**2.3. The group of britholite**

The structure and the crystal habit of the mineral fluorstrophite are shown in **Fig. 26**. It is a green, yellow-green or colorless mineral with vitreous-greasy luster that crystallizes in hexagonal system with the space group P63/M or P63. The crystallographic parameters of the unit cell are *a* = 9.565 and *c* = 7.115 Å, the ration *a*:*c* = 1:0.744, *V* = 563.74 Å3 and *Z* = 2. The hardness of the mineral on the Mohs scale is 5. Calculated and measured densities are 3.74

The mineral kuannersuite-(Ce) (Na2Ce2Ba6(PO4)6FCl [88]) was found and named according to the locality (Kuannersuit plateau) in the Ilímaussaq alkaline complex, South Greenland (**Fig. 27**). It occurs associated with the minerals including aegirine, analcime, beryllite

((Na,Ca)2(Si,Al)5O10·3H2O [94]), lovdarite (K2Na6Be4Si14O36·9H2O [95],[96]), nabesite (Na2BeSi4O10·4H2O [97],[98]), neptunite, pectolite, polylithionite (KLi2AlSi4O10F2 [99]), pyro‐

17 There are three minerals: gmelinite-(Ca), gmelinite-(K), and gmelinite-(Na) with the composition of

18 The member of the pyrochlore group ((Na,Ca)2Nb2O6(OH,F)). A new scheme of nomenclature for the pyrochlore supergroup, approved by the CNMNC–IMA, is based on the ions at the A, B, and Y sites. The subgroups should be changed to the groups: pyrochlore (1), microlite (2), roméite (3), betafite (4), and elsmoreite (5). The new names are composed of two prefixes and one root name (identical to the name of the group). The first prefix refers to the dominant anion (or cation) of the dominant valence [either H2O or □] at the Y site. The second prefix refers to the dominant cation of the dominant valence [either H2O or □] at the A site. The prefix "keno–" represents "vacancy." Where the first and the

Ca2(Si8Al4)O24·11H2O [90],[91], K4(Si8Al4)O24·11H2O [93], and Na4(Si8Al4)O24·11H2O [91],[93], respectively.

[90],[91],[92],[93], gonnardite

and 3.84 g·cm−3, respectively. It has imperfect cleavage to {1010}.

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

(Be3SiO4(OH)2·H2O [89]), chkalovite, galena, gmelinite17

[100], sphalerite (ZnS [101]) and tugtupite.

**Fig. 27** The locality for the mineral kuannersuite-(Ce).

second prefix are equal, only one prefix is applied [100].

**2.2.7. Kuannersuite-(Ce)**

chlore18

Britholites are typically phosphorus-bearing silicates with apatite structure and general formula: (REE,Ca)5[(Si,P)O4]3**Z**, where REE is usually yttrium and *Z* = OH<sup>−</sup> , F<sup>−</sup> or Cl<sup>−</sup> . The minerals from the group of britholite usually contain significant impurities of thorium and sometimes also uranium. These minerals are widespread in alkaline rocks such as pegma‐ tites and metasomites19 related to syenite15 and nepheline–syenite complexes [102]. The name of this group is derived from the Greek word *brithos* for weight in order to refer to the high density of the mineral. The following minerals are described below.

The structure and the crystallographic data of some of the minerals from the group of britholite were introduced in **Fig. 29** and **Table 1**, respectively. The structural, thermodynamic and electronic properties of britholites were investigated by NJEMA et al [103].

<sup>19</sup> The series of metamorphic processes whereby chemical changes occur in minerals or rocks as the result of the introduction of material, often in hot aqueous solutions, from external sources.

**Fig. 29** The structure (the view according to axis *c*) of mineral: (a) britholite-(Ce), (b) britholite-(Y) and (c) fluorbritho‐ lite-(Ce). Coupled heterovalent substitution at M and T site in the series apatite–calciobritholite–britholite [1].


**Table 1** The crystallographic data of minerals from the group of britholite

#### **2.3.1. Britholite-(Ce)**

The britholite-(Ce) (Lessignite-(Ce), (Ce,Ca)5(SiO4)3OH) [104],[105],[106]) mineral (**Fig. 30**) was first recognized as the new mineral by G. FLINK (1897) in the pegmatite form of the nepheline– syenite at Naujakasik, Ilímaussaq complex, Greenland. Known localities for the mineral britholite are shown in **Fig. 31**.

