Crystallographic morphology of heterogeneous fluoridated carbonate apatites

Carbonate-containing fluoridated apatites were synthesized with two different modes of fluoride incorporation: by supplying F-free solution initially, followed by a F-containing solution; and with the order of supply of these solutions reversed. Both of these heterogeneously synthesized fluoridated CO3 apatites showed typical apatitic x-ray diffraction patterns; and both had similar total fluoride contents (0.87 +/- 0.07 and 0.94 +/- 0.03 mmol/g, respectively), i.e., half of the maximum fluoride content of fluorapatite. However, they differed considerably in their crystallographic properties. The 300 reflection peaks of both apatites were split, and their patterns were slightly different. In the former, SEM observation revealed capsule-like crystals with a rounded hexagonal shape. However, the latter had many small crystals, similar to those of homogeneous CO3 apatite, adhering to the larger hexagonal crystals, which were also similar to homogeneous fluorCO3 apatite. Wavelength-dispersive spectroscopy attached to the SEM showed a higher fluoride intensity in the former spectrum than the latter. These results suggest that two different types of heterogeneous fluoridated CO3 apatites were formed, CO3 apatite covered with fluorCO3 apatite, and fluorCO3 apatite covered with CO3 apatite. The simple two-step supply system used in this study is helpful for the study of complicated heterogeneous formation of apatites.