**Author details**

Petr Ptáček

**10. Xenotime** [118]: monazite (**Fig. 26**) is isostructural with zircon (I4 <sup>1</sup> /AMD). Monazite atomic arrangement as well as that of xenotime is based on [001] chains of intervening phos‐ phate tetrahedra and RE polyhedra, with REO, polyhedron in xenotime that accommo‐ dates heavy lanthanides (Tb-Lu in the synthetic phases) and REO polyhedron in monazite that preferentially incorporates larger light rare-earth elements (La-Gd). As the struc‐ ture "transforms" from xenotime to monazite, the crystallographic properties are comparable along the [001] chains, with the structural adjustments to the different sizes

**Fig. 26.** The structure of xenotime-(Y): *a* = 6.8947 Å, *b* = 6.8947 Å and *c* = 6.0276 Å (a) and xenotime-(Dy) (b) *a* = 6.9052 Å,

Isostructural arsenate analogues of many phosphate minerals are known, and in some cases, vanadates too. Some orthophosphates capable of forming complete ranges of solid solutions

**• Fairfieldite group**: Ca2M(XO4)2·2H2O, where M = Mn, Fe, Mg, Ni, Zn, Co and X = P, As;

**• Vivianite group**: M3(XO4)2·8H2O, where M = Fe, Mn, Mg, Zn, Co, Ni and X = P, As;

of REE atoms occurring principally in (001).

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

*b* = 6.9053 Å and *c* = 6.0384 Å (b) [118] viewed along the b-axis.

with the corresponding orthoarsenates are [25]:

**• Variscite group**: MXO4·2H2O, where M = Fe, Al and X = P or As;

**• Monazite group**: MXO4, where M = Ce, La, Nd, Th, Bi and X = P, As;

**• Xenotime group**: MXO4, where M = Y, Ce, Bi and X = P, V;

**• Rhabdophane group**: MXO4·H2O, where M = Ce, La, Nd, Th and X = P, As;

Brno University of Technology, Czech Republic
