**6.1.8. Manganese-substituted apatites**

Naturally occurring manganese-substituted apatite is known as manganese-bearing apatite (Mn,Ca5(PO4)3F, **Section 1.1**). According to the findings of HUGHES et al [24], the symmetry of Mn-bearing apatite does not degenerate from P63/M to P6 or P3 with Mn substitution, nor degenerates the symmetry limit of Mn substitution to one atom/unit cell. Mn2+ substituent, which is smaller than Ca2+, preferentially occupies larger apatite Ca(l) site although not completely. Mn atom is underbonded at Ca site, less so at larger Ca(l) site; nine O atoms coordinating that site satisfy more effectively the Mn bond valence than seven ligands coordinating Ca(2) site. The M-O bond lengths of Mn-substituted sites reflect the substitu‐ tion of the smaller Mn ion.

It is interesting to note that apatite acts effectively as a geochemical sieve that traps Mn2+ and excludes Fe2+ elements, which are virtually inseparable in most geochemical systems. The bond valence sums for Fe2+ at apatite Ca sites yield 1.26 and 1.19 valence units for Ca(1) and Ca(2) sites, respectively; large discrepancy from the formal valence prohibits extensive substitu‐ tion of Fe2+ in the apatite structure (**Fig. 5**) [24].

The crystal structure of pale blue transparent Mn-rich fluorapatite (9.79 wt.% of MnO) with optimized formula (Ca8.56M2+1.41Fe2+0.01)P6O24F2 was resolved by HUGHES et al [24] to be of the

**Fig. 5.** The structure of natural Mn-bearing apatite refined by HUGHES et al [24] and viewed along the c-axis.

space group of P63/M with the cell parameters *a* = 9.3429 Å, *c* = 6.8110 Å and *Z* = 2. Mn is strongly ordered at Ca(1) site (Ca(1)0.72Mn0.28, Ca(2)0.96Mn0.04). The apatite structure also contains Mn5+ at X-site (P5.96Mn5+ 0.04).

#### **6.1.9. Substitution of REEs in apatite**

Crystals of La-, Gd- and Dy-bearing fluorapatite [La-FAP, Gd-FAP, Dy-FAP, Ca10−x−2y NayREE<sup>x</sup> +y(P1−ySixO4)Z2, where *x* = 0.24 – 0.29 and *y* = 0.32 – 0.36] were synthesized by hydrothermal route by FLEET and PAN [37]. The substitution of trivalent REE in apatite is principally com‐ pensated as follows:

$$\text{REE}^{3+} + \text{Si}^{4+} \Leftrightarrow \text{Ca}^{2} + \text{P}^{5+} \implies \text{Ca}\_{10-x} \text{REE}\_{x} \text{\(P}\_{1-x} \text{Si}\_{x} \text{O}\_{4} \text{\)}\_{6} \text{Z}\_{2} \tag{4}$$

$$\text{REE}^{\text{>}} + \text{Na}^{+} \Leftrightarrow 2\text{ Ca}^{2} \quad \Rightarrow \quad \text{Ca}\_{10-2\text{y}}\text{Na}\_{\text{y}}\text{REE}\_{\text{y}}\text{(PO}\_{4}\text{)}\_{6}\text{Z}\_{2} \tag{5}$$

$$\text{REE}^{5+} + \text{O}^{2-} \Leftrightarrow \text{Ca}^{2} + \text{Z}^{-} \implies \text{Ca}\_{10-2x} \text{REE}\_{x} \text{(PO}\_{4}\text{)}\_{6} \text{Z}\_{2-x} \text{O}\_{x} \tag{6}$$

$$\text{2 }\text{ REE}^{\text{3\*}} + \text{[V]} \Leftrightarrow \text{3 Ca}^{\text{2\*}} \implies \text{Ca}\_{10-\text{3w}}\text{REE}\_{\text{2w}} \text{[V]}\_{\text{w}} \text{(PO}\_{4}\text{)}\_{6}\text{Z}\_{2} \tag{7}$$

The structure of some REE-bearing apatites [37],[38] is shown in **Fig. 6**.

The partitioning of REE between two Ca positions in apatite contradicts usual first-order dependence on spatial accommodation, with LREE2 [39],[40],[41],[42],[43], in particular, favoring the smaller Ca(2) position. This behavior was variously ascribed to the control via [37]:

<sup>2</sup> Rare-earth elements or metals (REE or REM) are Sc, Y and lanthanoids [40]. Light rare-earth elements (LREE) are Sc, La, Ce, Pr, Nd, Pm, Sm, Eu and Gd (7 elements from La to Eu are known as the cerium group or cerium-group lanthanides). Heavy rare-earth elements (HREE) are Y, Tb, Dy, Ho, Er, Tm, Yb and Lu. The definition of LREE and HREE is based on the electron configuration. LREEs possess unpaired 4*f* electron from 0 to 7 (half-filled 4*f* electron shell). HREEs have paired electron (the clockwise and counterclockwise spinning election) [39],[41]. The element with half-filled *f*-electron shell (Eu) shows enhanced stability of its particular electron configuration [43]. In some cases, REEs are divided into three groups including (1) LREE (La – Pm), (2) MREE (middle rare-earth element, Sm – Dy) and (3) HREE (Ho – Lu) [42].

