Apatite structure

Vanadates For the isostructural strontium and barium ortho-vanadates the V-—0 vibrational frequencies decrease with increasing effective nuclear charge [51]. Similar orthovanadates with the apatite structure have for a given halogen ion decreasing frequencies of the VO4 ion in the order Ca>Sr >Ba ((62) and Table 4]. However, it is impossible to see any trend towards the tiivalent lanthanide cations (59). The IR bands are so wide in this case that small changes cannot be measured. 

Table 4. IR data (in cm i) for orthovanadates with the apatite structure (62)...

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],... 

The XRD pattern of FA is reported in Fig. 4. It is characteristic of the apatite structure. This pattern is very analogous to that of HA. Fluoride substitution for OH is related to a decrease in the a unit-cell dimension and a very slight increase in the c unit-cell parameter. 

The vibration bands relative to phosphate groups in the apatite structure differ from the normal modes of the P04 isolated ion, due to distortions of the PO4 tetrahedra in the apatite lattice and vibrational coupling [4]. Therefore, site-group and factor-group analyses were applied [15,16,18,21] to elucidate the vibrational spectra observed (Fig. 5) and band assignments of infrared (IR) and Raman bands have been given (Table 3). 

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]). 

The synthesis of B-type carbonate-fluoroapatites was reported by Montel et al. with fluoride ions present in two kinds of sites in the apatitic structure [126]. Moreover, the correlation between the respective amounts of fluoride and... 

E. R. Kreidler, F.A. Hummel, The crystal chemistry of apatite Structure fields of fluor-and chlorapatite. Am. Mineral. 55 (1970) 170-184. 

R. LeGeros, O.R. Trautz, E. Klein, J.P. LeGeros, Two types of carbonate substitution in the apatite structure, Experientia 25 (1969) 5-7. 

M. lijima, D.G.A. Nelson, Y. Pan, A.T. Kreinbrink, M. Adachi, T. Goto, Y. Mori waki, Fluoride analysis of apatite crystals with a central planar OCP inclusion Concerning the role of F" ions on apatite/OCP/apatite structure formation, Calcif. Tissue Int. 59 (1996) 377-384. 

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. 

Thus different luminescence takes place after activation in air and in vacuum. As was already mentioned, in the apatite structure Ca5(P04)3F there are two types of Ca site Ca(I) with C3 symmetry and Ca(II) with Cs sym-... 

It is interesting to note that in magmatic apatites the luminescence of uranium containing centers have not been discovered before or after oxidizing heating. Thus it is reasonable to suppose that uranium is present mainly in the 11" + form. The U with an ionic radius of 0.97 A may be located in the apatite structure instead of Ca with the ionic radius of 0.99 A. The most likely way for achieving the excess charge compensation is the Na" for Ca " structural substitutions. 

The relations involve introducing into the apatite structure mirror planes parallel to (0001) at z/c = 0 n,odi ot ll lmodi- The ( Mn ) octahedra that were at OOz, and centred at the mirror plane level, are thus converted to trigonal prisms. In nasonite these mirrors are at intervals of z/c = 1 (of the apatite structure), so that octahedra and trigonal prisms... 

The apatite structures, Laio- fM60 26+j/ (M=Si or Ge), conduct well at very high... 

The so-called Ling oxalate titration indicates that CCP consists of 80% Ca3(P04)2 and 20% CaHP04, with an overall Ca P ratio of 1.4 1 (Pyne, 1962). However, the oxalate titration procedure has been criticized because many of the assumptions made are not reliable. Pyne and McGann (1960) developed a new technique to study the composition of CCP. Milk was acidified to about pH 4.9 at 2°C, followed by exhaustive dialysis of the acidified milk against a large excess of bulk milk this procedure restored the acidified milk to normality in all respects except that CCP was not reformed. Analysis of milk and CCP-free milk (assumed to differ from milk only in respect of CCP) showed that the ratio of Ca P in CCP was 1.7 1. The difference between this value and that obtained by the oxalate titration (i.e. 1.4 1) was attributed to the presence of citrate in the CCP complex, which is not measured by the oxalate method. Pyne and McGann (1960) suggested that CCP has an apatite structure with the formula ... 

Halophosphates crystallize in the apatite structure. The chemical composition can be expressed most clearly as 3Ca3(P04)2 Ca(F, Cl)2 Sb3+, Mn2+. 

Vanadium is deposited more slowly in bone than in other tissues and clears more slowly as well1721. In this tissue it is possible that the metal is immobilized through a substitution of VO2- for PO2- in the apatite structure. Alternatively, V02+ has been shown to be adsorbed onto synthetic apatites, but this is relatively easily displaced177- 178). 

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