**Abstract**

The previous chapters were dedicated to description of structure and properties of minerals from supergroup of apatite and introduction of method for identification and investigation of properties of phosphate minerals. The first synthesis of apatite was performed by Daubrée by passing the PCl3 vapor over red-hot lime. The fourth chapter of this book introduces techniques for the preparation of synthetic analogs of apatite minerals including solid-state synthesis, wet chemical methods, hydrothermal synthesis as well as methods for preparation of single crystals. Chapter continues with descrip‐ tion of structure and properties of synthetic compounds of apatite type and ends with incorporation of 3d-metal ions into the hexagonal channel of apatite.

**Keywords:** Apatite, Solid-State Synthesis, Wet Chemical Methods, Hydrothermal Synthesis, Single crystal, Lacunar Apatite

The first synthesis of apatite was performed by DAUBRÉE [1], who obtained it in crystals by passing the vapor of phosphorus trichloride (PCl3) over red-hot lime. The synthetic mineral analogues of chlorapatite, fluorapatite, or the mixtures of these phases were prepared by MANROSS [2] via fusing of sodium phosphate either with calcium chloride, calcium fluoride, or both together. The similar process was also successfully used by BRIEGLEB [3]. FORCHHAMMER [4] prepared chlorapatite by fusing calcium phosphate with sodium chloride. When bone ash or marl was used instead of artificial calcium phosphate, mixed apatite was formed. Similar results were reported by DEVILLE and CARON [5], who fused bone ash with ammonium chloride and either calcium chloride or fluoride, and also by DITTE [6], who repeated Forchhammer's

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experiment [7]. ZAMBONINI and FERRUCIO [8] found that the fusion of Ca3(PO4)2 with CaCl2 produced apatite with very weak birefringence. Fusing Ca3(PO4)2 with an excess of NaCl gave crystals with the birefringence of 0.0050–0.0058.

By heating calcium phosphate with calcium chloride and water, under the pressure at 250°C, DEBRAY prepared chlorapatite [9]. WEINSHENK [10] also prepared chlorapatite by heating calcium chloride, ammonium phosphate and ammonium chloride at the temperature of 150 to 180°C in a sealed tube.1 CAMERON AND MCCAUGHEY [11] prepared fluorapatite by dissolving calcium fluoride in fused disodium phosphate and lixiviating2 the cooled melt. Artificial fluorapatite sometimes exhibits the peculiarity of re-entrant pyramidal ends or phantom crystals. Spodio‐ site (Ca2(PO4)F) is orthorhombic, chlorspodiosite (Ca2(PO4)Cl [12],[13]) being much less developed along the *c*-axis than fluorspodiosite [14]. Chlorapatite was formed when dicalci‐ um phosphate was added in excess to molten calcium chloride. When precipitated calcium phosphate was used, chlorspodiosite3 (Ca3(PO4)2·CaCl2, Ca2(PO4)Cl [14]) was obtained. Apatite was reported by HUTCHINS [15] and VOGHT [16] as present in lead-furnace slag [7].

The history of synthesis of various apatite compounds and substitution in the apatite struc‐ ture including the preparation of didymium bearing chlorapatite and chlorspodiosite is de‐ scribed in the work of ZAMBONINI and FERRUCIO [17]. The paper describes three of those apatite syntheses using the mixture of Ca3(PO4)2, DiPO4 4 and CaCl2 in the weight ratio of 1:0.07:2.19 (1), 1:0.15:8.76 (2) and 1:0.67:2 (3). These mixtures were heated to the temperature of 1180, 1000 and 1100°C, respectively. Transparent, colorless apatite with the content of 3% DiPO4 results from the first experiment. The second experiment leads to the pale-violet crys‐ tal of chlorspodiosite3. The crystals of DiPO4 and only little amount of chlorapatite and chlorspodiosite with 9% of DiPO4 were recognized in the third mixture.

HENDRICKS et al [19] reported the preparation of hydroxylapatite by the hydrolysis of tricalci‐ um phosphate5 with neutral ammonium citrate. Oxoapatite was then prepared by the igni‐ tion of hydroxylapatite to constant weight at 50°C.6

<sup>1</sup> The method is described in **Section 4.1.1**.

<sup>2</sup> The separation techniques based on leaching of minerals from a solid by dissolving them in a liquid.

<sup>3</sup> Chlorine analog of spodiosite [14]. Calcium chlorspodiosite is colorless crystalline compound structurally related to the mineral wagnerite (Mg2(PO4)F). It was reported by NACKEN in his study of the phase relationships which was obtained in the system CaCl2-CaO-P2O5 [12]. The synthesis of the compound by dissolving Ca3(PO4)2 in fused CaCl2 was reported by KLEMENT and GEMBRUCH [13]. Since the mineral was recognized as the mixture of fluorapatite, calcite and serpentine, it was discredited (IMA action 2003-03-B).

<sup>4</sup> Didymium (Di) was recognized as the mixture of element of neodymium and praseodymium [18].

<sup>5</sup> Tricalcium phosphate was prepared by slow addition of Na3PO4 solution to the solution with the excess of Ca(NO3)2. The precipitate was washed with saturated solution of Ca3(PO4)2 until the filtrate was free of nitrates. The salt was then dried at 50°C [19],[18].

<sup>6</sup> Hydroxylapatite is very difficult to dehydrate, even at high temperature (**Section 1.5.2**). As a consequence of this, many workers have mistaken it for oxyapatite [18].

**Fig. 1.** The phase equilibrium in carbonate-apatite–CaCO3 (a) and carbonate-apatite–CaCO3–Na2CO3 systems (b) [20].

EITEL [20] investigated the binary systems Ca3(PO4)2–CaCO3 (**Fig. 1**(**a**)) and Ca3(PO4)2–CaCO3– Na2CO3 7 (**b**) and claimed to have crystallized carbonate-apatite from a melt, but he deter‐ mined CO2 only qualitatively and on a sample that may have been impure. The structure of prepared carbonate-apatite is not known [18].

The equilibrium given by the following scheme was predicted- for carbonate-apatite [20]:

$$\text{CaCO}\_3 + \text{liquid} \leftrightarrow \text{carbonate} - \text{apatie} + \text{Na}\_2\text{Ca} \text{(CO}\_3\text{)}\_2 \tag{1}$$

$$\begin{aligned} \text{liquid} & \leftrightarrow \text{carbonate} - \text{apatie} + \text{Na}\_2\text{Ca}\{\text{CO}\_3\}\_2 \\ & + \text{Na}\_2\{\text{CO}\_3\}\_2 \text{ (mix of crystals)} \end{aligned} \tag{2}$$

The prepared crystals are typically long or short lengths with hexagonal prism {101 ¯0}, dipyramid {101 ¯1} and pinacoid {0001}. The skeletal crystals had inclusions along the central canals parallel to the *c*-axis. The refractive index is uniaxial (-), *ε* = 1.635, *ω* = 1.626, *δ* = 0.009.
