The synthesis and solubility of carbonate fluorapatite

Solubility of laboratory synthesized carbonate fluorapatite (CFAP) determined from supersaturated and undersaturated initial conditions. A constant composition technique was used to synthesize the CFAP resulting in solids that appear to be homogeneous, crystalline, and of accurately known stoichiometry. The CFAP dissolves incongruently when exposed to solutions that differ greatly from that in which the solid formed. By analogy to solid solution systems, the results from undersaturation are extrapolated to congruent dissolution conditions. Results demonstrate that the solubility of apatite increases dramatically with increasing carbonate content. The seawater phosphate concentration in equilibrium with carbonate fluorapatite containing 1.4 carbonate ions per unit cell is 90 to 100 times greater than that that would be in equilibrium with pure fluorapatite. Experiments also suggest that fluorapatite of relatively high carbonate content is the thermodynamically stable apatite phase in high carbonate solutions such as seawater.\--Modified journal abstract.