Carbonate fluoroapatite

Phosphate is remineralized during the oxidation of organic matter and dissolution of hard parts, such as bones and teeth, that are composed of the minerals hydroxyapatite and fluoroapatite. Unlike the other products of remineralization, pore-water phosphate concentrations are regulated only by mineral solubility, such as through vivianite (iron phosphate) and francolite (carbonate fluoroapatite). Redox reactions are not significant because phosphorus exists nearly entirely in the h-5 oxidation state. 

The cycling of phosphorus in marine sediments is quite different from that of nitrogen in that its remineralized form, HPO , is readily sequestered in several mineral phases, namely carbonate fluoroapatite (CFA) and iron-rich clays and oxyhydroxides. 

Fig. 6. 3F1 -D MAS-NMR spectra of type B carbonated fluoroapatite weakly hydroxylated (a) F projection of C F HetCor MAS-NMR spectra, (b) F projection of 2-D P F HetCor MAS NM, (c) F SPE-MAS NMR spectrum.

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... 

Perrone et al. (2001) modelled Ni(II) adsorp-tion to synthetic carbonate fluoroapatite (CaI0 ((P04)5(C03))(0H,F). The solid phase had a pHIEP of 6.3 and a ZPC of 6.4 with an SSA of 8.8m2/g, an estimated sorption site density of 3.1 sites/nm2. They conducted 8-day isotherms in closed vessels at Ni concentrations of 5 x 10-10 to 1 x 10 8 M, constant I (0.05, 0.1 or 0.5 M), constant solid phase concentrations of 10 g/dm3 at pH values of 4 to 12. As Ni sorption occurred, no significant release of Ca was seen. Sorption was reversible. Rather than precisely characterize surface functional groups, they elected to describe their sorbent surfaces using acid-base reactions for the average behaviour of all sites involved in protonation and deprotonation. Potentiametric titration data were used to estimate the constants with the FTTEQL computer code ... 

Perrone, J., Fourest, B. Giffaut, E. 2001. Sorption of nickel on carbonate fluoroapatites. Journal of Colloid and Interface Science, 239, 303-313. 

The biologically most relevant calcium phosphates are dahllite [carbonate hydroxyapatite, (Na,Ca)10(PO4,CO3)6(OH)2] francolite [carbonate fluoroapatite] hydroxyapatite [Ca10(PO4)6(OH)2]... 

Authigenic minerals minerals formed in situ such as carbonate fluoroapatite and pyrite. 

Regnier, P. et al.. Mechanism of CO, substitution in carbonate-fluoroapatite Evidence from FUR spectroscopy, NMR, and quantum mechanical calculations, Am. Mineral., 19, 809, 1994. 

SrS04>. Carbonate-rich fluoroapatite minerals, such as francolite, are the major component of bones. As noted in Chapter 11, hard and soft parts can contain significant amoimts of trace metals. 

Apatites are complexes of cationic Ca " matched by HP04, COa ", OH , or F as anions. Depending on the counter-ion, apatite can occur in the forms carbonate apatite Caio(P04)6C03, as hydroxyapatite Caio(P04)e (0H)2, or fluoroapatite Caio(P04)6F2. In addition, alkaline earth carbonates also occur in bone. In adults, more than 1 kg calcium is stored in bone. 

The most common apatite is Ca5(P04)30H and is called hydroxyapatite. Other forms include chloroapatite (Ca5(P04)3Cl), fluoroapatite (Ca5(P04)3F) and carbonate apatite or dahllite (Ca5(P04)3C03). These minerals are in pure forms, but it is also possible to generate them by partial replacement of one anion by another or one cation by another. For example, Ca may be replaced by Pb by ionic substitution, yielding pyromorphites [Pb5(P04)3(0H,Cl,F)]. As we shall see in Chapter 16, this mineral is very important in stabilizing the hazardous metal Pb. Also as discussed in Chapter 2 and shall be seen in later chapters, Mg-based CBPCs have many applications, and hence minerals such as Mg5(P04)3(0H,Cl,F) are also very common. 

Typically, bone has a solid outer portion called cortical bone and a porous inner part called cancellous bone. The amounts of each vary with location in the body. The cortical bone is a ceramic containing calcium compounds and viscous liquids, a protein called collagen , and an organic polymer. In addition to HAP, bone consists of calcium carbonate and calcium phosphate. HAP is 69 wt.% of total calcium phosphate compounds [4]. Part of the Ca in these compounds is substituted by Na, K, Mg, and Sr. Hydroxyl ions in the HAP are also substituted by F, CO3, or Cl, which makes the apatite a fluoroapatite, dahllite or chloroapatite, respectively. These substitutions are considered to play significant roles in the structure and mechanical properties of bones. 

The fluorine in fluoroapatite can be substituted by chlorine on treatment with an N02-N0C1 mixture at 1000°C,534 and the B-type carbonate apatites lose carbon dioxide on heating between 700 and 1000 °C, giving CaO and hydroxylapatite.535 X-Ray data indicate strong similarities between the structure of strontium chloroapatite and the fluoro- and hydroxy-analogues.536... 

In their publication, LeGeros and LeGeros (1993) oudine the different apatites, ranging from natural apatite (minerals) to biological (human dentin, enamel, and bone) and synthetic (chemically synthesized) apatite. This publication clearly shows that apatite is a group of crystalline compounds. The most important of these compounds is calcium hydroxyapatite. All the related crystal structures, such as fluoroapatite, chloroapatite, and carbonate apatite are derived from it. 

Fluoroapatite (F-apatite), chloroapatite (Cl-apatite) and carbonate apatite (CO -apatite) are derived from hydroxyapatite. In F-apatite and Cl-apatite, the F and Cl ions assume the position of the OH ions. When F and Cl ions are inserted in the Ca triangle, their position in relation to the OH in hydroxyapatite changes (Fig. 1-23). As a result, the lattice parameters change compared with those of hydroxyapatite (Young and Elliot, 1966). When F ions are inserted in place of OH ions, the -axis is reduced and the c-axis remains constant (F = apatite a = 9.382 A, c = 6.880 A). The insertion of Cl ions enlarges the unit cell (Cl apatite a = 9.515 A, c = 6.858 A). The crystal structure of fluoroapatite is shown in Appendix 19. 

Fiuoride. Fluoride is the most effective agent available for strengthening tooth resistance to acid demineralization. The mechanism by which fluorine increases caries resistance of the teeth is not fully understood. However, it appears that crystals of fluoroapatite can replace some of the calcium phosphate crystals of hydroxyapatite that are normally deposited during tooth formation, and that it may also replace some of the carbonate normally found in the tooth. Apparently these fluoride substances are more resistant to mouth acids. Fluorine may also inactivate oral bacterial enzymes which create acids from carbohydrates. 

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