**2.4.1. Fluorellestadite**

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

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

**Fig. 44** Known localities for the mineral 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**.

**2.3.7. Tritomite-(Y)**

The mineral fluorellestadite (formerly called ellestadite-(F) [85],[118],[119] Ca5(SiO4)1.5 (SO4)1.5F [1],[120]) is a rare mineral found in nature in skarns or metamorphosed limestones21 [121]. It was named according to American analytical chemist R.B. ELLESTAD and fluorine in the chemical composition. The mineral occurs as needles, as hexagonal prismatic, poorly termi‐ nated crystals up to 3 mm long, and as fine-grained aggregates. Thin needles are colorless, crystals are transparent and aggregates are translucent. Known localities of fluorellestadite are introduced in **Fig. 45**. The structure of mineral ellestadite is shown on **Fig. 46**.

<sup>21</sup> Limestone is a name used for sedimentary rock composed mainly of calcium carbonate, usually in the form of calcite (trigonal CaCO3) or aragonite (orthorhombic CaCO3), but there could also be considerable amounts of magnesium carbonate (MgCO3, trigonal mineral magnesite) or dolomite (trigonal CaMg(CO3)2) [121].


**Fig. 45** Known localities for the mineral fluorellestadite.

Fluorellestadite is colorless, blue or pale bluish green hexagonal mineral belonging to the space group P63/M. The unit cell parameters are *a* = 9.485, *c* = 6.916 Å, *Z* = 2 and *V* = 538.84 Å3 . Calculated density is 3.10 g⋅cm−3. The hardness of the mineral on the Mohs scale is 4½.

The mineral is also known from burned coal dumps, where its formation is possible in the presence of carbonaceous and carbonate rocks such as the rests of pyrometamorphism22 [9] of sedimentary rocks. The generalized formula of this mineral can be expressed as Ca10(SiO4)3−*<sup>x</sup>*(SO4)3−*<sup>x</sup>*(PO4)2*x* (OH,F,Cl)2, where the parameter *x* varies from 0 (ellestadite) to 3 (apatite).

<sup>22</sup> The term pyrometamorphism, which is derived from the Greek word *pyr/pyro* (fire), *meta* (change), and *morph* (shape or form) was first used by BRAUNS to describe high-temperature changes which take place at immediate contact of magma and country rock with or without interchanges of material. Tyrrell defined pyrometamorphism as pertaining to the "effect of the highest degree of heat possible without actual fusion." There are a number of rock terms commonly used in association with the phenomenon of pyrometamorphism including hornfels,3 buchite, porcellanite, sanidinite, emery, paralava, clinker, fulgurite, or with other general terms such as fused or burnt rock [121].

**Fig. 46** The structure of the mineral fluorellestadite (perspective view along the *c*-axis).

## **2.4.2. Hydroxylellestadite**

**Fig. 45** Known localities for the mineral fluorellestadite.

(apatite).

Fluorellestadite is colorless, blue or pale bluish green hexagonal mineral belonging to the space group P63/M. The unit cell parameters are *a* = 9.485, *c* = 6.916 Å, *Z* = 2 and *V* = 538.84 Å3

The mineral is also known from burned coal dumps, where its formation is possible in the presence of carbonaceous and carbonate rocks such as the rests of pyrometamorphism22

sedimentary rocks. The generalized formula of this mineral can be expressed as Ca10(SiO4)3−*<sup>x</sup>*(SO4)3−*<sup>x</sup>*(PO4)2*x* (OH,F,Cl)2, where the parameter *x* varies from 0 (ellestadite) to 3

<sup>22</sup> The term pyrometamorphism, which is derived from the Greek word *pyr/pyro* (fire), *meta* (change), and *morph* (shape or form) was first used by BRAUNS to describe high-temperature changes which take place at immediate contact of magma and country rock with or without interchanges of material. Tyrrell defined pyrometamorphism as pertaining to the "effect of the highest degree of heat possible without actual fusion." There are a number of rock terms commonly used

in association with the phenomenon of pyrometamorphism including hornfels,3

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

paralava, clinker, fulgurite, or with other general terms such as fused or burnt rock [121].

Calculated density is 3.10 g⋅cm−3. The hardness of the mineral on the Mohs scale is 4½.

.

[9] of

buchite, porcellanite, sanidinite, emery,

Hydroxylellestadite (formerly called ellestadite-(OH) [85], Ca5(SiO4)1.5(SO4)1.5(OH) [1],[117], [122],[123]) was first reported at cornet Hill by PASCAL et al [124] and MARINCEA et al [125]. Natural hydroxylellestadite23 occurrences were reported from pegmatite veins, skarn and pyrometamorphic deposits and from mine dumps, but this mineral has never been reported from a cave. The mineral forms aggregates of xenomorphic crystals which have a maximum length of 0.5 mm and a maximum width of about 0.1 mm.

<sup>23</sup> Synthetic analogs are known as "technical products," such as burnt industrial waste and cement [122].

Hydroxylellestadite is associated with berlinite24 (AlPO4 [126]) , another high-temperature mineral. It is likely to have formed within highly phosphatized, silicate-rich, carbonatemudstone sediments heavily compacted and thermally transformed due to *in situ* bat guano combustion. Known localities, where the mineral hydroxylellestadite can be found, and its structure are shown in **Fig. 47** and **Fig. 48**, respectively.

Hydroxylellestadite is a pink or purple-gray hexagonal mineral, which belongs to the space group P63/M. The unit cell parameters are *a* = 9.491, *c* = 6.921 Å, *Z* = 2 and *V* = 539.91 Å3 . Calculated density is 3.11 g⋅cm−3. The hardness of the mineral on the Mohs scale is in the range of 3½ to 4½. Hydroxylellestadite shows faded white-yellow fluorescence when irradiated with UV light, independently of the excitation frequency [122].

**Fig. 48** The structure (perspective view along the *c*-axis) and the crystal habit of mineral hydroxylellestadite.
