B-type carbonate-apatites

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... 

Both A-type and B-type carbonated apatite structure are chemically unstable yielding a HA structure that is more reactive than synthetic HA under physiological conditions (Habibovic et al. 2010). 

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... 

Apatites with large amounts of fluoride ions the B-type carbonated fluorapatite. 

LEHR and Me CLELLAN (3) demonstrated numerous natural apatites and a correlation between the amount of fluoride ions and that of the carbonate ions. This led them to propose that PO -ions can be replaced by COg - ions associated with F ions. Such a hypothesis could explain the abnormally high amount of fluoride in some FRANC0LITES. However this type of substitution was not proved by the authors. We studied some synthetic apatites where fluoride and carbonate ions were simultaneously introduced. Samples of B-type carbonated fluorapatite (COj - substituting PO -) were obtained as a powder from an aqueous medium rich in fluoride ions and also an aqueous medium poor in fluoride ions. 

In order to simulate the amorphous to crystalline transformation, Milev et al. developed a novel method to produce single-phase, nano-sized, plate-like, mixed A-B-type carbonate-containing apatite (CAp) similar to bone apatite by using sol-gel technology (Milev et al., 2003). The methodology emulates biomineralization, where topotactic transition from amorphous to octacalcium phosphate (OCP) than to hydroxyapatite (HAp), which is believed to occur in vivo. 

After setting of the cement, gradual phase transformation of the Ca-P compounds occurs into more apatitic phases. Due to the nonsintered nature and the higher surface area of Ca-P cements, dissolution of calcium and phosphate ions in physiological conditions occurs to a higher extent than in sintered ceramics. However, the relatively large surface area in combination with supersaturated body fluids also results in high Ca-P precipitation. Ishikawa et al. [23] reported that the total precipitation of (B-type) carbonate HA at the surface of Ca-P cement increased the total surface dissolution in simulated blood plasma in vitro for a period of 20 weeks. In vivo, formation of carbonated HA on cement surface has been reported to occur within 12 h of implantation [24]. This phenomenon could be one of the essential aspects of the exceptionally positive osteoconductive behavior of Ca-P cements. 

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 occupied by a second kind of fluoride ion (adapted from Ref. [4]). (See Colour Plate Section at the end of this book.)...

M. Vignoles, G. Bonel, Sur la localisation des ions fluorures dans les carbonate apatites de type B, C.R. Acad. Sci. (Paris) 287 (1978) 321-324. 

Figure 4.17 FUR spectra of Na-bearing type A-B carbonate apatites characterised by OH stretching bands nears 3600 cm-1 (v0H) and asymmetric stretching (v3) and out-of-plane...

In type A carbonate apatite, maximum substitution is attained when one COj" has replaced two OH and created one channel vacancy . In type B carbonate apatite, CO replaces PO ", and cation vacancies are created in order to maintain charge balance. In type AB , both types of substitution are found. 

Other phases of Ca-P than ACP reveal a crystalline structure with characteristic peaks on XRD analysis. There is a broad range in crystal morphology depending on composition and preparation characteristics such as temperature, pH, impurity, and the presence of macromolecules. Impurities, as commonly occur in bone mineral, greatly influence crystallinity (reflecting crystal size and crystal strain) but depend on the type of substitution. For example, type B carbonated apatite (CO3 for PO4 substitution) has a lower crystallinity and increased solubility, whereas F substitution (F for OH) give the opposite effects due to a better fit of the F ion in the apatite crystal structure. 

As shown in Table 3.2, bone mineral contains a high amount of carbonate. Biological apatites as present in human bone can therefore be referred to as carbonated apatite (CHA). Carbonate in biological apatites primarily substitutes for phosphate groups (B-type substitution). This type of substitution in synthetic COj-Aps can be obtained by synthesis at lower temperatures (60-100°C) (LeGeros 1965). On the other hand, when the reaction is performed at high temperatures (800-1000°C) for several hours in dry COj, the substitution preferentially takes place on OH" molecular sites (A-type substitution) (Elliott 1994). 

Abstract Solid-state NMR studies on bone, bone mineral standards and collagen are reviewed. NMR spectroscopy was mostly applied to the bone mineral and confirmed that the structure resembles that of calcium carbonatoapatite of type B. Apatite in bone was found to be deficient in structural hydroxyl groups. Concentration and distribution of hydrogen-phosphate and carbonate ions, and of water in apatite crystals (interior vs surface and crystal defects vs structural positions) were closely investigated. The NMR characterization of the organic matrix still remains a challenge for future research. 

Fleet, M.E. and Liu, X. (2007) Coupled substitution of type A and B carbonate in sodium-bearing apatite. Biomaterials, 28, 916-926. 

Le Geros RZ (1965) Effect of carbonate on the lattice parameters of apatite. Nature 206 403-404 Le Geros RZ, Miravite MA, Quirolgico GB, Curzon ME (1977) The effect of some trace elements on the lattice parameters of human and synthetic bone. Calc Tiss Res 22 362-367 Le Geros RZ, Bond B, Le Geros R (1978) Types of H2O in human enamel and in precipitated apatites. CalcifTiss Inti 26 111-118... 

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