3.1. Zn2(SeO3)Cl2 Polymorphs
Sophiite, α-Zn
2(SeO
3)Cl
2, was first discovered in fumaroles of the Tolbachik volcano, Kamchatka peninsula, Russia [
12]. Its β-polymorph was synthesized by chemical transport reactions [
45], which has repeatedly been used for simulation of fumarolic mineralization [
54]. It is noteworthy that both polymorphs of Zn
2(SeO
3)Cl
2 are highly hygroscopic. Their crystal structures are very close to each other and both are based upon the same type of electroneutral [Zn
2(SeO
3)Cl
2]
0 layers formed by (ZnO
2Cl
2) tetrahedra, (ZnO
4Cl
2) octahedra, and (SeO
3) trigonal pyramids (
Figure 1a). The (ZnO
4Cl
2) octahedra share common O corners to form chains running parallel to
c in both polymorphs. Each octahedron shares one O–Cl edge with an adjacent (ZnO
2Cl
2) tetrahedron and one O–O edge with (SeO
3) pyramid, which provide linkage of the chains into two-dimensional layers. The layers are parallel to (010) in sofiite (
Figure 1b) and to (100) in the β-polymorph (
Figure 1c). The structures of both modifications may be considered as polytypes, since they are based upon the same type of layers. According to the modern nomenclature, sofiite as α-Zn
2(SeO
3)Cl
2 should be regarded as 2
O-polytype (orthorhombic structure with two layers per unit cell), whereas β-Zn
2(SeO
3)Cl
2 should be considered as 1
M-polytype (monoclinic structure with one layer per unit cell). Since the layers in the structures of both polytypes are electroneutral, the linkage between the layers is achieved through the secondary bonds that involve Cl
− anions and Se
4+ cations (
Figure 1).
The lists of bcps found for sofiite and β-Zn
2(SeO
3)Cl
2 are given in
Table 2 and
Table 3, respectively. It can be clearly seen that the interatomic interactions in the two structures can be subdivided into two groups: interactions with
H(
rc) < 0 and interactions with
H(
rc) > 0. The first kind of interactions is observed for the Se–O, Zn–O, and Zn–Cl bonds. These interactions correspond to the intralayer bonding and can be characterized as belonging to the intermediate type (since ∇
2ρ(
rc) > 0 for all interactions in the two structures). According to the
ρ(
rc) values, these bonds can be separated into Se–O bonds (
ρ(
rc) = 0.18–0.20 a.u.) and Zn–X bonds (
ρ(
rc) = 0.02–0.08 a.u.).
The most interesting are the bcps, for which ∇
2ρ(
rc) > 0 and
H(
rc) > 0. These bond paths correspond to the Se–Cl, Cl–Cl, and O–O closed-shell interactions, which we shall analyse in more detail. The geometrical environments around the atoms participating in weak closed-shell interactions in sofiite are shown in
Figure 2. There are three Se–Cl interactions (
Figure 2a), from which two interactions (Se–Cl1 and Se–Cl2) are intralayer and one Se–Cl2 interaction is interlayer. The same situation is also observed in β-Zn
2(SeO
3)Cl
2 (
Figure 3a). In sofiite, the interlayer interaction corresponds to the shortest Se–Cl distance of 3.317 Å, whereas, in β-Zn
2(SeO
3)Cl
2, the respective Se–Cl distance is the longest among three Se–Cl contacts with its length equal to 3.514 Å. Taking into account weak interactions, the coordination of Se atom in both polymorphs can be considered as a distorted trigonal (SeO
3Cl
3) prism with the O
3 and Cl
3 triangular bases. The Se–O interactions are more than one order stronger than the Se–Cl interactions. The number of Cl–Cl interactions in the two Zn
2(SeO
3)Cl
2 is notably different, despite the fact that the number of symmetrically independent bcps is the same (
Table 2 and
Table 3). The point is that two Cl–Cl bcps in sofiite are located on the twofold axes, whereas similar bcps in β-Zn
2(SeO
3)Cl
2 are in general positions. In general, the structure of β-Zn
2(SeO
3)Cl
2 contains 12 Cl–Cl bcps per interlayer, whereas the structure of sofiite contains only 8. The difference results in different coordinations of the Cl atoms in the structures of the two polymorphs.
