**4.5 Chlorapatites**

#### **4.5.1 Synthetic analogues of the mineral chlorapatite**

The stoichiometric Ca:P ratio in the composition of chlorapatite, the mole ratio of calcium to phosphorous was equal to 1.67 [139]. The reaction of CaCl2 with H3PO4 under hydrothermal conditions including the temperature of 400°C and the pressure < 3 kbar leads to chlorapa‐ tite (**Eq. 21**) [82],[140].

The mechanochemical synthesis of chlorapatite in a high energy planetary mill should be described by the reaction [139]:

$$\text{9CaO} + \text{CaCl}\_2 + \text{3 P}\_2\text{O}\_3 \rightarrow \text{Ca}\_{10} \text{ (PO}\_4\text{)}\_6\text{Cl}\_2 \tag{29}$$

NACKEN [141] determined the phase diagram for the section Ca3(PO4)2–CaCl2 of the ternary system CaO–P2O5–CaCl2 (**Fig. 24**). Chlorapatite crystallized from melts of its own composi‐ tion is highly deficient in Cl, while lower temperatures near 1040°C lead to the crystalliza‐ tion of stoichiometric chlorapatite [133].

**Fig. 24.** Phase equilibrium in the system Ca3(PO4)2–CaCl2 by NACKEN [133].

The mechanosynthesis and the characterization of chlorapatite nanopowders were per‐ formed by FAHAMI et al [139]. The formation of chlorapatite takes place according to the reaction 29. At the beginning of milling, the main products were stoichiometrically deficient chlorapatite and calcium oxide. Eventually, high crystalline CAP nanopowder was obtained after 300 min of milling. By increasing the milling time to 300 min, the lattice strain signifi‐ cantly increased.
