Virtually all iron objects recovered from a 7th century Byzantine shipwreck off the coast of Turkey were encrusted with a carbonate-rich layer. Mineralogical and chemical examination reveals limonite, siderite, and aragonite as dominant authigenic phases. The encrustations can be explained by oxidation (corrosion) of the metal in sea water. Analogous processes are suggested for many concretions formed around metallic nuclei in sea water.

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