**Abstract**

Apatite is the generic name, which was first introduced by German geologist A.G. Werner. These minerals and their synthetic analogs represent a major class of ionic compounds and the most common crystalline form of calcium phosphates, which are of interest of many industrial branches and scientific disciplines. Since, apatite (fluora‐ patite) is the most abundant phosphate mineral, apatite bearing phosphate rocks represents an important source of inorganic phosphorus. First chapter of this book introduces the basic concepts of nomenclature, composition, classification, crystal structure, mineralogy and properties of minerals from the supergroup of apatite. Furthermore, the minerals from the group of apatite and polysomatic apatites are described. Since, the most of the topics mentioned in this chapter will be developed in the following chapters, the key concepts provided in this chapter are important to understood before proceeding further.

**Keywords:** Apatite, Group of Apatite, Polysomatic Apatites, Fluorapatite, Hydroxyla‐ patite, Chlorapatite, Vanadinite

The minerals1 [1],[2],[3],[4],[5] from the apatite group2 [6] are classified as hexagonal or pseudo‐ hexagonal monoclinic anhydrous phosphates containing hydroxyl or halogen of the generic formula3 :

<sup>3</sup> The variable formula should be written as: (Ca,Sr,Pb,Y,Mn,Na)5(PO4,AsO4,SO4CO3)3(F,Cl,OH).

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

<sup>1</sup> **Minerals** are individual components comprising rocks formed by geological processes classified according to their crystal structure and chemical composition. The total number of minerals accepted by mineralogical community is about 4000. **Mineraloids** are mineral-like phases including synthetic materials, human-treated substances, and some biological materials, which do not fulfill the criteria for the definition of mineral species [2]. **Anthropogenic substances** are not considered as minerals. If such substances are identical to minerals, they can be referred as the "synthetic equivalents" of given mineral. If the synthetic substance has a simple formula, then the preference should be given to the use of a chemical formula instead of a mineral name. **Biogenic substances** can be accepted as minerals if geological processes were involved in the genesis of these compounds [1],[3],[4].

<sup>2</sup> The mineral classification system developed by German mineralogist K.H. STRUNZ.

$$\mathbf{M}\_{\rm s}(\rm XO\_{4})\_{\rm s}\,\mathbf{Z}\_{\rm q}$$

(**DANA CLASSIFICATION** [7],[8]) or as phosphates, arsenates and vanadates with additional anions without water (**STRUNZ CLASSIFICATION**<sup>4</sup> [8],[9]), where *M* = Ba, Ca, Ce, K, Na, Pb, Sr and Y; *X* = As, P,5 and Si; and *Z* = F, Cl, O, OH and H2O. Apatite minerals form a numerous and diverse group of minerals, while in addition a large number of synthetic compounds with the apatite type structure are known [10].

Apatite minerals can be formally derived from phosphoric acid6 [11] (or H3AsO4 and H3VO4 for arsenates and vanadates, respectively). According to the Werner's coordination theory [12], [13], e.g. fluorapatite was considered as a compound formed by the substitution of calcium phosphate (Ca3(PO4)2) into halide mineral fluorite (calcium fluoride, CaF2 [14]). The coordina‐ tion formula of fluorapatite can then be written as follows:

$$\begin{bmatrix} \begin{pmatrix} \mathbf{O} & \mathbf{PO\_3} & \mathbf{Ca} \\\\ \mathbf{Ca} & \mathbf{O} & \mathbf{O\_2} \\\\ \mathbf{O} & \mathbf{PO\_3} & \mathbf{Ca} \end{pmatrix} \end{bmatrix}\_3 \mathbf{F\_2}$$

The ratios of the mean sizes of ions "*M*" to "*X*" vary in the range from 1.89 to 4.43 for apatite compositions, but there are discontinuities between the ratios 2.50–2.60 and 3.25–3.35. These two gaps provide the structural base, which was used for the suggestion of classifying7 apatites into three groups named after well-known mineral species occurring in each group [15],[16]:


The summary of some apatite species is listed in **Table 1**. The current nomenclature of minerals from apatite supergroup is described in Section **1.1**.

