**4.6 Carbonated (biological) apatites**

It seems now to be generally accepted that CO3 2− dominantly replaces PO4 3− in biological apatite (BAP, BAp) [155]. Carbonate-hydroxyl-apatite (Ca10(PO4,CO3)6(OH)2), can be found mainly on islands and in caves, as a part of bird and bat excrements, guano [156].

#### **4.6.1 Carbonated hydroxyl<sup>−</sup> and fluorapatite**

Carbonated hydroxylapatite is the most important mineral in human dental enamel and bone [157],[158],[159],[160],[161],[162],[163],[164]. The presence of highly carbonated apatite was also proposed as a marker of the presence of bacteria (infectious microorganism) in kidney stones51 [165],[166]. According to the position of planar bivalent carbonate ion (CO3 2−) with anionic radius of 0.176 nm in the structure of apatite, three kinds of carbonate apatite are recognized in literature [157],[158],[167],[168],[169].

<sup>47</sup> The weight of liquid was measured instead of the weight of crystal.

<sup>48</sup> Since measured parameter is force (*F*, measured by tensiometer or microbalance), the Wilhelmy plate (or rod) method can be easily applied for small contact angles. The surface tension (*γ*) is calculated from the equation: *γ* = *F*/ (*l* cos *Θ*), where *l* = 2×length + 2×width of the plate, and wetting angle *Θ* is usually not determined but its value is taken for zero (complete wetting is assumed) or the value from either literature is used [149],[150],[151].

<sup>49</sup> The value should be affected by the estimation of value of aspect ratio.

<sup>50</sup> First-order hexagonal prism and first-order dipyramid, respectively.

<sup>51</sup> From the medical point of view, pathological calcifications refer to a concretion, e.g. a kidney stone, often associated with the tissue alteration. Additionally, normal physiological calcifications such as bone may become pathological through the influence of diseases such as arthrosis or osteoporosis Different chemical phases constitute the pathological calcifications, but calcium phosphate apatites are present in most of them [166]. Biological apatites (BAP) are described in **Section 7.1.3**.

**1. Type A**: CO3 2− substitutes for OH<sup>−</sup> (or *Z*<sup>−</sup> anion in general) in the apatite channel at z ≈ 0.5 (**Fig. 27**(**a**)) by two triad clusters of Ca2+ atoms z ≈ 0.25 (1/4) and 0.75 (3/4). The composi‐ tion of CCAP is given by the formula: Ca10(PO4)6(CO3)x(OH)2−2x. The TYPE-A substitution can be described as follows [170]:

SUZUKY and KIBE [148] used the NaCl flux method to prepare barium (Ba5(PO4)3Cl) and

for the determination of surface free energy (~26 mN·m−1 for both apatite crystals49

determination of specific surface free energies (surface tension) for single crystal of Sr5(PO4)3Cl [152] (aspect ratio is 3.2) via the measurement of contact angles of water and formamide (CH3NO) shows that ideal flat surface without a step should have uniform specific

tively. Experimentally obtained specific surface free energies roughly satisfy the Wulff's

surface free energy, estimated to ≤ 26 and ≤ 50 mN·m−1 for (101̅0) and (101̅1) faces,<sup>50</sup>

is the specific surface free energy of the *i*-th face of the crystal and *h*<sup>i</sup>

apatite (BAP, BAp) [155]. Carbonate-hydroxyl-apatite (Ca10(PO4,CO3)6(OH)2), can be found

Carbonated hydroxylapatite is the most important mineral in human dental enamel and bone [157],[158],[159],[160],[161],[162],[163],[164]. The presence of highly carbonated apatite was also proposed as a marker of the presence of bacteria (infectious microorganism) in kidney

[165],[166]. According to the position of planar bivalent carbonate ion (CO3

anionic radius of 0.176 nm in the structure of apatite, three kinds of carbonate apatite are

<sup>48</sup> Since measured parameter is force (*F*, measured by tensiometer or microbalance), the Wilhelmy plate (or rod) method can be easily applied for small contact angles. The surface tension (*γ*) is calculated from the equation: *γ* = *F*/ (*l* cos *Θ*), where *l* = 2×length + 2×width of the plate, and wetting angle *Θ* is usually not determined but its value is taken for zero

<sup>51</sup> From the medical point of view, pathological calcifications refer to a concretion, e.g. a kidney stone, often associated with the tissue alteration. Additionally, normal physiological calcifications such as bone may become pathological through the influence of diseases such as arthrosis or osteoporosis Different chemical phases constitute the pathological calcifications, but calcium phosphate apatites are present in most of them [166]. Biological apatites (BAP) are described

mainly on islands and in caves, as a part of bird and bat excrements, guano [156].

