Inorganic formation of apatite in brackish seawater from the Baltic Sea: an experimental approach

Long-term incubation experiments were conducted to study the formation of apatite in calcite-seeded brackish seawater from the Baltic Sea proper. Apatite failed to precipitate from solutions undersaturated with respect to the precursor octacalcium phosphate (OCP), despite a supersaturation with respect to hydroxyapatite (HAP), fluorapatite (FAP), and carbonate-fluorapatite (CFAP). It is suggested that the direct nucleation of apatite is blocked by the presence of dissolved magnesium in seawater. In fact, our marine solutions remained metastable throughout 700 days of incubation. However, formation of apatite took place in seawater supersaturated with respect to OCP, although dissolved magnesium slowed the overall rate of formation. Thus, the precipitation of OCP provides a kinetic boundary for the formation of apatite in natural seawater. These findings strongly suggest that the formation of authigenic apatite in the marine environment generally proceeds via a precursor pathway involving OCP rather than through direct nucleation. Reliable information on the kinetic boundary (even possible complete inhibition) is necessary for proper evaluation of sink estimates in budgets and models of the global phosphorus cycle.