sedimentary
Fossiliferous Limestone (Coquina / Shell Limestone)
Biogenic Limestone (CaCO3)
Hardness: 3 (Mohs scale); Color: Gray, tan, or white; Luster: Dull to earthy; Crystal structure: Hexagonal/Trigonal (calcite matrix); Cleavage: Perfect rhombohedral in calcite crystals, but usually presents as shell fragments; Specific gravity: 2.3-2.7.
- Hardness
- 3 (Mohs scale)
- Color
- Gray, tan, or white
- Luster
- Dull to earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: Gray, tan, or white; Luster: Dull to earthy; Crystal structure: Hexagonal/Trigonal (calcite matrix); Cleavage: Perfect rhombohedral in calcite crystals, but usually presents as shell fragments; Specific gravity: 2.3-2.7.
Formation & geological history
Formed in marine environments from the accumulation of organic debris such as shell fragments, coral, and algae. Over millions of years, these calcium carbonate remains are lithified through compaction and cementation. Often associated with the Paleozoic to Cenozoic eras.
Uses & applications
Used primarily in construction for building stone, aggregate for roads, and the production of cement and lime. It is also popular in landscaping and as a decorative stone in architecture.
Geological facts
Many historic forts in Florida, such as the Castillo de San Marcos, were built using a form of this rock called Coquina because it is surprisingly effective at absorbing cannon fire without shattering.
Field identification & locations
Identify in the field by the visible presence of fossilized shells or skeletons. It will react and fizz (effervesce) when exposed to a drop of dilute hydrochloric acid or vinegar due to the calcium carbonate content.
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