The kinetic and thermodynamic aspects of biomineralization are determined primarily by the degree of supersaturation (DS) of the fluid environment with respect to the biomineral concerned. In the case of a calcium phosphate, the DS can be expressed as the ratio of the ionic activity product (IP) of the ionic lattice constituents in solution to the solubility product (Ksp) of the solid, i.e., DS = IP/Ksp. In order to assess the driving forces for mineralization or dissolution taking place in various fluid environments, we focused our efforts on determination of appropriate stoichiometry and thermodynamic solubility product (Ksp) of human tooth enamel, dentin and bone apatites. The stoichiometry model used was (Ca)5-x(Mg)q(Na)u(HPO4)v(CO3)w(PO4)3-y(OH)1-z, which takes into account putative substituents in the lattice positions. These stoichiometric coefficients were analytically determined by assessing separately the exchangeable pool of each ionic species on the crystal surface and the ionic species in the bulk lattice. Their solubility was determined through a series of solid/solution equilibrium at 250C under constant CO2/N2 gas environment. The results obtained disclosed that the solubility behaviour of each of the biogenic apatities remained relatively constant in the range of 1 through 3% CO2/N2, whereas the discrete features were appreciable below and above the corresponding CO2/N2 range. The diversities of tooth and bone mineral regarding their morphology, stoichiometric composition and solubility properties most likely reflect differences in the mineralizing environments and complex nucleation-growth processes including the transition from unstable intermediates to thermodynamically stable phases.
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