Hydroxyapatite fluorapatite

Most foods and drinking waters contain enough fluoride to result in the incorporation of significant amounts of fluoride into this mineral whereby the solubility decreases. Therefore, the system hydroxyapatite-fluorapatite is primarily of importance for the prevention of dental caries. However, in this context its theoretical treatment is important for geochemists who may be confronted with so-called subregular solid solutions. 

Phosphorus is the eleventh element in order of abundance in crustal rocks of the earth and it occurs there to the extent of 1120 ppm (cf. H 1520 ppm, Mn 1060 ppm). All its known terrestrial minerals are orthophosphates though the reduced phosphide mineral schrieber-site (Fe,Ni)3P occurs in most iron meteorites. Some 200 crystalline phosphate minerals have been described, but by far the major amount of P occurs in a single mineral family, the apatites, and these are the only ones of industrial importance, the others being rare curiosities. Apatites (p. 523) have the idealized general formula 3Ca3(P04)2.CaX2, that is Caio(P04)6X2, and common members are fluorapatite Ca5(P04)3p, chloroapatite Ca5(P04)3Cl, and hydroxyapatite Ca5(P04)3(0H). In addition, there are vast deposits of amorphous phosphate rock, phosphorite, which approximates in composition to fluoroapatite. " These deposits are widely... 

By far the most abundant phosphate mineral is apatite, which accounts for more than 95% of all P in the Earth s crust. The basic composition of apatite is listed in Table 14-2. Apatite exhibits a hexagonal crystal structure with long open channels parallel to the c-axis. In its pure form, F , OH , or Cl occupies sites along this axis to form fluorapatite, hydroxyapatite, or chlor-apatite, respectively. However, because of the "open" nature of the apatite crystal lattice, many minor substitutions are possible and "pure" forms of apatite as depicted by the general formula in Table 14-2 are rarely found. 

Once fluoride ions react with bone, they are not easily dissolved out or exchanged by other elements. If bone is buried for long periods of time, the relative amount of fluorine in the bone gradually increases as a function of time the "fluoridation" process continues until the maximum amount of fluorine (necessary to convert all the hydroxyapatite to fluorapatite) is reached. The total concentration of fluor in carbonated fluorapatite can reach levels as high as above 3%. There is ample room, therefore, for an increase in the relative amount of fluorine in buried bone. Determining the relative amount of fluorine in buried bone may thus serve as a tool for dating bone. 

CCP in milk is mentioned in connection with casein above (Section VI.C). Fluorapatite is a major constituent of phosphate rocks, and a constituent, probably important, of human tooth enamel for those whose drinking water contains significant amounts of naturally occurring or added fluoride. Fluorapatite is significantly less soluble than hydroxyapatite - the relationship between the solubilities of fluorapatite and hydroxyapatite parallels (but is much less extreme than) that between calcium fluoride (Ksp — 3.9 x 10 11 mol3 dm-9) and calcium hydroxide (Ksp = 7.9 x 10 6 mol3 dm 9). Calcium diphosphate, Ca2P207, is believed to be the least soluble of the calcium phosphates. 

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. 

On the other hand, Wier et al (59) have shown that fluoride ions react with the surface of hydroxyapatite particles so that a state of equilibrium is reached as if the aqueous solution is in equilibrium with pure fluorapatite, provided that enough fluoride ions occur in the aqueous solution. Therefore, one should expect, that particles of solid solutions of hydroxyapatite and fluorapatite will react similarly with fluoride ions from an aqueous solution, and that a surface layer is formed which has a composition closer to that of pure fluorapatite than that of the original solid solution. 

In that study 5 ), the average of the logarithms of the solubility products for pure hydroxyapatite (log Kg / ) and pure fluorapatite (log Kp/ ) appeared to be - 59.16 and - 60.52 respectively, both with an uncertainty of about + 0.30. In the present study the solubility data found for equilibration of solid solutions are expressed as the negative logarithms for the ionic products of hydroxyapatite and fluorapatite, i.e. 

Subsequently, the apparent activities of the quasibinary components hydroxyapatite OHA and fluorapatite FA were derived as follows ... 

The data of Table III show that the surface layer of the solid particles is indistinguishable from pure fluorapatite in all equilibrations at x = 0.110, 0.190 and 0.435 and 0.595. However, some equilibrations at x = 0.763 and all at x = 0.868 do deviate significantly from the behavior of pure fluorapatite. A peculiar aspect is that the activity of fluorapatite becomes significantly larger than 1. Simutaneously, the activity of hydroxyapatite approaches unity. This would mean that at all values of x both activities would become smaller than 1, and thus an ideal behavior of the solid solutions would not explain the observed 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). 

Fig. 1. Thermal stability of hydroxyapatite (HA) and fluorapatite. HA decomposes through a series of reactions into very stable CaO (melting point. 2650 C) and a liquid phase. In the case of fluorapatite, the release of CaF2 has been reported at very high temperature, before melting however, the associated phases have not been identified.

V.M. Bhatnagar, Infrared spectra of hydroxyapatite and fluorapatite. Bull. Soc. Chim. Fr. (special issue) (1968) 1771-1773. 

Y. Pan, P- F rotational-echo double resonance nuclear magnetic resonance experiment on fluoridated hydroxyapatite. Solid State Nucl. Magn. Reson. 5 (1995) 263-268. L. Wu, W. Forsling, P.W. Schindler, Surface complexation of calcium mineral in aqueous solution, surface protonation at fluorapatite surface, J. Colloid Interface Sci. 147 (1991) 178-185. 

S. Ben Abdelkader, I. Khattech, C. Rey, M. Jemal, Synthesis, characterization and thermochemistry of calcium-magnesium hydroxyapatite and fluorapatite, Thermochim. Acta 376 (2001) 25-36. 

M.S. Tung, D. Skrtic, Interfacial properties of hydroxyapatite in fluorapatite and octa-calcium phosphate, in L.C. Chowm, E.D. Eanes (Eds.), Octacalcium Phosphate, Karger, Basel, 2001, pp. 112-129. 

S. Overgaard, K. Soballe, M. Lind, C. Biinger, Resorption of hydroxyapatite and fluorapatite coatings in man. An experimental study in trabecular bone, J. Bone Joint Surg. (British Volume) 79 (1997) 654-659. 

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. 

Bacteria on the surface of our teeth metabolize sugars to produce lactic acid, which lowers the pH enough to slowly dissolve tooth enamel. Fluoride inhibits tooth decay because it forms fluorapatite, Cal0(PO4)6F2, which is more acid resistant than hydroxyapatite. 

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