**1.5. Apatite group minerals**

The apatite group includes minerals listed in **Table 3**. The most important are three minerals with ideal apatite formula Ca5(PO4)3F, Ca5(PO4)3OH and Ca5(PO4)3Cl known as fluorapatite, hydroxylapatite and chlorapatite, respectively. They are recently renamed as apatite–(CaF), apatite–(CaOH) and apatite–(CaCl). The origin of those three distinct names to denote the F<sup>−</sup> , OH<sup>−</sup> and Cl<sup>−</sup> variants and their distinction with respect to the "original apatite" *sensu lato* is uncertain [41],[45]. The composition and the cell dimension of major end members of apatite group minerals are listed in **Table 7**.

**Fig. 12.** Optical properties of fluoro-, hydroxyl- and chlorapatite series based on the values of end-member apatites [39].

Apatite is optically negative and normally uniaxial, although biaxial varieties with optic angle up to 20° are known. Carbonate bearing apatites (e.g. francolite), in particular way, have anomalous optics. Large variations in the composition within the apatite group make the accurate correlation of optical data difficult. The refractive index is the highest for chlorapa‐ tite (**Fig. 12**) and decreases by the substitutions of OH and F [39].

<sup>37</sup> The distribution of atoms at cationic sites can be affected also by Coulombic effect as was recognized for the structure of mineral aiolosite (**Section 2.1.5**) and cesanite (**Section 2.1.7**).


**Table 7.** Composition and properties of calcium apatite end-member Ca5(PO4)3Z.

#### **1.5.1. Fluorapatite (Apatite–CaF))**

tory. Generally, **M**(1) sites (Wyckoff positions *4f* in P63/M structure, **Table 5**) are occupied by smaller cations (in particular Ca) and **M**(2) sites (Wyckoff positions *6h*) accommodate larger

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

The apatite group includes minerals listed in **Table 3**. The most important are three minerals with ideal apatite formula Ca5(PO4)3F, Ca5(PO4)3OH and Ca5(PO4)3Cl known as fluorapatite, hydroxylapatite and chlorapatite, respectively. They are recently renamed as apatite–(CaF), apatite–(CaOH) and apatite–(CaCl). The origin of those three distinct names to denote the F<sup>−</sup>

OH<sup>−</sup> and Cl<sup>−</sup> variants and their distinction with respect to the "original apatite" *sensu lato* is uncertain [41],[45]. The composition and the cell dimension of major end members of apatite

**Fig. 12.** Optical properties of fluoro-, hydroxyl- and chlorapatite series based on the values of end-member apatites

Apatite is optically negative and normally uniaxial, although biaxial varieties with optic angle up to 20° are known. Carbonate bearing apatites (e.g. francolite), in particular way, have anomalous optics. Large variations in the composition within the apatite group make the accurate correlation of optical data difficult. The refractive index is the highest for chlorapa‐

37 The distribution of atoms at cationic sites can be affected also by Coulombic effect as was recognized for the structure

tite (**Fig. 12**) and decreases by the substitutions of OH and F [39].

of mineral aiolosite (**Section 2.1.5**) and cesanite (**Section 2.1.7**).

,

cation37

[39].

such as Ba2+ or Pb2+ [45].

**1.5. Apatite group minerals**

group minerals are listed in **Table 7**.

Fluorapatite (Ca10(PO4)6F2, F-rich apatite, FAP and FAp) [45],[141] is the most common member in the group of apatite that can be found mainly in igneous rocks (fluorapatite is a common accessory mineral in syenites [39] and metamorphic environments, for example, in carbona‐

<sup>38</sup> In some works termed as hydroxylapatite.

<sup>39</sup> The symbol Ap, e.g. in HAp, reflects the abbreviation of mineral in rocks and ores, i.e. apatite = Ap.

<sup>40</sup> Depends on the type of carbonated apatite (**Section 2.6**), **Table 7** provides data for Ca5(PO4,CO3)3OH (ideal composition of TYPE-B).

<sup>41</sup> Apatite is known to be often nonstoichiometric, especially on the surface [41].

<sup>42</sup> Measured/calculated density of mineral.

