**2.5. Pieczkaite**

The mineral pieczkaite (Mn5 2+(PO4)3Cl [135],[136]) was found in the Southeastern shoreline of a small, unnamed island in Cross Lake, Manitoba, Canada (54°41′N, 97°49′W; **Fig. 52**) and classified as the member of the supergroup of apatite. It is isostructural with calcium fluora‐ patite (**Section 1.5.1**). The approximate composition of hydrothermally grown manganese chlorapatite is Mn5(PO4)3Cl0.9(OH)0.1 [136].

**Fig. 52** Locality for the mineral pieczkaite.

**Fig. 50** Known localities for the mineral mattheddleite.

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

**Fig. 51** The structure and the crystal habit of the mineral mattheddleite (perspective view along the *c*-axis).

**Fig. 53** The structure of the mineral pieczkaite (perspective view along the *c*-axis).

It is a hexagonal mineral that crystallizes in the space group P63/M with the crystallographic parameters of unit cell *a* = 9.532 and *c* = 6.199 Å, *a*:*c* = 1:0.6501, *V* = 587.78 Å3 and *Z* = 2. Calculated density of pieczkaite is 3.783 g·cm−3. The hardness of the mineral on the Mohs scale varies in the range from 4 to 5. The structure of the mineral pieczkaite is shown in **Fig. 53**.

The coordination polyhedron around Mn(1) has the point-group symmetry 3 and is a trigonal prism in which the two triangles of oxygen atoms are slightly rotated relative to each other. The coordination polyhedron around Mn(2) is a severely distorted octahedron. The phos‐ phate group is more distorted than in any of the other apatites. The chlorine atom is located in the center of an equilateral triangle formed by three Mn(2) atoms [136].
