Fluorapatite structure

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... 

Figure 5 (a) Fluorapatite. The crystal structure projected down the unique c-axis showing the hexagonal disposition of the Ca ions and PO4 groups around the F ion. Caj in sevenfold coordination, C 2 in ninefold coordination (Gaines et al., 1997, p. 855). (b) Plot of the best fit, 15%, and next best fit, 30%, of cations that could take the place of Ca in sevenfold ( ) and in ninefold ( ) coordination in fluorapatite structure (source Skinner et al., 2003). 

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. 

Kniep R, Simon P (2007) Fluorapatite-Gelatine-Nanocomposites Self-Organized Morphogenesis, Real Structure and Relations to Natural Hard Materials. 270 73-125 Koenig BW (2007) Residual Dipolar Couplings Report on the Active Conformation of Rhodopsin-Bound Protein Fragments. 272 187-216 Kolusheva S, see Jelinek R (2007) 277 155-180... 

Fluorapatite (FA) corresponds to the chemical formula Caio(P04)eF2 and crystallises in the hexagonal space group PGs/m, with Z = 1 and unit-cell parameters a = b = 9.367 A and c = 6.884 A [1] (Fig. 2). From a structural viewpoint, fluorapatite is often considered as a crystalline model for other apatites and is seen as a reference apatitic array [2]. It is one of the very first apatite structures to have been solved. It has been thoroughly studied since the 1930s [3] and is well documented in the literature. In particular, Sudarsanan et al. [1] reported the single crystal refinement of X-ray diffraction (XRD) data, and the detailed description of atomic positions and local symmetry is fully available [4,5],... 

Fig. 2. Structure of fluorapatite. Projection on the (001) cristallographic plane, perpendicular to the c axis of the hexagonal structure. (Reproduced by permission of lUCrfrom Ref. [2]). Purple Calcium green Fluorine red Oxygen yellow Phosphorus. (See Colour Plate Section at the end of this book.)...

The difference in the H MAS NMR results obtained by the different authors mentioned indicated that the structure of fluorapatite is not uniform and seems to depend greatly on the preparation conditions (pH, temperature treatment, etc.) of the samples. 

In addition to end-member phases, such as fluorapatite and HA, several studies have reported thermodynamic data related to solid solutions of apatite with various cations involving substitutions like Ca-Mg, Ca-Cd, Ca-Pb and Ca-Sr [72-74]. The related enthalpies of mixing were obtained, and their variation versus composition was generally indicative of a non-statistical occupancy of the cationic sites of the apatitic structure. In some instances, the limits of cationic substitution for calcium were estimated (e.g. in the range 0.073-0.101 for Ca-Mg fluorapatites according to Ben Abdelkader et al. [74]). 

H. Sfihi, C. Rey, 1-D and 2-D Double heteronuclear magnetic resonance study of the local structure of type B carbonate fluorapatite, in J. Fraissard, B. Lapina (Eds.), Magnetic Resonance in Colloid and Interface Science, Nato ASI Series II, Vol. 76, Kluwer Academic Publishers, Dordrecht, 2002, pp. 409-422. 

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. 

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. 

Chapter 6, Fig. 3. Splitted view of atoms along the c axis of the hexagonal structure showing the two possible fluoride ion locations. In stoichiometric fluorapatite, fluoride ions locate in the equilateral triangle formed by Ca(ll) ions. In type B carbonate apatite, the replacement of P04 ions by ions creates an oxygen atom vacancy which may be... 

The third type of compound used extensively as a structural component is apatite, CadPO jX. Hydroxyapatite (X = OH) is the major component of bone tissue in tbe vertebrate skeleton. It is also the principal strengthening naierial in teeth. Partial formation of fluorapatite (X = F) from application of fluondes strengthens the structure and causes it to be less soluble in the add formed from fermenting organic material, hence a reduction of caries. Fluorapatite is also used structurally in certain Brachiopod shells. 

The number of parameters adjusted was generally in the range 10-30 (7 to 51 in the x-ray cases reviewed by Young (6.)). The standard deviations in atom position parameters are typically 1-4 parts in 1,000 in the x-ray cases reviewed. Fig. 3 shows the x-ray patterns for a fluorapatite, CasfPOOsF, case in which 28 parameters were varied (20 of them structural), Rp= 12.1%, and RWn= 13.9% (4.). In this Fig., the observed pattern is indicated by the data points with vertical error bars, the calculated pattern is the continuous curve overlying them, the difference between observed and calculated patterns is shown by the lower curve, and the positions of Bragg peaks are shown by the vertical bars between the upper and lower curves. In this case the fit both looks to be very good and is so indicated by the R value. 

The structure of fluorapatite CajQ(P0 ) F2 was described in the 30 s owing to the existence of well-defined single crystals. 

The methods of synthesis of fluorapatite have been widely dis cussed (J ). It is for example possible to obtain fluorapatite by substituting the hydroxyl ion for the fluoride ion, either in a-queous solution at room temperature, or through a solid state reaction at 800°C. It can also be prepared by the action of 6-tricalcium phosphate on calcium fluoride at about 800°C. Its solubility and thermal stability have already been established. While much is known about fluorapatite, many questions still exist concerning the mechanism of their formation, their composition and the structure of some of them. Two of these problems are dealt with here. First, we discuss the formation mechanism of fluorapatite by a solid state reaction between calcium fluoride and apa-titic tricalcium phosphate. Then we present the preparation and the structure of a carbonated apatite rich in fluoride ions. 

Busch S, Schwarz U, Kniep R (2003) Chemical and structural investigations of biomimeti-cally grown fluorapatite-gelatin composite aggregates. Adv Funct Mater 13 189-198... 

Some of the ions shown in Table 3.1.6 have been proved to enter the structure of fluorapatite, either as synthetic products or as naturally occurring minerals. For example, S and Si replace P in about equal amounts in ellestadite (McConnell, 1938), and Al replaces both Ca and P in heated morinite, when converted to apatite (Fisher and McConnell, 1969). Presumably, these same situations can obtain for biologic apatites also that is, small amounts (traces or more) of the constituents shown in Table 3.1.6 are permissible theoretically. Whether they actually do substitute depends, among other factors, upon their relative availabilities within the biologic environment. Thus, the formation of an aluminium-rich apatite within the organic milieu seems highly improbable one would expect instead... 

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