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Effect of Solution Composition on Morphological and Structural Features of Carbonated Calcium Apatites

Forsyth Dental Center, 140 The Fenway, Boston, Massachusetts 02115

T. Aoba

Forsyth Dental Center, 140 The Fenway, Boston, Massachusetts 02115

E.C. Moreno

Forsyth Dental Center, 140 The Fenway, Boston, Massachusetts 02115

Y. Miake

Forsyth Dental Center, 140 The Fenway, Boston, Massachusetts 02115

The composition of enamel mineral corresponds to that of a calcium carbonato-apatite. For insight to be gained into the precipitation of carbonato-apatites having specific properties (crystal size, morphology, and carbonate incorporation into the crystal lattice), apatites were prepared at 80°C in aqueous systems having various CO₃ concentrations and pH values of around 7.5 or 10.5 (± 0.5). The various preparations had a wide range (0.005 to 0.19) of CO₃/Ca molar ratios that bracket the ratios found in porcine enamel mineral at various developmental stages. Fourier transform infrared spectroscopy (FTIR) and x-ray diffraction analyses showed that the calcium apatites precipitating at neutral pH incorporated the carbonate into both the hydroxyl and phosphate ion sites in their lattices (A,B-types), whereas the preparations made at the alkaline pH (high OH-CO₃^2- competition) or in the presence of fluoride (F^--CO₃^2- competition) yielded only the B-type carbonato-apatite. It was also ascertained that the size and morphology of the carbonato-apatites, assessed by specific surface area determination and high-resolution electron microscopy, were highly dependent on the driving force for precipitation and the presence of regulators (CO ₃^2- and F^-) in solution. In neutral media, early precipitates were thin-ribbon in appearance, but grew into crystals having flattened-hexagonal cross-sections. In the presence of fluoride or in alkaline media, acicular apatite crystals, precipitated initially, grew into large rod-like carbonato-apatites having a symmetric-hexagonal cross-section. In both neutral and alkaline solutions, carbonate inhibited the growth of apatite crystals along their c axis, leading to the formation of bulkier crystals. The formation of carbonato-apatites at the neutral pH and their properties are consistent with observations made on enamel minerals formed in the early developmental stages.

Journal of Dental Research, Vol. 69, No. 11, 1731-1740 (1990)
DOI: 10.1177/00220345900690110501


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