sedimentary
Coquina
Biogenic Limestone (Calcium Carbonate, CaCO3)
Hardness: 2-3 (Mohs scale); Color: Cream, tan, or white with brown shell fragments; Luster: Dull/Earthy; Crystal Structure: Amorphous/Bioclastic; Cleavage: None; Specific Gravity: 2.5-2.7
- Hardness
- 2-3 (Mohs scale)
- Color
- Cream, tan, or white with brown shell fragments
- Luster
- Dull/Earthy
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Physical properties
Hardness: 2-3 (Mohs scale); Color: Cream, tan, or white with brown shell fragments; Luster: Dull/Earthy; Crystal Structure: Amorphous/Bioclastic; Cleavage: None; Specific Gravity: 2.5-2.7
Formation & geological history
Formed in high-energy marine environments where shell fragments and calcified remains of marine organisms accumulate and are cemented together by calcium carbonate. Most deposits are relatively young, dating from the Pleistocene to Holocene epochs.
Uses & applications
Used primarily as a construction material (especially in coastal areas like Florida), road gravel, and historically for building fortifications due to its ability to absorb cannonball impacts.
Geological facts
The Castillo de San Marcos in St. Augustine, Florida, was built from coquina; its porous nature meant that instead of shattering, the walls would 'swallow' cannonballs shot at it.
Field identification & locations
Identify by its 'oatmeal-like' appearance and visible shell fragments. It will effervesce (fizz) vigorously when exposed to dilute hydrochloric acid or vinegar. Found commonly along coastal zones.
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