The specimen was named and described by CHR. WINTHER [104] as opaque, brown crystals of the composition of 3[4SiO2,2(Ce,La,Di,Fe)2O3,3(Ca,Mg)O,H2O,NaF],2[P2O5,Ce2O3], which are apparently hexagonal prisms with pyramids, but it actually consists of biaxial orthorhombic individuals twined together as in aragonite. The Th-rich britholite-(Ce) was also known as fenghuangshite [107]. Britholite-(Ce) (first described as britholite) is the forefather of the

**Fig. 30** The crystal (13 mm) of britholite-(Ce) from Ostkogen, Tvedalen, Norway.


Ca5 Ca3.5 REE1.5

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

X (PO4)1.5(SiO4)1.5 (PO4)3

Ca5(PO4)3F Fluorapatite

Measured/calculated

**2.3.1. Britholite-(Ce)**

britholite are shown in **Fig. 31**.

Apatite Fluorcalciobritholite

**Fig. 29** The structure (the view according to axis *c*) of mineral: (a) britholite-(Ce), (b) britholite-(Y) and (c) fluorbritho‐ lite-(Ce). Coupled heterovalent substitution at M and T site in the series apatite–calciobritholite–britholite [1].

The britholite-(Ce) (Lessignite-(Ce), (Ce,Ca)5(SiO4)3OH) [104],[105],[106]) mineral (**Fig. 30**) was first recognized as the new mineral by G. FLINK (1897) in the pegmatite form of the nepheline– syenite at Naujakasik, Ilímaussaq complex, Greenland. Known localities for the mineral

The specimen was named and described by CHR. WINTHER [104] as opaque, brown crystals of the composition of 3[4SiO2,2(Ce,La,Di,Fe)2O3,3(Ca,Mg)O,H2O,NaF],2[P2O5,Ce2O3], which are apparently hexagonal prisms with pyramids, but it actually consists of biaxial orthorhombic individuals twined together as in aragonite. The Th-rich britholite-(Ce) was also known as fenghuangshite [107]. Britholite-(Ce) (first described as britholite) is the forefather of the

**Mineral name Crystallographic parameters Hardness (Mohs)** *a c a***:***c Z V* **SG Density\***

**[Å] — — [Å3**

Britholite-(Ce) 9.63 7.03 1:0.730 2 564.60 P63/M 4.45/4.49 5½ Britholite-(Y) 9.43 6.81 1:0.722 524.45 4.25/4.07 5.0

Fluorbritholite-(Y) 9.44 6.82 1:0.722 526.68 —/4.61 5½ Fluorcalciobritholite 9.58 6.99 1:0.729 555.17 4.20/4.25 5½ Tritomite-(Ce) 9.35 6.88 1:0.736 520.89 4.20/5.02 5.5 Tritomite-(Y) 9.32 6.84 1:0.734 514.54 3.22/4.48 3.5-6.5

Fluorbritholite-(Ce) 9.52 6.98 1:0.734 547.74 4.66/4.67

**Table 1** The crystallographic data of minerals from the group of britholite

M Ca2.5 REE2.5 Ca2 REE3

Ideal composition

Fluorcalciobritholite Fluorbritholite

Ca3Ce2 (SiO4)2(PO4)F Ce3Ca2 (SiO4)3F

**] — [g·cm−3]**

Britholite

(PO4)0.5(SiO4)2.5 (SiO4)3


**Fig. 31** The localities for the mineral britholite-(Ce).

**Fig. 32** The crystal structure (perspective view according to *c*-axis) of britholite-(Ce) and some common crystal shapes.

**Fig. 33** Depiction of Ca(1)-O(3) triangles in apatite (a) and REE(1)-O(3) triangles in hexagonal (b) and monoclinic bri‐ tholite (*c*). The dashed and solid lines in the structure of britholite indicate short and long REE-O bonds, respectively [106].

britholite group [108]. The structure of monoclinic britholite-(Ce) is shown in **Fig. 32** and the crystallographic data are listed in **Table 1**.

The crystal structure of monoclinic dimorphs **Fig. 33** of the mineral britholite-(Ce) (and also of britholite-(Y)described below) was solved in **P21** space group by NOE et al [106]. The monoclinic britholite dimorph differs from its hexagonal counterpart principally in the ligation of the REE equivalent of the apatite Ca(l) site. Whereas in **P63** britholite each Ca(l) equivalent has either three short or three long REE-O(3) bonds; in the **P21** dimorph, the Ca(1) equiva‐ lents have either one long and two short REE-O(3) bonds or one short and two long REE-O(3) bonds. Arrangement of short and long bonds leads to **P63** symmetry in hexagonal britholite due to removal of **<sup>M</sup>** from symmetry elements of apatite, and **P21** symmetry in monoclinic britholite due to removal of symmetry elements **3** and **M**. The reduction in symmetry ex‐ plains the common observation of biaxial optical characteristics of britholite samples [106].
