**4.5.2 Cadmium chlorapatite**

**4.5 Chlorapatites**

tite (**Eq. 21**) [82],[140].

described by the reaction [139]:

tion of stoichiometric chlorapatite [133].

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

**4.5.1 Synthetic analogues of the mineral chlorapatite**

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

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‐

The mechanochemical synthesis of chlorapatite in a high energy planetary mill should be

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‐

2 2 5 10 4 2 ( )<sup>6</sup> 9CaO CaCl 3 P O Ca PO Cl ++ ® (29)

Large crystals of Cd5(PO4)3Cl (space group P63/M, *a* = 9.633 Å, *c* = 6.484 Å and *Z* = 2) grow hydrothermally at 500°C and 800–1400 atm. from Na- and NH4-containing solutions [142], [143]. The phase transition in cadmium chlorapatite from P63/M to P6C/MCM was confirmed through the temperature dependent Raman measurements. The phase transition tempera‐ ture from lower temperature phase (P63/M) to high temperature phase (P63/MCM) is approxi‐ mately 700°C and was detected through the disappearance of low-temperature phase *A*<sup>g</sup> Raman bands as the temperature approached the transformation temperature [144].
