often mistaken for fossils, eggs, or meteorites due to their unusual spherical shapes. Some famous examples include the Moeraki Boulders in New Zealand.
Field identification & locations
Identify by its spherical or disc-like shape, high density compared to surrounding rock, and often a 'shell' that can be cracked open to reveal a core. Found worldwide in sedimentary basins.
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