**1.5.2. Hydroxylapatite (apatite–(CaOH))**

Hydroxylapatite (Ca10(PO4)6(OH)2, HAP, pentacalcium monohydroxyorthophosphate) can be found mainly in igneous and metamorphic environments but also in biogenic deposits, e.g. in bone deposits [38],[44],[142]. Hydroxylapatite is very rare mineral. Wax yellow crystals up to 6 × 6 × 11 mm have been described from talc schist43 from the Old Verde Antique serpen‐ tine quarry near Holly Springs, Cherokee County, Georgia [38]. The properties of hydroxyla‐ patite are summarized in **Table 7**. The structure of hydroxylapatite and the comparison of sizes of ions is shown in **Fig. 18** and **Fig. 19**, respectively.

**Fig. 18.** The structure (view according to the *c*-axis) of hydroxylapatite showing the location of Ca(1) and Ca(2) sites.

**Fig. 19.** The comparison of size of ions in the structure of hydroxylapatite [38].

<sup>43</sup> Medium-grade metamorphic rock occurred in almost infinite varieties, which was formed by the metamorphosis at high temperatures and pressure which leads to preferred orientation of flat (sheet-like) grains. The schist is medium to coarse grained.

**Fig. 20.** Habit of hexagonal (a) and monoclinic (b) hydroxylapatite crystals.

The structure of monoclinic polymorph of fluorapatite is related to the space group P21/B with the crystallographic parameters *a* = 9.488, *b* = 18.963, *c* = 6.822Å, *β* = 119.97° and *Z* = 6 [145].

Hydroxylapatite (Ca10(PO4)6(OH)2, HAP, pentacalcium monohydroxyorthophosphate) can be found mainly in igneous and metamorphic environments but also in biogenic deposits, e.g. in bone deposits [38],[44],[142]. Hydroxylapatite is very rare mineral. Wax yellow crystals up

tine quarry near Holly Springs, Cherokee County, Georgia [38]. The properties of hydroxyla‐ patite are summarized in **Table 7**. The structure of hydroxylapatite and the comparison of sizes

**Fig. 18.** The structure (view according to the *c*-axis) of hydroxylapatite showing the location of Ca(1) and Ca(2) sites.

43 Medium-grade metamorphic rock occurred in almost infinite varieties, which was formed by the metamorphosis at high temperatures and pressure which leads to preferred orientation of flat (sheet-like) grains. The schist is medium to

from the Old Verde Antique serpen‐

**1.5.2. Hydroxylapatite (apatite–(CaOH))**

to 6 × 6 × 11 mm have been described from talc schist43

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

of ions is shown in **Fig. 18** and **Fig. 19**, respectively.

**Fig. 19.** The comparison of size of ions in the structure of hydroxylapatite [38].

coarse grained.

The crystal habit and the structure of monoclinic polymorphs is show in **Fig. 20**. The mono‐ clinic structure of hydroxylapatite (apatite–(CaOH)–M) appears to exist only in completely pure stoichiometric hydroxylapatite, and it is transformed to hexagonal form above about 250°C. The properties such as acid solubility and OH infusibility along the channels are related to the degree of disorder of OH positions. Hydroxyl anions lie in ordered positions in channels, whereas in hexagonal variety there is some disorder. Electrical properties are probably also dependent on exact channel position [106].

The atomic parameters (**Table 10**) and the length of bonds (**Table 11**) in the structure of hydroxylapatite were refined by POSNER et al [150] from the structural data collected on the crystal of synthetic hydroxylapatite specimen. The results of natural sample of near-end member hydroxylapatite were provided by HUGHES et al [33] and these data are also listed in **Table 10** and **Table 11**.

Hydroxylapatite is mainly used in the biomedical field for the preparation of bioceramics44 [151],[152]. Since hydroxylapatite (HAp) is chemically similar to inorganic component of bone matrix and has excellent biocompatibility and surface active properties with living tissues, it has become one of the most important materials for artificial bone and bone regeneration [153], [154]. HA ceramics together with β-tricalcium phosphate have been the most extensively used substitution materials for artificial bone grafts for nearly three decades [153] (described in **Section 10.9**).


<sup>44</sup> Bioceramics can be divided into two large groups: bioinert and bioactive ceramics. The bioinert ceramics have almost no influence on surrounding living tissues like ZrO2 and Al2O3. In contrast, the bioactive ceramics like calcium phosphates are able to bond with living tissues (**Section 10.9**).


**Table 10.** Atomic parameters for synthetic [150]/natural [33] hydroxylapatite: number of atoms per formula unit (*N*), positional parameters (*x*, *y* and *z*) and equivalent isotropic temperature factor (*B*).


**Table 11.** Length of bonds in the structure of synthetic [150]/natural [33] hydroxylapatite based on the parameters from **Table 10**.