<sup>4</sup> A mineral group consists of two or more minerals with the same or essentially the same structure (i.e. isotypic structure belonging to one structural type) and composed of chemically similar elements (i.e. elements with similar crystalchemical behavior). Crystal structures considered as being "essentially the same" can be denoted by the term *homeotypic*. The hierarchical scheme for the group nomenclature includes (1) mineral class, (2) mineral subclass, (3) mineral family, (4) mineral supergroup, (5) mineral group(s), and (6) mineral subgroup or mineral series [8].

<sup>5</sup> The phosphorus element was discovered by a Hamburg alchemist H. Brand. As was described in the book of G.E. LEIBNITZ (1646–1716) **Historia Invetions Phosphori**; phosphorus was extracted from "the spirit of urine" during the search for the philosopher´s stone. The name of phosphorus (in Latin means "morning star") was derived from Greek word "*phospho‐ ris*," which meaning "*bringing light*" [35].

<sup>6</sup> In 3 moles of H3PO4, 8 H+ ions were replaced by 4 Ca2+, and the last H+ was replaced by Ca-Z (Z = OH, F, Cl…).

<sup>7</sup> Despite the fact that this classification is out of fashion, there are some interesting structural consequences with general validity. Therefore, this classification was included to the introduction of this book.

The specific gravity of *sensu lato*<sup>8</sup> apatite ranges from 3.1 to 3.3 g·cm−3. Apatites show basal and imperfect cleavage, conchoidal and uneven fracture and the hardness on the Mohs scale is 5. The color of streak is white and luster vitreous to subresinous. Apatite occurs usually in the shades of green to gray-green, also white, brown, yellow, bluish, or reddish, transparent to translucent and some specimens can be multicolored. The habit of apatite crystals (Ca5(PO4)3(F,Cl,OH)) is usually prismatic, dipyramidal or tabular, and also massive compact or granular. Some varieties are phosphorescent when heated, and others become electric by friction. On the other hand, the morphology of apatite crystals is very complex, and there is large amount (~53) of described forms [17],[18].

5 4q ( ) M XO Z3

(**DANA CLASSIFICATION** [7],[8]) or as phosphates, arsenates and vanadates with additional anions

 and Si; and *Z* = F, Cl, O, OH and H2O. Apatite minerals form a numerous and diverse group of minerals, while in addition a large number of synthetic compounds with the apatite type

for arsenates and vanadates, respectively). According to the Werner's coordination theory [12], [13], e.g. fluorapatite was considered as a compound formed by the substitution of calcium phosphate (Ca3(PO4)2) into halide mineral fluorite (calcium fluoride, CaF2 [14]). The coordina‐

> O PO Ca <sup>3</sup> Ca Ca F2 O PO Ca <sup>3</sup> <sup>3</sup>

ç ÷ ë û è ø

The ratios of the mean sizes of ions "*M*" to "*X*" vary in the range from 1.89 to 4.43 for apatite compositions, but there are discontinuities between the ratios 2.50–2.60 and 3.25–3.35. These

into three groups named after well-known mineral species occurring in each group [15],[16]:

The summary of some apatite species is listed in **Table 1**. The current nomenclature of minerals

 A mineral group consists of two or more minerals with the same or essentially the same structure (i.e. isotypic structure belonging to one structural type) and composed of chemically similar elements (i.e. elements with similar crystalchemical behavior). Crystal structures considered as being "essentially the same" can be denoted by the term *homeotypic*. The hierarchical scheme for the group nomenclature includes (1) mineral class, (2) mineral subclass, (3) mineral family, (4)

 The phosphorus element was discovered by a Hamburg alchemist H. Brand. As was described in the book of G.E. LEIBNITZ (1646–1716) **Historia Invetions Phosphori**; phosphorus was extracted from "the spirit of urine" during the search for the philosopher´s stone. The name of phosphorus (in Latin means "morning star") was derived from Greek word "*phospho‐*

<sup>7</sup> Despite the fact that this classification is out of fashion, there are some interesting structural consequences with general

was replaced by Ca-Z (Z = OH, F, Cl…).

