Fluorapatite precipitation

The logarithm of the solubility product for hydroxyapatite is -58.6 and that of fluorapatite (CajtPO jF) is -60.6 (57), and thus, D = 0.01 in favour of fluoride incorporation into the solid apatite precipitate. Accordingly, it should be difficult to prepare solid solutions of these compounds by precipitation from aqueous solution and if prepared batchwise, they are expected to contain logarithmic gradients in their internal composition. Yet, Moreno et al.(M3) report linear changes in the lattice parameters of such solid solutions. They also determined their solubility behavior. 

In conclusion, the solubility data indicate that upon precipitation from aqueous solutions which have a F/OH molar ratio less than a certain value, slightly fluoridated hydroxyapatites will be formed (x .0.15), and above that ratio nearly pure fluor-apatite will be formed. Usually the F/OH ratio varies so that intimate mixtures of hydroxyapatite and fluorapatite will result (64). The effect of fluoride on teeth and bones are discussed elsewhere (52, 57). 

Solid solutions between carbonated HA and fluorapatite occur naturally in the body, in bone and teeth. In particular, the presence of fluoride ions offers the low solubility and good acid resistance needed for protecting teeth [57]. Note, however, that at very high pH levels, the HA end-member becomes less soluble than fluorapatite [58]. That is why it is often difficult to prepare stoichiometric fluorapatite by precipitation methods involving generally alkaline pH levels. 

Fluorhydroxyapatite can be synthesised by the traditional double-decomposition method generally used for apatite precipitation. An ammonium phosphate and fluoride solution (solution B) is added, dropwise, into a hot (generally at boiling temperature) calcium solution (solution A) at a basic pH level as previously published [122,123]. Fluorapatites close to stoichiometry are obtained (a = 2, see the following reaction equation) however, a very small residual amount of OH always seems to be present. Filtration and several washing operations are necessary to remove the counter-ions. The reaction is almost total due to the very low solubility of fluorhydroxyapatites. 

Okazaki etal. showed that for the same fluoride level supplied, the precipitation of fluorapatite using an organic phosphate source (phosphate ester monomer) instead of an inorganic phosphate source led to higher fluoride contents [127],... 

Finally, control of fluorapatite particle size (from 15 nm to 200 pm) and morphology can be achieved by varying precipitation conditions using aqueous precipitation from a micro-emulsion or by biomimetic synthesis in a gelatin matrix [129],... 

H. Chen, K. Sun, Z. Tang, R.V. Law, J.F. Mansfield, A. Czajka-Jakubowska, B.H. Clarkson, Synthesis of fluorapatite nanorods and nanowires by direct precipitation from solution. Crystal Gro wth and Design 6 (2006) 1504-1508. 

Under neutral conditions, fluoride is also able to induce nucleation and growth of apatite crystals without the involvement of OCP [72]. This requires fluoride concentrations of 0.5 ppm or higher, which are rarely achieved in vivo except in cases where fluorosis may result. It is significant that in severe cases of fluorotic enamel, ultra-structural studies [73] have shown the occurrence of a proliferation of apatite nuclei, suggesting that the presence of fluoride may act to encourage precipitation of crystals of fluorapatite. 

We also examined the possibility of precipitation of other F insoluble minerals, such as fluorapatite in these soils. However, total dissolved phosphate concentrations in soil water extracts were extremely low and in most cases phosphate was well... 

At pH values below about 6.0 the influence of HEDP and fluoride, an important environmental constituent is particularly interesting. It has been shown that the nature of the precipitating calcium phosphate phase can be controlled not only by the concentration of inoculating HAP seed ( )but also by the presence of additives. Thus fluoride ion accelerates the crystallization of calcium phosphate probably through the formation of fluorapatite (Ca5(P04)3 F, FAP) (, 60). Under... 

In general, saliva (as well as plaque fluid) is supersaturated with respect to calcium-phosphate salts, and they prevent tendency to dissolve mineral crystals of teeth. Moreover, precipitation of calcium-phosphate salts that include hydroxyapatite may also occur (remineralization) in early lesions of tooth surfaces injured by acidic bacterial products (i.e., lactic acid). Salivary fluoride facilitates calcium-phosphate precipitation, and such crystals (i.e., fluorapatite) show lower acid solubility properties that lead to an increased caries preventive effect. The increase of pFI (i.e., buffer capacity and pH of saliva, as well as ureolysis in dental plaque) also facilitates crystal precipitation and remineralization (4, 13). 

Fluoride in our drinking water and our toothpaste can help minimize the damage described above. The fluoride ion takes the place of the hydroxide ion to precipitate fluorapatite, Ca5(P04)3p, a compound very similar to the... 

Figure 2. Precipitated fluorapatite (a) at 65°C reaction temperature, (b) at 25°C reaction temperature as observed in transmission electron micrographs, and (c) a scanning electron micrograph of precipitate dried at 100°C showing agglomerated crystallites.

Figure 2-31 Precipitation of fluorapatite within the large CaO-PgOg-F-rlch droplet phase of mica-apatite glass-ceramic. TEM/replica. Fractured surface etched (HCI, 5 sec).

All niarine phosphorites consist mostly of microcrystalline apatite (carbonate fluorapatite) in the form of laminae, pellets, oolites, nodules and skeletal or shell debris. Uranium, considered syngenetic, may be present in carbonate fluorapatite as a substitute for calcium. Uranium in sea water was probably incorporated during or shortly after precipitation, and it is usually disseminated rather uniformly throughout a given bed or horizon. Primary uranium minerals are rarely present, but secondary uranium minerals (tyuyamunite, autunite, torber-nite) have been identified in a few localities. 

At equilibrium, the least soluble substance in a system that can form will precipitate. Much phosphate contained in sea water is precipitated as tricalcium orthophosphate or hydroxyl apatite, Caio(P04)6(OH)2, and fluorapatite, Caio(P04)6(F)2. Oceans floors are covered with these deposits and are referred to as marine pellets. There are many ways in which this problem may be approached, but it is obvious that if phosphates are to be leached from igneous rocks, large boulders will leach very slowly. Smaller particles of rock caused by grinding, weathering, and aging solubilize more rapidly than larger particles. As a first approximation, rates of solubilization are proportional to fresh surfaces of solubilized rocks. 

---