**Fig. 13.** Fluorapatite from (a) Cerro de Mercado Mine, Victoria de Durango, Mexico and (b) Sljudjanka, Bajkal, Russia.

tites and mica-pyroxenites [142]. The mineral crystallizes in the form of well-formed hexago‐ nal crystals elongated on [0001], forms tabularto discoidal crystals flattened on {0001} or occurs as granular, globular to reniform, nodular and massive. The properties and chemical compo‐ sition of fluorapatite mineral are listed in **Table 7**.

The color of fluorapatite mineral (**Fig. 13**) is pale green, green, light blue, yellow, purple, or white. Some varieties are colorless. Very brittle mineral gets broken to small fragments showing conchoidal fractures. Apatite shows poor cleavage on {0001} and {101 ¯1}. Fluorapa‐ tite is the most insoluble from the phosphate minerals [143],[144]. Apatite forms rare contact twins on {112 ¯1} or {101 ¯3}. Some examples of fluorapatite crystals morphology are introduced in **Fig. 14**.

**Fig. 14.** Habit of fluorapatite crystals.

As was mentioned above (**Section 1.2**), fluorapatite was the first apatite with established structure [107],[108]. There is slight disagreement on the position of fluoride anion [145] Fluorapatite crystallizes in hexagonal dipyramidal crystal system with the parameters listed in **Table 7**. Atomic parameters including the number of atoms (*N*, where Σ*N* = 42) with equivalent site symmetry, their positions (coordinates *x*, *y* and *z*) in fractional coordinate system and equivalent isotropic temperature factor (*B*) for fluorapatite by HUGHES et al [33] are listed in **Table 8**.



**Table 8.** Positional parameters and equivalent isotropic factor for fluorapatite [33],[148].

## The lengths of bonds in the fluorapatite structure are listed in **Table 9**.


**Table 9.** Bond lengths in the structure of fluorapatite [33].

tites and mica-pyroxenites [142]. The mineral crystallizes in the form of well-formed hexago‐ nal crystals elongated on [0001], forms tabularto discoidal crystals flattened on {0001} or occurs as granular, globular to reniform, nodular and massive. The properties and chemical compo‐

**Fig. 13.** Fluorapatite from (a) Cerro de Mercado Mine, Victoria de Durango, Mexico and (b) Sljudjanka, Bajkal, Russia.

The color of fluorapatite mineral (**Fig. 13**) is pale green, green, light blue, yellow, purple, or white. Some varieties are colorless. Very brittle mineral gets broken to small fragments

tite is the most insoluble from the phosphate minerals [143],[144]. Apatite forms rare contact

As was mentioned above (**Section 1.2**), fluorapatite was the first apatite with established structure [107],[108]. There is slight disagreement on the position of fluoride anion [145] Fluorapatite crystallizes in hexagonal dipyramidal crystal system with the parameters listed in **Table 7**. Atomic parameters including the number of atoms (*N*, where Σ*N* = 42) with equivalent site symmetry, their positions (coordinates *x*, *y* and *z*) in fractional coordinate system and equivalent isotropic temperature factor (*B*) for fluorapatite by HUGHES et al [33] are

**Atom** *N* **Site symmetry** *x y z B* **[Å2**

Ca(1) 4 C3h 2/3 1/3 0.0010 0.91 Ca(2) 6 C3 -0.00712 0.24227 ¼ 0.77 P 6 Cs 0.36895 0.39850 ¼ 0.57

¯3}. Some examples of fluorapatite crystals morphology are introduced

¯1}. Fluorapa‐

**]**

showing conchoidal fractures. Apatite shows poor cleavage on {0001} and {101

sition of fluorapatite mineral are listed in **Table 7**.

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

¯1} or {101

**Fig. 14.** Habit of fluorapatite crystals.

listed in **Table 8**.

twins on {112

in **Fig. 14**.

**Fig. 15.** Primitive unit cell of fluorapatite (C10(PO4)6F2) with atoms labeled according to symmetric type of element (a). The crystal structure of apatite (b, perspective view along the *c*-axis) and structure of columns (c, d).

The hexagonal crystal structure of fluorapatite of P63/M space group is shown in **Fig. 15**. The atoms of Ca occupy two distinct sites [146], [147]:

**i.** Ca(I)O9 polyhedra in sevenfold coordination
