ATOMIC STRUCTURE OF BIOLOGICAL APATITES

It is clear from the earlier discussion, particularly spectroscopic studies, that the mineral in bones and teeth differs from HAP in many respects, though it is clearly apatitic. Quantitatively, the main differences are the variable Ca/P ratio, and the HP04 and CO3 contents found in biological apatites. 

Models of various degrees of complexity have been proposed for Ca-def Aps (see Elliott 1994). One of the earliest and simplest, 

In apatite, there are two crystallographically independent Ca sites. Cal and Ca2 (see Pan and Fleet, this volume, p. 13). Rietveld analysis of XRD powder data shows that it is predominantly Ca2 that is lost in Ca-def Aps (Jeanjean et al. 1996). This preferential loss has recently been confirmed in a study of well-crystallized Ca-def Aps prepared at 80-95°C precipitated from solution by a slow increase in pH caused by the hydrolysis of urea (Wilson, Elliott, Dowker and Rodriguez—unpublished results) or formamide (Wilson, Elliott and Dowker— unpublished results). 

Rietveld structure refinements have also been undertaken on synthetic precipitated COsAps as model systems for biological apatites. The reduction in apparent PO4 volume, and P and Ca2 occupancies seen in dental enamel were also seen in Na-free precipitated COsAps with CO3 contents of 4 wt % produced by the reaction between CaCOs and CaHP04 at 100°C (Morgan et al. 2000). COsAps precipitated in the presence of Na ions 

Mineralized tissues contain a significant amount of Mg (Table 3). Although Mg seems to be important in mineralized tissues, there has always been doubt about whether it was all adsorbed on the apatites crystal surfaces, or whether a significant amount replaced Ca ions in the apatite lattice (Glimcher 1998). Lattice substitution is doubted because the radius of the Mg ion (0.72 A) is much less than that of the Ca ion (1.00 A) and because definitive proof of the possibility of such a substitution was lacking. However, Bigi et al. (1996) have used Rietveld analysis of XRD data to show that a limited replacement (at most 10 atom %) appeared to be possible. It seemed that Mg had replaced some Ca ions in Ca2 sites. 

---

Biological apatite crystals are too small to allow direct measurement of their density. However, the calculated density from estimated unit cell contents (see later section on atomic structure of biological apatites) is about 5% less than HAP (Elliott 1997). 

---