**2.3.6. Tritomite-(Ce)**

The mineral fluorbritholite crystallizes in hexagonal systems of the space group P63/m with the crystallographic parameters of unit cell *a* = 9.444 and *c* = 6.819 Å, *a*:*c* = 1:0.722, *V* = 526.68 Å3 and *Z* = 2. It is a brittle mineral of light-pink or brown color and calculated density of 4.61 g·cm

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‐

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

−3. It has a pale brownish or white streak and its hardness on the Mohs scale is 5.

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

**2.3.5. Fluorcalciobritholite**

britholite is shown in **Fig. 40** and **Fig. 41**, respectively.

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

Tritomite-(Ce) (Ce5(SiO4,BO4)3(OH,O) [105],[111]) was first found by WEIBYE in 1849 at the island of Låven in Langesundsfjord as dark tetrahedral crystals in leucophanite ((Na,Ca)2BeSi2(O,OH,F)7 [112] or analcime. Chemically and structurally, it is very similar to melanocerite (melanocerite-(Ce),20 Ce5(SiO4,BO4)3 (OH,O) [1],[105],[111]) and caryocerite [111]. The pyramidal crystal of mineral tritomite-(Ce) is shown in **Fig. 42**.

**Fig. 42** The crystal habit of the mineral tritomite-(Ce) and tritomite-(Y).

The mineral was named from the Greek *tρitομοs* meaning "cut in three parts" in allusion to the triangular and pseudo-tetrahedral crystal habit [113],[114]. Known localities forthe mineral tritomite-(Ce) are shown in **Fig. 43**.

Tritomite-(Ce) crystallizes as hexagonal mineral in the space group P63/M with crystallograph‐ ic parameters *a* = 9.35 and *c* = 6.88 Å, *a*:*c* = 1: 0.736, *V* = 520.89 Å3 and *Z* = 2. It is a very brittle mineral of dark brown color with a hardness (on the Mohs scale) of 5½.

<sup>20</sup> Since the mineral is equal to tritomite-(Ce), the name of melanocerite-(Ce) is discredited [1].

**Fig. 43** Known localities for the mineral tritomite-(Ce).

#### **2.3.7. Tritomite-(Y)**

The mineral tritomite-(Y) ((Y5(SiO4,BO4)3(OH,O,F), [Y3+(Cr, Pr, Th)4+Ca](Si2B)O12O [111],[113], [115]) was first described by FRONDEL. It is also known as the hexagonal mineral spencite (named after Canadian geologist H.S. SPENCE) [116]. The mineral tritomite-(Y) is formed in the nepheline syenite pegmatites of the area, which carries rare earths predominantly from the yttrium group. Known localities of mineral tritomite-(Y) are introduced in **Fig. 44**.

**Fig. 44** Known localities for the mineral tritomite-(Y).

When heated in air to temperatures ranging from 600°C to 1000°C, tritomite-(Y) recrystalliz‐ es to the structure of apatite and amorphous phase, presumably to a calcium borosilicate glass [60]. The pyramidal crystals of the mineral tritomite-(Y) are similarto tritomite-(Ce) ones, which are shown in **Fig. 42**.

Tritomite-(Y) crystallizes as hexagonal in the space group P63/M with unit cell parameters *a* = 9.32 and *c* = 6.84 Å, *a*:*c* = 1:0.734, *V* = 514.54 Å3 and *Z* = 2. It is dark green-black, red-brown, nearly black mineral with vitreous or resinous luster and average density of 3.22 g⋅cm−3. It is a brittle mineral forming small fragments with conchoidal fracture. The hardness of trito‐ mite-(Y) on the Mohs scale ranges from 3.5 to 6.5 [111],[113],[115].
