A resolution-enhanced Fourier transform infrared spectroscopic study of the environment of the CO3(2-) ion in the mineral phase of enamel during its formation and maturation.

A resolution-enhanced Fourier Transform Infrared (FTIR) Spectroscopic study of the CO3(2-) ion in pig enamel of increasing age and maturity has demonstrated the existence of four different, main carbonate locations. The major CO3(2-) site arises as a result of the substitution of CO3(2-) ions in the positions occupied by PO4(3-) ions in the apatitic lattice. In addition, two minor locations have been identified in positions in which the CO3(2-) ions substitute for OH- ions. The fourth carbonate group appears to be in an unstable location. Its concentration has been found to decrease with aging and maturation, during which there is a progressive increase in the amount of mineral deposited in the enamel. The distribution of the carbonate ions in the different apatitic sites varies randomly during the formation of the mineral phase in enamel and during its maturation. Although these changes have been shown to be related to changes in the composition of the mineral phase, a comparison of the parameters assessing the degree of crystallinity of the mineral phase from upsilon 2CO3(2-) and upsilon 4PO4(3-) infrared absorption data reveals a significant discrepancy related to the nonhomogeneous partition of the CO3(2-) ion in the mineral phase. After maximum mineralization is reached, the composition of the mature mineral phase is decidedly different than that of the initial mineral deposited; the changes affect principally the concentrations of Ca2+, OH-, and HPO4(2-) ions, but not the CO3(2-) ions.