 **and fluorapatite**

(complete wetting is assumed) or the value from either literature is used [149],[150],[151].

i i Wilhelmy method [149],[150],

<sup>γ</sup> const, <sup>h</sup> <sup>=</sup> (30)

2− dominantly replaces PO4

). The

respec‐

is the distance

3− in biological

2−) with

strontium chlorapatite (Sr5(PO4)3Cl) crystals and modified.47

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

of face from the Wulff's (central) point of crystal.

**4.6 Carbonated (biological) apatites**

**4.6.1 Carbonated hydroxyl<sup>−</sup>**

It seems now to be generally accepted that CO3

recognized in literature [157],[158],[167],[168],[169].

47 The weight of liquid was measured instead of the weight of crystal.

49 The value should be affected by the estimation of value of aspect ratio. 50 First-order hexagonal prism and first-order dipyramid, respectively.

[151]48

where *γ*<sup>i</sup>

stones51

in **Section 7.1.3**.

relationship [153],[154]:

$$\begin{aligned} \text{Ca}\_{10}\text{(PO}\_{4}\text{)}\_{6}\text{(OH)}\_{2} + \text{CO}\_{3}^{2-} &\rightarrow \text{Ca}\_{10}\text{(PO}\_{4}\text{)}\_{6}\text{CO}\_{3} + 2\text{ OH}^{-}\\ \text{or} \quad \left[\text{\$\,^{\text{A}}\text{CO}\_{3}^{2-} + \text{}^{\text{A}}\text{[V]} \Leftrightarrow 2\text{(OH}^{-}, \text{F}^{-}, \text{Cl}^{-}\text{)}\right] \end{aligned} \tag{31}$$

The crystal structure of type-A carbonate apatite is controversial [172]. There are three different structures: with the space group*P*6 ¯in hexagonal symmetry (**Fig. 27**) [171], with the space group*P*3 ¯[159],[173] and with the space group Pb in monoclinic symmetry [174], [175].

**Fig. 26.** The atomic configuration of type-A carbonated apatite on the (010) plane (a) and (001) plane (b) according to SUETSUGU et al [171].

**2. Type B**: CO3 2− substitutes for phosphate ion (PO<sup>4</sup> 3−). Different chemical formula is used to describe B-type carbonate apatite, the simplest and often used is [176]: Ca10−x(PO4)6−x(CO3)x(OH)2−x. The interpretation of the location of type-B carbonate ion is also problematical [158]. The carbonate ion is located in the vicinity of substituted phosphate group and occupies as many phosphate oxygen sites as possible. Some examples of the B-TYPE substitutions are described by equations [170],[177]:

$$\begin{aligned} \text{Ca}\_{10} \text{(PO}\_4\text{)}\_{6} \text{(OH)}\_2 + \text{Na}^+ + \text{CO}\_3^{2-} &\rightarrow \text{Ca}\_9\text{Na} \text{(PO}\_4\text{)}\_{5}\text{CO}\_3 \text{(OH)}\_2 \\ + \text{Ca}^{2+} + \text{PO}\_4^{3-} &\end{aligned} \tag{32}$$

$$\begin{aligned} \text{Ca}\_{10}\text{(PO}\_{4}\text{)}\_{6}\text{(OH)}\_{2} + \text{CO}\_{3}^{2-} &\rightarrow \text{Ca}\_{9}\text{[V}\_{\text{Cu}}\text{]}\text{(PO}\_{4}\text{)}\_{3}\text{CO}\_{3}\text{(OH)}\text{[V}\_{\text{OH}}\text{]}\\ + \text{PO}\_{4}^{3-} + \text{OH}^{-} &\rightarrow \text{Ca}^{2+} \end{aligned} \tag{33}$$

$$\begin{aligned} \text{Ca}\_{10} \text{(PO}\_4\text{)}\_6 \text{(OH)}\_2 + \text{HPO}\_4^{2-} + \text{CO}\_3^{2-} &\rightarrow\\ \text{Ca}\_9 \text{[V}\_{\text{Cu}}\text{]}\text{HPO}\_4 \text{(PO}\_4\text{)}\_4 \text{CO}\_3 \text{(OH)} \text{[V}\_{\text{OH}}\text{]} + 2 \text{ PO}\_4^{3-} + \text{Ca}^{2+} \end{aligned} \tag{34}$$

