Solubility Properties of Carbonatoapatites with Discrete Stoichiometric Compositions

Acquisition of substantial amounts of carbonate, acid phosphate and other impurities is encountered universally in biogenic apatites in vertebrates. To date, however, much still remains to be elucidated about the driving force behind carbonatoapatite formation in physiological environments and whether the solubility of carbonatoapatites depends on their stoichiometric composition and types of carbonate substitution. We here used discrete groups of carbonatoapatites prepared under various aqueous conditions and determined their solubility during acid equilibration. We selected a stoichiometry model to describe these crystal compositions, i.e., (Ca) 5-x (HPO4) v (CO3) w (PO4) 3-x (OH)1-x. All apatite products were equilibrated in phosphoric acid solutions at 25° for a maximum of 28 days under a constant partial pressure of CO2 i.e., 1.8% CO2/N2. After this long-term equilibration, each of the apatite samples was verified to reach a plateau of the ionic activity product (IP), which was calculated on the basis of the experimentally determined stoichiometries. Notably, the solubility product values of carbonatoapatites were confirmed to depend primarily on their carbonate contents. Importantly, analysis of the solubility data and electron microscopic observation validated that the incongruent equilibration, i.e., dissolution of the original crystal and coprecipitation of projections formed on the original crystals, was most prominent for batches of B-type carbonatoapatites originally prepared at alkaline conditions. In contrast, the solubility of A, B-type carbonatoapatites, which were originally prepared at neutral conditions, yielded the so-called central-core dissolution without morphologic evidence for projection-type precipitation. The results obtained support the hypothesis that the stoichiometry and solubility properties of carbonatoapatites depend primarily on the solution environment, including the gas atmosphere, where crystal precipitation has occurred, so that a congruent equilibration of biogenic or synthetic carbonatoapatites might be pursued under conditions similar to the originally precipitating milieu.