Coprecipitation of Fluoride into Jewelry Coral (Corallium) Skeletons

The fluoride content in jewelry coral skeletons (Mg-calcite) was analyzed using ion chromatography in order to elucidate the factors that control coprecipitation of fluoride into carbonate skeletons. The ratio of F/Ca in jewelry coral was higher than that in shallow marine carbonates. Assuming that a carbonate ion in coral Mg-calcite is substituted with two fluoride ions, the apparent equilibrium constant of the ion-exchange reaction can be expressed as: KF'=[(Ca, Mg)F2] [CO32−]/[CaCO3] [F−]2. By rearranging this equation, the coral F/Ca ratio can be expressed as : [(Ca, Mg)F2]/[CaCO3]=KF'[F−]2/[CO32−]. This equation suggests that the fluoride content in jewelry coral skeletons is inversely proportional to carbonate concentration in seawater. Accordingly, the fluoride content in jewelry coral skeletons was higher than that in shallow marine carbonates because the carbonate concentration in sea water decreases with an increase in water depth.