where *V* denotes the vacancy. There is no clear energetic preference of CO3 2− to substi‐ tute for any particular PO4 3− group [178]. Sodium (**Eq. 33**) or other alkali metal cation (AM = Li, Na, K, Rb and Cs) is also known to increase the maximum ratio of carbonate substitution in B-site because its incorporation in calcium sites induces favorable electrical charge balance [176]:

$$\left[\mathrm{^{\text{B}}CO\_{3}^{2-}} + \mathrm{AM}^{\*} \Leftrightarrow \mathrm{Ca}^{2+} + \mathrm{PO}\_{4}^{3-}\right] \tag{35}$$

**3. Type AB**: mixed A-B type of apatite, where the composition can be described as: Ca10(PO4)6−y(CO3)x+(3/2)y(OH)2−2x. If both PO4 3− (*z* ≈ 0.25 or 0.75) and OH<sup>−</sup> anions were replaced by two CO3 2−, the process can be described as follows:

$$\begin{\begin{aligned} \text{Ca}\_{10} \text{(PO}\_4\text{)}\_6 \text{(OH)}\_2 + 2\text{ CO}\_3^{2-} &\rightarrow \text{Ca}\_{10} \text{(PO}\_4\text{)}\_5 \text{(CO}\_3\text{)}\_2 \text{(OH)}\\ + \text{OH}^- + \text{PO}\_4^{3-} &\end{aligned} \tag{36}$$

Two different structural roles of CO3 2− anion result in characteristic infrared (IR) signatures: type A carbonate having a doublet band at about 1545 and 1450 cm−1 (asymmetric stretching vibration, *ν*3) and a singlet band at 880 cm−1 (out-of-plane bending vibration, *ν*2), and type B having these bands at about 1455, 1410 and 875 cm−1, respectively [158].

Published structural studies [158] of carbonated apatites were performed with the synthetic phases. REN et al [171] investigated the structure of carbonated apatite using the AB INITIO simulation (**Fig. 27**) with the conclusion that the most energetically stable substitutions is TYPE-AB in which two carbonate ions replace one phosphate group and one hydroxyl group respectively. The crystal structure of A-TYPE of carbonated apatite is energetically more favorable than B-TYPE of substitution. The most stable configuration of TYPE-A is carbonate triangular plane almost parallel to the *c*-axis at *z* = 0.46. TYPE-A substitution tends to increase the lattice parameter *a* but decreases *c* whereas TYPE-B substitution shows the opposite effect. The lowest energy configuration of TYPE-B has calcium ion replaced by a sodium ion to balance the charge (**Eq. 33**) and the carbonate lying almost flat on the *b*/*c* plane.

( )( ) [ ]( ) ()[ ] <sup>2</sup> 10 4 6 2 3 9 Ca 4 3 5 OH

2 2

+ +®


(AM = Li, Na, K, Rb and Cs) is also known to increase the maximum ratio of carbonate substitution in B-site because its incorporation in calcium sites induces favorable electrical

+++ (33)

3 2

3− group [178]. Sodium (**Eq. 33**) or other alkali metal cation


3− (*z* ≈ 0.25 or 0.75) and OH<sup>−</sup>

2− anion result in characteristic infrared (IR) signatures:


+ + (34)

2− to substi‐

anions were

Ca PO OH CO Ca V PO CO OH V


+ ®

[ ] ( ) ()[ ]

9 Ca 4 4 3 4 OH 4

Ca V HPO PO CO OH V 2 PO Ca

where *V* denotes the vacancy. There is no clear energetic preference of CO3

B 2 2 3 CO AM Ca PO 3 4

**3. Type AB**: mixed A-B type of apatite, where the composition can be described as:

2−, the process can be described as follows:

( )( ) ( )( )( ) <sup>2</sup> 10 4 6 2 3 10 4 3 5 2

type A carbonate having a doublet band at about 1545 and 1450 cm−1 (asymmetric stretching vibration, *ν*3) and a singlet band at 880 cm−1 (out-of-plane bending vibration, *ν*2), and type B

Published structural studies [158] of carbonated apatites were performed with the synthetic phases. REN et al [171] investigated the structure of carbonated apatite using the AB INITIO simulation (**Fig. 27**) with the conclusion that the most energetically stable substitutions is TYPE-AB in which two carbonate ions replace one phosphate group and one hydroxyl group respectively. The crystal structure of A-TYPE of carbonated apatite is energetically more favorable than B-TYPE of substitution. The most stable configuration of TYPE-A is carbonate triangular plane almost parallel to the *c*-axis at *z* = 0.46. TYPE-A substitution tends to increase the lattice parameter *a* but decreases *c* whereas TYPE-B substitution shows the opposite effect. The lowest energy configuration of TYPE-B has calcium ion replaced by a sodium ion to balance

+ + (36)

Ca PO OH 2 CO Ca PO CO OH


+ ®

having these bands at about 1455, 1410 and 875 cm−1, respectively [158].

the charge (**Eq. 33**) and the carbonate lying almost flat on the *b*/*c* plane.

