The occurrence of aragonite needles in the carbonate mud of the surface sediments of Florida Bay, the Florida Keys, and the Great Bahama Bank is of interest to problems of carbonate deposition. Older views regarding the origin of the aragonite needles support inorganic precipitation. More recent theories favor a biogenic formation and link the mud particles to the aragonite sheath of algae, mainly Penicillus, Rhipocephalus, and Halimeda. Electron microscope studies of mud suspensions and of aragonite crystals from Penicillus reveal a remarkable similarity between the aragonite formed on the algal surface, and the aragonite in the carbonate mud. Surface replicas of aragonite sheath peeled from Penicillus show needle-like crystals scattered between a network of fiber- or fi m-like algal material. The closely interwoven system of plant material and aragonite needles supports the suggestion of the algal substrate acting as a matrix for the aragonite formation.

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