**2.3.2. Britholite-(Y)**

The mineral britholite-(Y) ((Y,Ca)5(SiO4)3OH, abukumalite, [105], [109]) occurs similarly to britholite-(Ce) in granite, alkaline rocks, skarns and hydrothermal veins [107]. The structure of monoclinic (P21, referthe discussion to **Fig. 33**) britholite-(Y) and known localities are shown in **Fig. 35** and **Fig. 34**, respectively.

**Fig. 34** The structure (perspective view along the *c*-axis) of britholite-(Y).


britholite group [108]. The structure of monoclinic britholite-(Ce) is shown in **Fig. 32** and the

c cc

**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

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

Symmetry: 3/m 3 1

Hexagonal Britholite

O3d

O3b

O3c

O3a

Monoclinic Britholite

REE(1b)

O3f

O3e

REE(1a)

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

crystallographic data are listed in **Table 1**.

Column

m

m

[106].

Fluorapatite

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

O3

Ca1

Britholite-(Y) is very brittle mineral with reddish brown or black color, pale brown streak and resinous luster that crystallizes as hexagonal in the space group P63/M with the unit cell parameters *a* = 9.43 and *c* = 6.81 Å, *a*:*c* = 1:0.722, *V* = 524.45 Å3 and*Z* = 2. Calculated and measured densities of the mineral are 4.07 and 4.25 g·cm−3, respectively.