ñ é ù æ ö ê ú ç ÷

two gaps provide the structural base, which was used for the suggestion of classifying7

**2.** Apatite–mimetite group with the *M*:*X* ratio in the range from 2.60 to 3.25;

[8],[9]), where *M* = Ba, Ca, Ce, K, Na, Pb, Sr and Y; *X* = As,

[11] (or H3AsO4 and H3VO4

apatites

without water (**STRUNZ CLASSIFICATION**<sup>4</sup>

Apatite minerals can be formally derived from phosphoric acid6

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

tion formula of fluorapatite can then be written as follows:

**1.** Vanadinite–svabite group with the *M*:*X* ratio less than 2.5;

**3.** Pyromorphite group with the *M*:*X* ratio higher than 3.25.

mineral supergroup, (5) mineral group(s), and (6) mineral subgroup or mineral series [8].

validity. Therefore, this classification was included to the introduction of this book.

ions were replaced by 4 Ca2+, and the last H+

from apatite supergroup is described in Section **1.1**.

*ris*," which meaning "*bringing light*" [35].

In 3 moles of H3PO4, 8 H+

structure are known [10].

P,5

4

5

6

Apatite occurs in a wide range of igneous and sedimentary rocks (**Chapter 7**) and deposits as isolated crystals in grains, usually as small as 1–2 mm. The largest known apatite deposit is in Kirovsk, Russia. The largest individual crystals were found in Renfrew, Ontario, Canada [19], [20].


8 Latin phrase (abbreviated as s.l.) used, which means "in the broad sense."

**Table 1.** The classification of synthetic (marked by bold) and natural apatites into three groups [15].

**Fig. 1.** Some of known forms of apatite crystals: c (0001), m (111 ¯ 0), a (112 ¯ 0), x (101 ¯ 1), s (112 ¯ 1), r (10r2) and u (2131).

Some examples of morphology of apatite crystals are shown in **Fig. 1**. The most abundant crystal faces of apatite possess the Miller–Bravais indices9 [21],[22],[23],[24],[25] (0001), i.e. basis (or basal pinacoid10), (111 ¯0),11 i.e. protoprism (or the prism of the first order), (101 ¯1) and (202 ¯1), i.e. the first-order dipyramids, but the faces such as (112 ¯0), i.e. deuteroprism (or the prism of the second order), (112 ¯1), i.e. dipyramid of the second order as well as (101 ¯2), i.e. the dipyramids of the first order, and (213 ¯1), i.e. dipyramid of the third order, are also common. Other faces such as (314 ¯1), (314 ¯2), (213 ¯0), (123 ¯2), etc., i.e. dipyramids of the third order, are also possible but rare (**Fig. 2**).

**Fig. 2.** Apatite from Gletsch (Switzerland): *y* (202 ¯ 1), *n* (314 ¯ 1), *o* (314 ¯ 2), *i* (123 ¯ 2) and *μ* (213 ¯ 1).

**Group of apatites**

Ca6Ce4(GeO4)4(PO4)2Cl2 2.41 Ba10(MnO4)6(OH)2 2.86 Ca4Y6(SiO4)6(OH)2 2.45 Ba10(CrO4)6(OH)2 2.86 Ba2La8(GeO4)6O2 2.45 Pb8K2(AsO4)6 2.89 Ba3La7(GeO4)6O1.5 2.49 Pb10(SiO4)2(AsO4)4 2.91 Ca4Ce6(GeO4)2(SiO4)4Cl2 2.49 Sr4La6(SiO4)6(OH)2 2.93 Ba10(VO4)6(OH)2 2.50 Pb8Rb2(AsO4)6 2.96 Pb10(SiO4)2(VO4)4 2.51 Pb10(SiO4)2(VO4)2(PO4)2 3.02 Sr10(MnO4)6(OH)2 2.52 Ca9Mg(PO4)6Cl2 3.02

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

a) 2.13–2.32 b) 2.70–2.95 c) 3.66–3.94

**Fig. 1.** Some of known forms of apatite crystals: c (0001), m (111

**Table 1.** The classification of synthetic (marked by bold) and natural apatites into three groups [15].

¯ 0), a (112

¯ 0), x (101

¯ 1), s (112

¯ 1), r (10r2) and u (2131).

