sedimentary
Coquina
Biogenic Limestone (Calcite/Aragonite base, CaCO3)
Hardness: 3 (Mohs scale); Color: Tan, beige, light brown; Luster: Dull/Earthy; Structure: Clastic/Bioclastic; Cleavage: None (friable); Specific Gravity: 2.3-2.5
- Hardness
- 3 (Mohs scale)
- Color
- Tan, beige, light brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: Tan, beige, light brown; Luster: Dull/Earthy; Structure: Clastic/Bioclastic; Cleavage: None (friable); Specific Gravity: 2.3-2.5
Formation & geological history
Formed in high-energy marine shoreline or lagoon environments where wave action accumulates shell fragments. Associated with the Pleistocene to Holocene epochs (approx. 2.5 million years ago to present day).
Uses & applications
Used as a historical building material (source for forts and coastal homes), road base aggregate, landscape stone, and as a source of lime for agriculture.
Geological facts
The Castillo de San Marcos in St. Augustine, Florida, was built from coquina; its porous nature allowed mineral walls to absorb cannon fire rather than shattering. It is essentially a 'natural concrete' made of shells.
Field identification & locations
Identify in the field by its high concentration of visible, poorly sorted shell fragments cemented together. Found commonly in coastal regions like Florida, North Carolina, and Australia. Collectors should look for intact fossil imprints.
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