The dehydroxylation of stoichiometric hydroxylapatite to oxyapatite takes place within the temperature range from 900°C to 1200°C [114]:

$$\mathrm{Ca}\_{10}\mathrm{(PO}\_{4}\mathrm{)}\_{6}\mathrm{(OH)}\_{2} \stackrel{900-1000^{\circ}\mathrm{C}}{\rightarrow} \mathrm{Ca}\_{10}\mathrm{(PO}\_{4}\mathrm{)}\_{6}\mathrm{O} + \mathrm{H}\_{2}\mathrm{O}\mathrm{(g)}\tag{1}$$

Further heating to temperatures higher than 1450°C leads to the thermal decomposition of oxyapatite into tricalcium phosphate (TCP, *α*-Ca3(PO4)2) and tetracalcium phosphate (TTCP, Ca4P2O9):

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_{6}\text{O} \overset{\text{Tə1459°C}}{\rightarrow} \text{2 }\text{Ca}\_3\text{(PO}\_4\text{)}\_{2} + \text{Ca}\_4\text{P}\_2\text{O}\_9 \tag{2}$$

Calcium deficient hydroxylapatite decomposes at lower temperatures (at about 800°C) to stoichiometric hydroxylapatite and tricalcium phosphate according to their stoichiometry:

#### Introduction to Apatites http://dx.doi.org/10.5772/62208 33

$$\begin{aligned} \text{Ca}\_{10-x} \text{(PO}\_4\text{)}\_{6-x} \text{(HPO}\_4\text{)}\_{\text{x}} \text{(OH)}\_{2-x} &\xrightarrow{\text{T}\ast 800^{\circ}\text{C}} \text{(1}-\text{x}) \text{ Ca}\_{10} \text{(PO}\_4\text{)}\_{6} \text{(OH)}\_2\\ + 3\text{x Ca}\_3 \text{(PO}\_4\text{)}\_2 + \text{x H}\_2\text{O} \text{(g)} \end{aligned} \tag{3}$$

Another important option for utilization of hydroxylapatite is the preparation of porous high-temperature sorbent for carbon dioxide [155]. Apatite materials can be employed in discontinuous operations for removing CO2 from gaseous streams in the form of structured monoliths or foams, with reduced pressure drops and enhanced refractory properties. Hightemperature capture of carbon dioxide by hydroxylapatite proceeds via following reversible chemical reaction45 [155]:

$$\mathrm{Ca\_{10}(PO\_4)\_6(OH)\_2 + CO\_2(g) \leftrightarrow Ca\_{10}(PO\_4)\_6CO\_3 + H\_2O(g)}\tag{4}$$

A similar reaction is also possible with oxy-apatites:

**Atom** *N x y z B* **[Å2**

positional parameters (*x*, *y* and *z*) and equivalent isotropic temperature factor (*B*).

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

Ca(1)-O3 2.802/2.802 – –

temperature range from 900°C to 1200°C [114]:

**Bond Length [Å] Bond Length [Å]** P-O1 1.533/1.534 Ca(2)-OH 2.354/2.3851 P-O2 1.544/1.537 Ca(2)-O1 2.712/2.711 P-O3 1.514/1.529 Ca(2)-O2 2.356/2.353 Ca(1)-O1 2.416/2.404 Ca(2)-O3 2.367/2.343 Ca(1)-O2 2.449/2.452 Ca(2)-O3 2.511/2.509

OH

**Table 10**.

Ca4P2O9):

O2 6 0.589/0.4649 0.466/0.5871 0.250/1/4 0.496/1.25 O3 12 0.348/0.2580 0.259/0.3435 0.073/0.0703 0.632/1.57

**Table 10.** Atomic parameters for synthetic [150]/natural [33] hydroxylapatite: number of atoms per formula unit (*N*),

**Table 11.** Length of bonds in the structure of synthetic [150]/natural [33] hydroxylapatite based on the parameters from

The dehydroxylation of stoichiometric hydroxylapatite to oxyapatite takes place within the

( ) ( ) ( ) ( ) 900 1000 C Ca PO OH Ca PO O H O g 10 4 6 2 10 4 <sup>6</sup> <sup>2</sup> - °

Further heating to temperatures higher than 1450°C leads to the thermal decomposition of oxyapatite into tricalcium phosphate (TCP, *α*-Ca3(PO4)2) and tetracalcium phosphate (TTCP,

Calcium deficient hydroxylapatite decomposes at lower temperatures (at about 800°C) to stoichiometric hydroxylapatite and tricalcium phosphate according to their stoichiometry:

( ) ( ) T 1450 C Ca PO O 2 Ca PO Ca P O 10 4 6 2 3 4 42 9

³ °

20.0/0.0 0.0/0.0 0.250/0.1979 O(H) 0.875/1.31

**]**

–/0.04 H –/3.3

® + (1)

® + (2)

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{O} + \text{CO}\_2\text{(g)} \leftrightarrow \text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{CO}\_3\tag{5}$$

The highest CO2 carrying capacity of HA macrogranules was detected at temperatures from 1000°C to 1100°C, achieving the values close to the theoretical limits. The changes in the HA microstructure induced during the thermal treatment (sintering) reduce the reactivity [155].