Ca9Ni(PO4)6O 3.03 **Ca10(SiO4)8(SO4)3(OH)3 3.03** Pb10(SiO4)(GeO4)(PO4)2 (AsO4)2 3.05 Ca5Cd5(PO4)6F2 3.09 Ba3La7(SiO4)6(OH)2 3.11 Ca9Cd(PO4)6F2 3.11 **Ca10(PO4)6(F,Cl,Br,OH)2 3.12** Pb8Bi2(SiO4)4(PO4)2 3.16 Ca9Sr(PO4)6O 3.16 Ca9Pb(PO4)6(O,Cl2) 3.18 Ca9Ba(PO4)6(O,Cl2) 3.23

This means that the shapes pinacoid {0001}, hexagonal prisms {101 ¯0} and {112 ¯0}, dihexagonal prism {213 ¯0} and hexagonal dipyramid12 {101 ¯1}, {101 ¯2}, {202 ¯1} and {112 ¯1} are possible to can be found on apatite crystals.

According to the crystal system, apatite minerals belong to the hexagonal13 dipyramidal class (sometimes also termed as apatite type). The point group of apatite is 6/M (**HERMAN**–**MAUGUIN SYMBOLS**, H-M) or C6h (**SCHÖNFLIES SYMBOLS**), and it is centrosymmetric [26],[27],[28], [29],[30], [31],[32]. The symmetry operation of this point group includes hexad perpendicular to mirror plane and the center of symmetry (**Fig. 3**).

<sup>9</sup> The orientation of planes in crystal was determined by Miller indices, i.e. three integers (hkl), which referee to the plane (or planes *hb*1 + k*b*2 + *lb*3, where *b*1, *b*2 and *b*<sup>3</sup> are reciprocal lattice vectors). Negative integer, e.g. -i is written as ̅*i* ¯ . The group of planes equivalent to (*hkl*) is written as {*hkl*}. The square brackets [*hkl*] denote a direction on the basis of the direct lattice vectors instead of the reciprocal lattice and the set of equivalent planes (*ha*1 + *ka*2 + *la*3, where *a*1, *a*2 and *a*3 are direct lattice vectors) is written as <*hkl*>.

<sup>10</sup> Pinacoid {0001} consists of two opposite faces perpendicular to the 6-fold axis. It commonly occurs in combination with hexagonal prism and hexagonal or dihexagonal truncated pyramids [25]. The single face is termed as pedion.

<sup>11</sup> The hexagonal (and rhombohedral) crystal system uses four (Miller–Bravais) indices (*hkil*), where *i* is termed as redundant index and *h* + *k* + *i* = 0, e.g. (202 ¯ 1) is equivalent to (201) and (110) to (112 ¯ 0).

<sup>12</sup> Hexagonal dipyramids {*h*0*h* ¯*<sup>l</sup>*} and {*hh*(2*<sup>h</sup>* ¯)*<sup>l</sup>*} consist of 12 isosceles faces which intersect in a point along the vertical axis. Those two dipyramids differ only in their orientation with respect to three horizontal axes.

<sup>13</sup> In hexagonal crystal system *a* = *b* ≠ *c*, *α* = *β* = 90°, and *γ* = 120°. The hexagonal system is usually referred to four crystallographic axes designated *a*1, *a*2, *a*3, and *c*. The *c*-axis is vertical and the three *a*-axes lie in horizontal plane (*c*-axis is perpendicular to this plane) with the angle of 120° between their positive ends [25].

**Fig. 3.** Point group–subgroup relationship of point groups [31] (a) and stereogram of point group 6/M [36] (b) showing a sixfold rotation axis (hexad) plus the centre of symmetry and the mirror plane.