10 4 6 2 4 3

Ca PO OH HPO CO

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

3 2


( )( )

Ca10(PO4)6−y(CO3)x+(3/2)y(OH)2−2x. If both PO4

3 4

OH PO

Two different structural roles of CO3


PO OH Ca

4

tute for any particular PO4

charge balance [176]:

replaced by two CO3

**Fig. 27.** The model of channel structure for ideal carbonate ion geometry of TYPE-A (a) [101] and the energetically fa‐ vored configuration of TYPE-A carbonated apatite (b), the most stable structure of TYPE-B carbonated apatite (c) and the configuration of TYPE-AB after the geometry optimization (d) [170].

The type-A of carbonated apatite in which carbonate ion was completely substituted for the hydroxyl site, was synthesized by heating low crystalline and stoichiometric synthetic analogue of hydroxylapatite powder in the flow of dry carbon dioxide gas at 1000°C for 24 h by TONEGAWA et al [172]. The chemical composition of this phase can be described by the formula: Ca10(PO4)6(CO3)0.93±0.06. The crystal structure was determined to be of monoclinic symmetry with the space group PB in the temperature range from 25 to 500°C.

The synthesis of type-A carbonate apatite can be performed by heating of pure HAP at temperatures from 800 to 1000°C for several hours in dry CO2 atmosphere according to the reaction [176],[179]:

$$\begin{aligned} \text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{(OH)}\_2 + \text{y CO}\_2 &\leftrightarrow \text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{(OH)}\_{2-\text{y}}\text{(CO}\_3\text{)}\_\text{y} \\ + \text{y H}\_2\text{O} &\end{aligned} \tag{37}$$

Type-B carbonated apatite powders are generally synthesized from the precipitation reac‐ tion in aqueous media [176]. The reaction **38** (or **Eq. 4** as was described in **Section 1.5.2**) can be used for the capture of carbon dioxide at high temperature over the operation limit of CaObased sorbents.

The mechanochemical synthesis of B-type carbonated fluorapatite under argon atmosphere using high-energy planetary ball mill was described by N.-TABRIZI and FAHAMI [180]. The process can be described by the reaction:

$$\begin{aligned} \text{(9-x/2)} & \text{ CaCO}\_3 + \text{(3-x/2)} \; \text{P}\_2\text{O}\_5 + \text{CaF}\_2 \rightarrow \text{Ca}\_{10-\text{x/2}} \left( \text{PO}\_4 \right)\_{6-\text{x}} \text{(CO}\_3 \right)\_x \text{F}\_2 \\ &+ \text{(9-3x/2)} \text{CO}\_2 \end{aligned} \tag{38}$$

#### **4.6.2 Carbonate-chlorapatites**

Carbonated chlorapatite nanopowders can be synthesized by the mechanochemical process under argon atmosphere using the mixture of calcite (CaCO3), phosphorus pentoxide (P2O5) and calcium chloride (CaCl2) as raw materials [181],[182]:

$$\begin{aligned} \left(9-\text{x}/2\right) \text{CaCO}\_3 + \left(3-\text{x}/2\right) \text{P}\_2\text{O}\_3 + \text{CaCl}\_2 &\rightarrow\\ \text{Ca}\_{10-\text{x}/2} \left(\text{PO}\_4\right)\_{6-\text{x}} \left(\text{CO}\_3\right)\_\text{Cl} &\rightarrow \left(9-\text{3x}/2\right) \text{CO}\_2 \end{aligned} \tag{39}$$

The substitution degree of PO4 3− was given by the *x* value in the general formula of TYPE-B of Ca10−x/2(PO4)6−x(CO3)xCl2.

The high-pressure (1 GPa) synthesis of sodium-bearing carbonate chlorapatite of TYPE A-B (CCLAP, Ca10−(y+z)Nay[V]z[(PO4)6−(y+2z)(CO3)y+2z][Cl2−2x(CO3)x], where *x* ≈ *y* ≈ 4*z* ≈ 0.4) from carbonate rich melt in the temperature range from 1000 to 1350°C, was described by FLEET et al [169].

