sedimentary
Coquina with Calcite Crystals
Calcareous Bioclastic Grainstone (CaCO3)
Hardness: 3 (Mohs scale), Color: Cream, tan, or white, Luster: Vitreous to pearly, Crystal structure: Hexagonal (Trigonal), Cleavage: Perfect rhombohedral
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Physical properties
Hardness: 3 (Mohs scale), Color: Cream, tan, or white, Luster: Vitreous to pearly, Crystal structure: Hexagonal (Trigonal), Cleavage: Perfect rhombohedral
Formation & geological history
Formed in high-energy marine environments where shell fragments and coral debris accumulate and are later cemented together. The interior crystals formed via groundwater precipitation in remaining pore spaces (vugs).
Uses & applications
Used primarily in construction (limerock), road beds, and landscaping. High-quality crystalline specimens are popular for educational purposes and mineral collecting.
Geological facts
Many of the oldest Spanish fortifications in Florida, like the Castillo de San Marcos, were built using coquina because its porous structure absorbs cannonball impacts without shattering.
Field identification & locations
Identify by its visible shell-fragment texture and reaction to dilute hydrochloric acid (it will fizz). Commonly found in coastal areas, particularly Florida and the Carolinas.
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