The Cl1 atom (
Figure 2b) is linked to two Zn atoms and participates in three Cl–Cl and one Se–Cl interactions. The overall coordination of the Cl1 atom can be described as distorted octahedral. From four weak closed-shell interactions, only one Cl–Cl interaction has an interlayer character. In β-Zn
2(SeO
3)Cl
2, the analogue of the Cl1 position in sofiite is the Cl2 site (
Figure 3c). Its intralayer coordination (by two Zn, one Se, and two Cl atoms) is very similar to that observed in sofiite. However, there are two interlayer Cl–Cl interactions versus one in sofiite. Thus, the total coordination of the Cl2 site in the β-polymorph is sevenfold. The coordination of the Cl2 site in sofiite is shown in
Figure 2c. There are two intralayer Cl–Zn bonds and one intralayer Cl–Se interaction. The interlayer interactions are characterized by one Cl–Se and one Cl–Cl bond paths. The situation in β-Zn
2(SeO
3)Cl
2 is again different (
Figure 3b). The number of intralayer interactions is the same as in sofiite, but there are three interlayer interactions, one Cl–Se and two Cl–Cl. In total, the coordination of the Cl1 site in the β-polymorph is sixfold versus fivefold in sofiite.
In both polymorphs, there is a bcp between the two O atoms located in the middle of the eight-membered atomic ring (
Figure 2d). This bcp is located in the inversion center and corresponds to a very weak closed-shell interaction.
The higher relative number of bcps in β-Zn2(SeO3)Cl2 compared to sofiite might be related to the higher stability of the β-polymorph. Its physical density, 3.68 g.cm−3 is slightly higher than that of sofiite, 3.65 g.cm−3, which manifests the higher efficiency of layer packing, which, in turn, may lead to the formation of more interlayer interactions, thus stabilizing the β-polymorph. It is noteworthy that, among the closed-shell interactions in the two polymorphs, the Se–Cl interactions are stronger than others, pointing out their important role in the linkage of the [Zn2(SeO3)Cl2]0 in the structures. Nevertheless, the Cl–Cl interactions are also of importance, providing additional contributions to the structural stability.
3.2. Cu5O2(SeO3)2Cl2 Polymorphs
In fact, α-Cu
5O
2(SeO
3)
2Cl
2 was the first selenite chloride reported by Galy et al. back in 1979 [
55]. Its dark-brown crystals were prepared by the chemical vapor transport reactions method. In 1999, it was described by Vergasova et al. [
16] as a natural mineral species from fumaroles of the Great fissure Tolbachik eruption (Kamchatka, Russia). In 2006, Vergasova et al. [
20] described another natural polymorph of Cu
5O
2(SeO
3)
2Cl
2, which was named parageorgbokiite in order to recognize its chemical similarity to georgbokiite. It is noteworthy that, in contrast to georgbokiite, the crystals of parageorgbokiite are green and thus the two polymorphs can easily be distinguished. The crystal structures of georgbokiite and parageorgbokiite are closely related and the best way to understand these relations are to describe them in terms of structural units based upon oxocentered tetrahedra formed around ‘additional’ O atoms not bonded to Se
4+ cations. This approach was first proposed in 1968 [
56] and recently developed into a coherent crystal chemical theory (see [
57,
58] for reviews and historical remarks). In general, description of certain crystal structures in terms of anion-centered coordination polyhedra became more popular over the years, in part due to the recent reports on interesting structural and physical properties and mineralogical importance of antiperovskites, i.e., materials with structures based upon anion-centered octahedra [
59,
60,
61,
62,
63,
64,
65,
66].
In the crystal structures of both polymorphs of Cu
5O
2(SeO
3)
2Cl
2, there are additional O atoms not bonded to Se and tetrahedrally coordinated by four Cu atoms thus forming (OCu
4) oxocentered tetrahedra. In both structures, (OCu
4) tetrahedra share Cu–Cu edges and Cu corners to form [O
2Cu
5] chains shown in
Figure 4a. The (SeO
3) groups are attached to (OCu
4) tetrahedra in a face-to-face fashion [
67,
68], which means that the triangular bases of both units are parallel to each other and have the same orientation. The complex 1-dimensional structural units with the composition {[O
2Cu
5](SeO
3)
2}
2+ shown in
Figure 4b are the basic structural modules for both polymorphs and it is the mode of their combination that generates the structural difference.
Figure 5 shows a successive (step-by-step) construction of the crystal structure of georgbokiite, α-Cu
5O
2(SeO
3)
2Cl
2, if viewed in terms of anion-centered tetrahedra. The [O
2Cu
5] chains are running parallel to the
c axis and grouped into layers parallel to the (100) plane (
Figure 5a). The (SeO
3) groups are attached to the chains and are also responsible for the linkage of the {[O
2Cu
5](SeO
3)
2}
2+ in the (100) and (010) directions through the formation of the Cu–O bonds (
Figure 5b). The metal-oxide bonding network based upon the Cu–O and Se–O bonds contains the strongest chemical bonds in the structure. It can be seen, however, that this network possesses 1-dimensional channels parallel to (001) (i.e., the direction of extension of the [O
2Cu
5] chains) and these channels are occupied by Cl
− anions (
Figure 5c). These channels are also remarkable by the fact that they provide spatial localization of the weak closed-shell interactions in the crystal structure (
Figure 5d).