**Fig. 28.** The structure of carbonate chlorapatite showing one of 12 possible orientations of the type-A carbonate ion in apatite channel: the unit-cell origin is in the center of figure, shaded phosphate polyhedra and Ca(2) atoms are cen‐ tered at *z* = 3/4 (a) and the fragment of CCLAP structure showing the location of B carbonate ion close to the sloping faces of substituted phosphate tetrahedron (b) according to FLEET AND LIU [169 ].

The structure of Na-bearing CCLAP crystals (**Fig. 28**(**a**)) with the contents of Na and A and Btype of carbonate ranges between those of Na-bearing carbonated fluorapatite (CFAP) and carbonated hydroxylapatite (CHAP). The stoichiometric amount of Na and A-type of carbonate is consistent with the near linear (1:1) correlation reported for CHAP and CFAP and provides the evidence of active role of Na in the substitution of carbonate into the apatite channel, even if Na does not appear in usual charge-balanced substitution scheme [169]:

#### Synthetic Phase with the Structure of Apatite http://dx.doi.org/10.5772/62212 217

$$\left[\,^{\Lambda}\mathrm{CO}\_{3}^{2-} \Leftrightarrow 2\,\mathrm{Cl}^{-}\right] \tag{40}$$

On the other hand, the B : Na ratio is higher than one (approximately 1 : 1.5) and is located between the values determined for CHAP (B : Na = 1) and CFAP (B : Na = 2). The substitu‐ tions of B carbonate ion into CCLAP seem to be more complex than those into CHAP, which is expressed by **Fig. 28** or by the following charge-balanced substitutions scheme:

$$
\left[\text{Na}^{\text{+}} + \text{^{\text{B}}} \text{CO}\_{3}^{2-} \Leftrightarrow \text{Ca}^{2+} + \text{PO}\_{4}^{3-}\right] \tag{41}
$$

There should be additional vacancies including the charge-balancing mechanism:

() () ( )( )


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

9 x / 2 CaCO 3 x / 2 P O CaF Ca PO CO F

Carbonated chlorapatite nanopowders can be synthesized by the mechanochemical process under argon atmosphere using the mixture of calcite (CaCO3), phosphorus pentoxide (P2O5)

3 25 2

+ - (39)

O(3)

Na

<sup>B</sup> <sup>8</sup>

9 2

1.21

5

O(3)

[110] *c*

A

7

**Fig. 28.** The structure of carbonate chlorapatite showing one of 12 possible orientations of the type-A carbonate ion in apatite channel: the unit-cell origin is in the center of figure, shaded phosphate polyhedra and Ca(2) atoms are cen‐ tered at *z* = 3/4 (a) and the fragment of CCLAP structure showing the location of B carbonate ion close to the sloping

The structure of Na-bearing CCLAP crystals (**Fig. 28**(**a**)) with the contents of Na and A and Btype of carbonate ranges between those of Na-bearing carbonated fluorapatite (CFAP) and carbonated hydroxylapatite (CHAP). The stoichiometric amount of Na and A-type of carbonate is consistent with the near linear (1:1) correlation reported for CHAP and CFAP and provides the evidence of active role of Na in the substitution of carbonate into the apatite channel, even if Na does not appear in usual charge-balanced substitution scheme [169]:

6

3− was given by the *x* value in the general formula of TYPE-B of

( )( ) ( )

9 x / 2 CaCO 3 x / 2 P O CaCl Ca PO CO Cl 9 3x / 2 CO - - - +- + ®

10 x / 2 4 6x x 3 2 2

The high-pressure (1 GPa) synthesis of sodium-bearing carbonate chlorapatite of TYPE A-B (CCLAP, Ca10−(y+z)Nay[V]z[(PO4)6−(y+2z)(CO3)y+2z][Cl2−2x(CO3)x], where *x* ≈ *y* ≈ 4*z* ≈ 0.4) from carbonate rich melt in the temperature range from 1000 to 1350°C, was described by FLEET et

3 2 5 2 10 x / 2 4 6x x 3 2

+ - (38)

( )

**4.6.2 Carbonate-chlorapatites**

The substitution degree of PO4

Ca(2)

Ca(1)

Ca10−x/2(PO4)6−x(CO3)xCl2.

al [169].

