sedimentary
Coquina
Biogenic Limestone (Calcite / CaCO3)
Hardness: 3 (Mohs scale); Color: tan, white, or light gray; Luster: dull to earthy; Structure: poorly consolidated mass of shells and shell fragments; Cleavage: none (clastic texture); Specific Gravity: 2.3-2.7
- Hardness
- 3 (Mohs scale)
- Color
- tan, white, or light gray
- Luster
- dull to earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: tan, white, or light gray; Luster: dull to earthy; Structure: poorly consolidated mass of shells and shell fragments; Cleavage: none (clastic texture); Specific Gravity: 2.3-2.7
Formation & geological history
Formed in high-energy marine environments (beaches/shallow reefs) where shell fragments accumulate rapidly and are lightly cemented by calcite. Most significant deposits range from the Pleistocene to Holocene epochs.
Uses & applications
Historically used as a building stone (notably for forts due to its energy-absorbing properties), road base material, and source of lime for agriculture.
Geological facts
The Castillo de San Marcos in Florida was built from coquina; it is so porous that it absorbed cannonballs rather than shattering, making the fortress nearly indestructible during historical sieges.
Field identification & locations
Identify by its 'oatmeal-like' appearance and visible shell fragments. Found primarily on coastal plains and barrier islands in Florida, Australia, and Mexico. It will effervesce (fizz) vigorously when in contact with dilute hydrochloric acid.
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