Since in the next decades the exploitation of fossil resources will continue and is expected to increase, rising the impact of fossil energy on the pollution and greenhouse effect, current technologies must be improved to become less harmful to the environment and more sustainable (zero emissions). The capture and the sequestration of CO2 generated from the conversion of fossil fuels are being investigated as effective measures to reduce greenhouse gas emissions [156]. The apatite materials seem to be suitable sorbents for this purpose [155].

#### **1.5.3. Chlorapatite (apatite–CaCl)**

Calcium chlorapatite, as mineral, is relatively much less frequent than fluorapatite or hydrox‐ ylapatite and is formed primarily in flour-deficient environment [157]. The mineral crystalli‐ zes in the hexagonal system and the crystals are prismatic in habit, usually long, sometimes short and may have rounded ends or be terminated by pyramidal faces. Sometimes it occurs in granular massive to compact form [158]. The crystal habit and the structure of chlorapa‐ tite and monoclinic polymorphs chlorapatite–M are shown in **Fig. 21**. The crystallographic parameters and the properties are listed in **Table 7**.

<sup>45</sup> The preparation of carbonated apatites (**Section 4.6.1**) is based on the same principle as shows more general equation **Eq. 37**.

Chlorapatite mineral can be found as a mineral in calcium silicate marble, is an accessory mineral in layered mafic intrusions, occurs in veins such as "diabase," and replaces "triphy‐ lite" in some granite pegmatites. Such deposits are found in the USA; Quebec, Canada; Bushveld complex of Transvaal, South Africa; Angarth-Sud Tazenekht Plain of Morocco; Rajagarth, India; Kurokura, Japan; and Snarum, Norway [157].

**Fig. 21.** Structure (perspective view according to the *c*-axis) and crystal habit of chlorapatite ([33], a) and chlorapatite– M ([116], b).


**Table 12.** Positional parameters and equivalent isotropic factor for chlorapatite [33].

The atomic parameters and equivalent isotropic temperature factorfor chlorapatite by HUGHES et al [33] are listed in **Table 12**. The lengths of bonds in the chlorapatite structure are listed in **Table 13**.


**Table 13.** Bond lengths in the structure of chlorapatite [33].

Chlorapatite mineral can be found as a mineral in calcium silicate marble, is an accessory mineral in layered mafic intrusions, occurs in veins such as "diabase," and replaces "triphy‐ lite" in some granite pegmatites. Such deposits are found in the USA; Quebec, Canada; Bushveld complex of Transvaal, South Africa; Angarth-Sud Tazenekht Plain of Morocco;

**Fig. 21.** Structure (perspective view according to the *c*-axis) and crystal habit of chlorapatite ([33], a) and chlorapatite–

The atomic parameters and equivalent isotropic temperature factorfor chlorapatite by HUGHES et al [33] are listed in **Table 12**. The lengths of bonds in the chlorapatite structure are listed in

**]**

**Atom** *N x y z B* **[Å2**

Ca(1) 4 2/3 1/3 0.0027 0.99 Ca(2) 6 0.00112 0.25763 1/4 1.14 P 6 0.37359 0.40581 1/4 0.77 O1 6 0.4902 0.3403 1/4 1.34 O2 6 0.4654 0.5908 1/4 1.47 O3 12 0.2655 0.3522 0.0684 1.88 Cl 2 0 0 0.4323 2.68

**Table 12.** Positional parameters and equivalent isotropic factor for chlorapatite [33].

Rajagarth, India; Kurokura, Japan; and Snarum, Norway [157].

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

M ([116], b).

**Table 13**.

**Fig. 22.** The structure field of chlorapatite (Ahrens's radii) [149].

Together with fluorapatite, chlorapatite forms the solution46 ([Ca5(PO4)3F]1−x·[ Ca5(PO4)3Cl]*x*, where parameter *x* varies in the range from 0 to 1) on whole range of composition [157].

The chlorapatite structural field (**Fig. 22**) was investigated by KREIDLER and HUMMEL [149]. It has the boundaries of 0.19 ≤ *R*p ≤ 0.60 Å and 0.80 ≤ *R*c ≤ 1.35 Å differing from those of fluora‐ patite in two aspects:


Both of these differences are probably related to the difference in the position of halide ions in the fluor- and chlorapatite structure, but more detailed explanation of how the structural

<sup>46</sup> Activated by Sb3+ and Mn2+, the resulting phosphor was used in fluorescent lamps until about 1990 when it was replaced by rare-earth activated alkaline earth aluminates [157].

difference enables chlorapatite to accept smaller cations without the distortion will not be attempted.