9 3x / 2 CO

2

and calcium chloride (CaCl2) as raw materials [181],[182]:

() ()

Ca(2)

*a*1

*a*2

Ca(1)

**(a) (b)**

O(7) O(6)

O(5)

faces of substituted phosphate tetrahedron (b) according to FLEET AND LIU [169 ].

$$\left[1/2\left[\text{V}\right] + \,^\text{B}\text{CO}\_3^{2-} \Leftrightarrow 1/2\,\text{Ca}^{2+} + \text{PO}\_4^{3-}\right] \tag{42}$$

This leads to the formula of sodium-bearing carbonate chlorapatite mentioned above.

Similar profiles of *ν*<sup>3</sup> bands in FT-IR spectra for all carbonate apatite composition series and carbonated contents, together with common X-ray structure suggest that Na cation and A and B carbonate ion substituents are present as randomly distributed defect clusters within host apatite structure. The defect cluster depicted in **Fig. 28** (**b**) facilitates local charge compensa‐ tion by Na-for-Ca substitution, explains the linear 1:1 correlation between Na and A carbo‐ nate, and minimizes the effects of spatial accommodation [169].

The synthesis of hydroxyl-chlorapatite solid solution via the precipitation method can be presented as follows [183]:

$$\begin{aligned} &10\ \text{Ca}\left(\text{NO}\_3\right)\_2 + 6\ \left(\text{NH}\_4\right)\_2\text{HPO}\_4 + 2\ \text{NH}\_4\text{Cl} + 6\ \text{NH}\_4\text{OH} \rightarrow\\ &\text{Ca}\_{10}\left(\text{PO}\_4\right)\_6\left[\text{(OH)}\_{2\text{-x}}\text{Cl}\_x\right] + 20\ \text{NH}\_4\text{NO}\_3 + \text{H}\_2\text{O} \end{aligned} \tag{43}$$

Also fluorine and chlorine co-substituted hydroxylapatites can be prepared by aqueous precipitation method [184]:

$$\begin{aligned} \text{10 }\text{Ca}(\text{NO}\_3)\_2 &+ 6\left(\text{NH}\_4\right)\_2\text{HPO}\_4 + \text{NH}\_4\text{F} + \text{NH}\_4\text{Cl} + 6\text{ NH}\_4\text{OH} \rightarrow\\ \text{Ca}\_{10}(\text{PO}\_4)\_6(\text{F,Cl}) + \text{20 }\text{NH}\_4\text{NO}\_3 + 6\text{ H}\_2\text{O} \end{aligned} \tag{44}$$

$$\begin{aligned} &10 \text{ Ca} \left( \text{NO}\_{3} \right)\_{2} + 6 \left( \text{NH}\_{4} \right)\_{2} \text{HPO}\_{4} + \text{x} \text{ NH}\_{4}\text{F} + \\ &3 \text{ y } \text{NH}\_{4}\text{Cl} + \left[ 8 - \left( \text{x} + \text{y} \right) \right] \text{NH}\_{4}\text{OH} \rightarrow \text{Ca}\_{10} \left( \text{PO}\_{4} \right)\_{6} \left( \text{OH}\_{2\cdot \text{(x+y)}} \text{F}\_{\text{x}} \text{Cl}\_{\text{y}} \right) \\ &+ 20 \text{ NH}\_{4} \text{NO}\_{3} + 6 \text{ H}\_{2}\text{O} \end{aligned} \tag{45}$$

Carbonate can be introduced into the structure of carbonated barium-chlorapatite by stirring apatite in an (NH4)2CO3 solution for 1 week [185]:

$$\text{Ba}\_{10} \left( \text{PO}\_4 \right)\_6 \text{Cl}\_2 + \text{CO}\_3^{2-} \rightarrow \text{Ba}\_9 \left( \text{PO}\_4 \right)\_5 \text{Cl} + \text{PO}\_4^{3-} + \text{Cl}^- + \text{Ba}^{2+} \tag{46}$$

The attempts to prepare carbonated barium-chlorapatite in a one-step synthesis results in a mixture of BaCO3 and Ba3(PO4)3. The variations in the manner in which carbonate was added to the reaction mixture, such as co-titrating a carbonate solution along with BaCl2 and NH4H2PO4, pre-mixing it with NH4H2PO4, or adding it first or last did not eliminate the precipitation of simple salts. The inability at 60°C and at the pH of 10 to produce carbonated barium-chlorapatite at any ratio of carbonate to phosphate in the aqueous solution is proba‐ bly due to close molar solubility of simple salts [185].
