**2.3.5. Fluorcalciobritholite**

The mineral fluorcalciobritholite ((Ca,REE)5(SiO4,PO4)3F; [1],[102]) was found at Mount Kukisvumchorr, Khibiny alkaline complex, Kola Peninsula, Russia and differs from fluorbri‐ tholite and fluorapatite in the content of calcium (Ca > *Σ* REE) and phosphorus (Si > P), respectively. The main crystal form is a hexagonal prism. The mineral is transparent, with a pale pinkish to brown color and a white streak. The structure and the locality of fluorcalcio‐ britholite is shown in **Fig. 40** and **Fig. 41**, respectively.

**Fig. 40** The structure (perspective view along the *c*-axis) and the crystal habit of the mineral fluorcalciobritholite.

**Fig. 41** The localities for the mineral fluorcalciobritholite.

The ideal chemical formula for fluorcalciobritholite may be written as (Ca3REE2) [(SiO4)2(PO4)]F. In the view of coupled heterovalent substitutions occurring at the M and T sites in the series apatite–calciobritholite–britholite, it is more practical in this case for nomenclature purposes to consider the total abundance of M cations as a single, composite site [1].

Pale pinkish brown or brown mineral fluorcalciobritholite crystallizes as hexagonal in the space group P63/M with the crystallographic parameters *a* = 9.58 and *c* = 6.985 Å, *a*:*c* = 1:0.729, *V* = 555.17 Å3 and *Z* = 2. Calculated and measured densities of the mineral are 4.25 and 4.2 g·cm −3, respectively. It has white streak and vitreous luster. The mineral is brittle and its hardness on the Mohs scale is equal to 5½.
