**2.4. The group of ellestadite**

Ellestadites *sensu lato* are sulfato-silicates. For stoichiometric reasons, the incorporation of the sulfate anion (SO4) 2− in the structure of apatite in the place of (PO4) 3− or (AsO4) 3− must be coupled with a concurrent substitution by silicate anions (SiO4) 4−. This holds in all cases in which the M sites are occupied by divalent cations. Pure sulfates with an apatite structure may occur only by reducing overall positive charge associated with the **M** cations, as is the case in cesanite and caracolite from the group of hedyphane (**Section 2.1**) [1].

The structural formula of ellestadite and (with slight modification) of wilkeite can be ex‐ pressed as follows [117]:

$$\left[\text{Ca}\_6\text{(F, Cl, O, OH)}\_2\text{][(S, Si, P, C)O}\_4\text{]}\_6\text{(Ca, C)}\_4\right]$$

This formula indicates that two-fifths of the Ca2+ ions are located on threefold axes and can be replaced by carbon. Three-fifths of the Ca2+ ions are tied to F<sup>−</sup> , Cl<sup>−</sup> and O<sup>−</sup> anions or OH<sup>−</sup> groups and cannot be replaced by carbon. All Ca2+ ions are tied to O-ions, which are arranged in tetrahedral coordination with S-, Si-, P- or C-ions at the centers.