**Fig. 6.** Rare-earth-element ordering and structural variations in natural rare-earth-bearing fluorapatites (a), LaFAP (b), NdFAP (c), GdFAP (d) and DyFAP (e) [37],[38].


space group of P63/M with the cell parameters *a* = 9.3429 Å, *c* = 6.8110 Å and *Z* = 2. Mn is strongly ordered at Ca(1) site (Ca(1)0.72Mn0.28, Ca(2)0.96Mn0.04). The apatite structure also contains Mn5+

**Fig. 5.** The structure of natural Mn-bearing apatite refined by HUGHES et al [24] and viewed along the c-axis.

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

Crystals of La-, Gd- and Dy-bearing fluorapatite [La-FAP, Gd-FAP, Dy-FAP, Ca10−x−2y NayREE<sup>x</sup> +y(P1−ySixO4)Z2, where *x* = 0.24 – 0.29 and *y* = 0.32 – 0.36] were synthesized by hydrothermal route by FLEET and PAN [37]. The substitution of trivalent REE in apatite is principally com‐

( ) 3 4 25

( ) 3 2

[ ] [ ] ( ) 3 2

The partitioning of REE between two Ca positions in apatite contradicts usual first-order

favoring the smaller Ca(2) position. This behavior was variously ascribed to the control via [37]:

 Rare-earth elements or metals (REE or REM) are Sc, Y and lanthanoids [40]. Light rare-earth elements (LREE) are Sc, La, Ce, Pr, Nd, Pm, Sm, Eu and Gd (7 elements from La to Eu are known as the cerium group or cerium-group lanthanides). Heavy rare-earth elements (HREE) are Y, Tb, Dy, Ho, Er, Tm, Yb and Lu. The definition of LREE and HREE is based on the electron configuration. LREEs possess unpaired 4*f* electron from 0 to 7 (half-filled 4*f* electron shell). HREEs have paired electron (the clockwise and counterclockwise spinning election) [39],[41]. The element with half-filled *f*-electron shell (Eu) shows enhanced stability of its particular electron configuration [43]. In some cases, REEs are divided into three groups

including (1) LREE (La – Pm), (2) MREE (middle rare-earth element, Sm – Dy) and (3) HREE (Ho – Lu) [42].

( ) 32 2

The structure of some REE-bearing apatites [37],[38] is shown in **Fig. 6**.

dependence on spatial accommodation, with LREE2

10 x x 1 x x 4 2 <sup>6</sup> REE Si Ca P Ca REE P Si O Z ++ + +Û+ Þ - - (4)

10 2y y y 4 2 <sup>6</sup> REE Na 2 Ca Ca Na REE PO Z + + +Û Þ - (5)

10 2z z 4 2 z z <sup>6</sup> REE O Ca Z Ca REE PO Z O +- - +Û+ Þ - - (6)

10 3w 2w <sup>w</sup> 4 2 <sup>6</sup> 2 REE V 3 Ca Ca REE V PO Z + + +Û Þ - (7)

[39],[40],[41],[42],[43], in particular,

at X-site (P5.96Mn5+ 0.04).

pensated as follows:

2

**6.1.9. Substitution of REEs in apatite**

The preference of individual REE among multiple Ca positions in minerals (the site occupan‐ cy of individual REE) was not extensively studied because of the inability of conventional diffraction methods to distinguish among individual elements at multiply-occupied sites. The site preference for individual LREE from theoretical bond-valence sums was estimated by HUGHES et al [38], reasoning that La → Pr should preferentially substitute into Ca(2), whereas Pm → Sm should selectively substitute into Ca(1).

The isomorphic substitutions of neodymium for strontium in the structure of synthetic Sr5(VO4)3OH apatite structure type (P63/M) were reported by GET'MAN et al [44]. The synthe‐ sis of apatite specimen was performed via the solution thermolysis on the assumption of the following reaction:

$$\begin{aligned} \text{s(S-x)Sr(NO\_3)\_2} &+ \frac{\text{x}}{2} \text{Nd}\_2\text{O}\_3 + 3 \text{ NH}\_4\text{VO}\_3 \rightarrow \\ \text{Sr}\_{5-x}\text{Nd}\_x\text{(VO}\_4\text{)\_3(OH)}\_{\text{l-x}}\text{O}\_x + ... \end{aligned} \tag{8}$$

where x = 0, 0.02, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18 and 0.20. The substitution scheme can be expressed as:

$$\text{Sr}^{2+} + \text{OH}^- \Longleftrightarrow \text{Nd}^{3+} + \text{O}^{2-} \tag{9}$$

The procedure includes three stages:


The solutions for the thermolysis were prepared by dissolving Sr(NO3)2 in water; Nd2O3 was dissolved in water with nitric acid added; NH4VO<sup>3</sup> was dissolved in water with hydrogen peroxide added. Dry residues after concentrating the solutions were pestled in an agate mortar and calcined with the temperature steadily raised from 600 to 800°C and intermittent grind‐ ings [44].
