**2.6. Carbonate-apatites**

As mentioned previously (**Section 1.1**) the name of both most typical examples, i.e. carbo‐ nate-hydroxylapatite (Ca5(PO4,CO3)3OH) and carbonate-fluorapatite (Ca5(PO4,CO3)3F), was discredited from the IMA list of minerals [1]. The structure and the crystal shape of carbo‐ nate-apatite and carbonate-fluorapatite are shown in **Fig. 54**. The carbonate-apatites, the properties of which are listed in **Table 7** (**Chapter 1**), are intensively studied as the mineral constituents of bones and teeth as described in **Section 10.9.**

The carbonate-rich apatites are:

**1. Francolite** (Ca10−*x*−*<sup>y</sup>*Na*x*Mg*y*(PO4)6−*<sup>z</sup>*(CO3)*z*F0.4*z*F2 or Ca5(PO4,CO3)3F) [137] is the name used for massive, cryptocrystalline or amorphous varieties of carbonate-rich hydroxyl- and fluorapatite. Francolite and staffelite are the synonyms for carbonate-fluorapatite.

This complex carbonate-substituted apatite is found only in marine environments, and, to a much smaller extent, in weathered deposits, forinstance above carbonatites [138]. The mineral was named according to its occurrence at Wheal Franco, Whitchurch, Tavistock District, Devon, England.

**Fig. 54** The structure and the crystal shape of carbonate-hydroxylapatite (a) and fluorapatite (b).

	- **•** Radiating (previously incorrectly termed as staffelite)
	- **•** Optically amorphous

It is a hexagonal mineral that crystallizes in the space group P63/M with the crystallographic parameters of unit cell *a* = 9.532 and *c* = 6.199 Å, *a*:*c* = 1:0.6501, *V* = 587.78 Å3 and *Z* = 2. Calculated

density of pieczkaite is 3.783 g·cm−3. The hardness of the mineral on the Mohs scale varies in

The coordination polyhedron around Mn(1) has the point-group symmetry 3 and is a trigonal

prism in which the two triangles of oxygen atoms are slightly rotated relative to each other.

The coordination polyhedron around Mn(2) is a severely distorted octahedron. The phos‐

phate group is more distorted than in any of the other apatites. The chlorine atom is located

As mentioned previously (**Section 1.1**) the name of both most typical examples, i.e. carbo‐

nate-hydroxylapatite (Ca5(PO4,CO3)3OH) and carbonate-fluorapatite (Ca5(PO4,CO3)3F), was

discredited from the IMA list of minerals [1]. The structure and the crystal shape of carbo‐

nate-apatite and carbonate-fluorapatite are shown in **Fig. 54**. The carbonate-apatites, the

properties of which are listed in **Table 7** (**Chapter 1**), are intensively studied as the mineral

**1. Francolite** (Ca10−*x*−*<sup>y</sup>*Na*x*Mg*y*(PO4)6−*<sup>z</sup>*(CO3)*z*F0.4*z*F2 or Ca5(PO4,CO3)3F) [137] is the name used

fluorapatite. Francolite and staffelite are the synonyms for carbonate-fluorapatite.

for massive, cryptocrystalline or amorphous varieties of carbonate-rich hydroxyl- and

This complex carbonate-substituted apatite is found only in marine environments, and,

to a much smaller extent, in weathered deposits, forinstance above carbonatites [138]. The

mineral was named according to its occurrence at Wheal Franco, Whitchurch, Tavistock

the range from 4 to 5. The structure of the mineral pieczkaite is shown in **Fig. 53**.

98 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

in the center of an equilateral triangle formed by three Mn(2) atoms [136].

constituents of bones and teeth as described in **Section 10.9.**

**2.6. Carbonate-apatites**

The carbonate-rich apatites are:

District, Devon, England.

The mineral is usually gray or brown due to the content of organic, humic or ferrugi‐ nous impurities. Sometimes, it is white or black colored. Pure kurskite has a specific gravity of 3 g·cm−3.

**4. Collophane** (3Ca3(PO4)2·*n*Ca(CO3,F2,O)·*x*H2O [137]) this type of phosphate minerals is typical for marine phosphate sediments [138]. Apatite is a principal constituent of fossil bones and other organic matter. The name cellophane is sometimes used for such phosphatic material [143].

According to the accommodation of carbonate ion in the apatite structure, three basic types of apatites (**Fig. 55**) can be recognized [144]:


**Fig. 55** Part of the *c*-axis channel showing the accommodation of carbonate ion in the structure of hydroxylapatite [144].

Individual types of carbonate apatite and their importance for bone and dental enamel are described in **Section 10.9.2.**

Carbonate apatites have distinctive *X*-ray patterns and rather small cell parameter *a*. An empirical relationship between the content of CO2 in apatite and the separation (Δ [Å]) of the 211 and 112 *X*-ray diffraction lines has been given by O'BRIEN et al [145],[146]:

$$\text{1CO}\_2\ \text{[wt.\%]} = \text{17.335} - \text{(615.524} \cdot \text{\AA}) \tag{1}$$