The projection of hexagonal crystal divided to twelve parts (dodecant) by the lateral axial planes is shown in **Fig. 4**(**a**). The pole (×) of each plane requires five other points above as well as six other located on the same place below the projection plane (b). This leads to the formation of hexagonal dipyramid. The cross section provides hexagon (c) inclined to right or left. Therefore, the right or right-handed form (d) and the left or left-handed form (e) of dipyra‐ mids were recognized. One is formed from the poles marked by sign (×) and the second one is in the void dodecants of projections (b). The right form can be turned left by the rotation of 180° along one of the crystallographic axis *a*.

**Fig. 4.** Hexagonal dipyramidal crystal system: hexagonal lattice (a), stereographic projection (b), hexagon (c) and right (d) and left (e) dipyramids.

The crystal shaper related to the hexagonal–dipyramidal crystal system can be derived via extend or skip altering planes (the same up and down) according to **Table 2**.


**Table 2.** Derivation of shapes in hexagonal–dipyramidal crystal system.

**Fig. 3.** Point group–subgroup relationship of point groups [31] (a) and stereogram of point group 6/M [36] (b) showing

The projection of hexagonal crystal divided to twelve parts (dodecant) by the lateral axial planes is shown in **Fig. 4**(**a**). The pole (×) of each plane requires five other points above as well as six other located on the same place below the projection plane (b). This leads to the formation of hexagonal dipyramid. The cross section provides hexagon (c) inclined to right or left. Therefore, the right or right-handed form (d) and the left or left-handed form (e) of dipyra‐ mids were recognized. One is formed from the poles marked by sign (×) and the second one is in the void dodecants of projections (b). The right form can be turned left by the rotation

**Fig. 4.** Hexagonal dipyramidal crystal system: hexagonal lattice (a), stereographic projection (b), hexagon (c) and right

The crystal shaper related to the hexagonal–dipyramidal crystal system can be derived via

extend or skip altering planes (the same up and down) according to **Table 2**.

a sixfold rotation axis (hexad) plus the centre of symmetry and the mirror plane.

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

of 180° along one of the crystallographic axis *a*.

(d) and left (e) dipyramids.

The space group14 of apatite is P63/M (**HERMAN–MAUGUIN SYMBOLS**15), where P denotes primi‐ tive type of Bravais lattice,14 63 is sixfold (360°/6 = 60°) screw rotation axis parallel to (001) followed by a translation through a distance 3*t*/6 (*t* is the magnitude of the shortest lattice vector along the axis), and the symbol /M refers to the mirror plane perpendicular to 63 screw axis (i.e. the mirror plane normal vector is parallel to the hexad16 that coincides with glide plane17 at z = ¼ for the monoclinic variant in the space group P21/B (**Fig. 9**, the structure of hexago‐ nal and monoclinic apatite was described in **Section 1.2**) [28],[29],[30], [31],[32],[38],[39].

The composition of natural apatites (Ca5(PO4)3(F,Cl,OH)) exhibits large variations in the content of F, Cl and OH. Pure end members, e.g. hydroxylapatite (Ca5(PO4)3OH), fluorapa‐ tite (Ca5(PO4)3F) and chlorapatite (Ca5(PO4)3Cl), are uncommon in nature, but binary and ternary compositions are widely reported in igneous, metamorphic and sedimentary rocks. Petrologists proposed that the variations in OH–F–Cl ratio in apatites or between biotite and apatite may be used as a geothermometer [33],[34],[35] and an indicator of volatile fugacity of halogens and water in magmatic and hydrothermal processes [33],[36]. The concentration of OH, F and Cl also directly correlates with the properties such as etching rates, annealing characteristics of U fission track in apatite (**Section 7.3.3**) and investigation of paleoenviron‐ ment and diagenesis (**Section 6.5**) [33].

Apatite minerals represent a major class of ionic compounds [37] of interest to many disci‐ plines, including medical and biomaterial sciences (**Section 10.9**), geology (**Chapter 7**), cosmology ([40], **Section 7.3.4**), environmental (**Chapters 7** and **9**) and nuclear sciences (**Chapter 10**). Apatite also represents an important source of inorganic phosphorus for natural ecosystems and may favor the establishment of microbial communities able to exploit it [42].
