Calcium fluorapatite crystal structure

Fig. 4.1 Crystal structure of fluorapatite showing the two different calcium sites Ca(l) and Ca(2)...

The accurate determination of the spatial relationships of ions in a crystal lattice requires that large well-formed single crystals be studied using a variety of techniques including X-ray diffraction, spectroscopic and optical methods. The best apatite crystals for such studies are those of artificially prepared calcium fluorapatite and, the next best, articifically prepared calcium hydroxyapatite. The description which follows is therefore principally of calcium fluorapatite, but the structure of calcium hydroxyapatite will also be mentioned. 

Figure 28.2 Basic structure of the calcium fluorapatite lattice. The large hexagonal cylinders have been formed by joining the centres of Ca " ions in the crystal lattice. The projection of a phosphate tetrahedron is a triangle...

Hydroxyapatite (HA) is the term commonly used for calcium hydroxyapatite Caio(P04)e(OH)2, the most stable form of Ca-P under physiological pH and body temperature. With fluorapatite and chlorapatite, hydroxyapatite forms the group of apatite minerals that share a similar hexagonal monoclinic crystal structure and the general formula Aio(P04)6(OH,F,Cl)2. The A cation can be several metal ions besides calcium, such as barium, strontium, and magnesium. The phosphate anion can be substituted by a carbonate anion to a limited extent. 

When HA is in contact with a fluoride-containing solution at low concentration, this F /OH substitution mostly occurs at the crystal surface [60] and can be accompanied by the formation of CaF2 as a secondary phase in acidic media (Fig. 8). The fluoride ions fixed on the HA crystal surface after exposure to a fluoride-containing solution were found to be coordinated by three calcium ions, as with the regular fluorapatite structure [61]. The fluoride uptake was shown to... 

In the presence of fluoride, calcium ions have been found to be more firmly anchored than in pure hydroxyapatite [67]. This enhances the overall resistance to dissolution. Thus, the presence of a thin stable film of fluorapatite on the surface of hydroxyapatite crystals has two effects, namely (i) resistance to diffusion and dissolution of the anion and (ii) firmer binding of calcium ions into the surface. Both of these make the resulting apatite structure more resistant to dissolution, regardless of the pH of the external medium, and they thereby increase the resistance of the mineral phase to the onset of caries. 

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