The mode of linkage of the basic structural modules in the structure of parageorgbokiite, β-Cu
5O
2(SeO
3)
2Cl
2, is different, which is illustrated in
Figure 6.
In contrast to georgbokiite, the [O
2Cu
5] chains in parageorgbokiite have different relative orientations (
Figure 6a) and are parallel to the
a axis. Their linkage through (SeO
3) groups (attached to the (OCu
4) tetrahedra in the face-to-face fashion) results in the formation of a 3-dimensional framework of relatively strong bonds (
Figure 6b) with channels occupied by Cl
− anions (
Figure 6c) and hosting the bcps corresponding to weak closed-shell interactions (
Figure 6d).
Table 4 and
Table 5 provide lists of bcps found for georgbokiite and parageorgbokiite, respectively. According to their properties, all interatomic interactions in the two minerals can be classified into three groups:
- (1)
the Cu–O, Cu–Cl and Se–O interactions with ∇2ρ(rc) > 0 and H(rc) < 0 (intermediate bonding interactions);
- (2)
the Cu–O and Cu–Cl interactions with ∇2ρ(rc) > 0 and H(rc) > 0 (closed-shell interactions); the Cu3–Cl interaction (2.739 Å) deserves special attention as this interaction lies exactly on the border between intermediate and closed-shell interactions (H(rc) = 0);
- (3)
the closed-shell Se–Cl, Cl–Cl, and Cl–O interactions with ∇2ρ(rc) > 0 and H(rc) > 0.
The appearance of the second group of interactions (which is absent in the Zn
2(SeO
3)Cl
2 polymorphs) is due to the Jahn–Teller distortion of Cu
2+ coordination [
69], which results in the splitting of Cu–
X interactions (
X = O, Cl) into short and long bonds. It is noteworthy that the empirical bond-valence theory [
70] would make no distinction between the different long Cu–
X bonds, whereas the data given in
Table 4 and
Table 5 indicate that the AIM theory classifies them into closed-shell and intermediate interactions (taking into account the conditional character of the adopted classification (see
Section 2)). In particular, for the Cu–Cl bonds, the border between the two types of interactions corresponds to the Cu–Cl distance of 2.739 Å. It is of interest that the
ρ(r
c) for the closed-shell Cu–
X interactions is of the same order as for other closed-shell interactions in the two structures.
The configuration of closed-shell interactions inside the channels in the crystal structures of the Cu
5O
2(SeO
3)
2Cl
2 polymorphs is shown in
Figure 7. In georgbokiite (
Figure 7a), the channels are more compact and have (SeO
3) groups on one side and Cl
− anions on another, each Se and each Cl atom participates in two Se–Cl interactions, and there are no Cl–Cl interactions. In contrast, in parageorgbokiite (
Figure 7b), the channels are larger and have both (SeO
3) groups and Cl
− anions on both sides. Each Se atom participates in two Se–Cl interactions, whereas each Cl atom participates in two Se–Cl, two Cl–Cl and one Cl–O interactions. The structure of the channels in the two polymorphs is therefore remarkably different, which can be explained by the different arrangements of basic structural modules.
The more open character of the crystal structure of parageorgbokiite compared to that of georgbokiite is reflected in the values of physical densities, which are equal to 4.88 and 4.69 g
.cm
−3 for the α- and β-polymorphs, respectively. According to Krivovichev et al. [
47], parageorgbokiite is most likely a high-temperature modification of Cu
5O
2(SeO
3)
2Cl
2, however, the relative stabilities of the two minerals are unknown. It is of interest that, whereas georgbokiite can easily be synthesized by the chemical vapor transport reactions method [
55,
71], no synthetic analogue is known for parageorgbokiite.
3.3. Burnsite, KCdCu7O2(SeO3)2Cl9
Burnsite, KCdCu
7O
2(SeO
3)
2Cl
9, was discovered in Tolbachik fumaroles in 2002 [
18]. By analogy with Cu
5O
2(SeO
3)
2Cl
2 polymorphs, its crystal structure contains additional O atoms tetrahedrally coordinated by four Cu atoms [
48]. Two (OCu
4) tetrahedra share a common Cu atom to form a [O
2Cu
7] dimer (
Figure 8a) similar to those recently observed in the crystal structure of Na
2Cu
7O
2(SeO
3)
4Cl
4 [
72,
73].
In the crystal structure of burnsite, the [O
2Cu
7] dimers are surrounded by two (SeO
3) groups each of them attached to the (OCu
4) tetrahedra in a face-to-face fashion (
Figure 8b). The resulting {[O
2Cu
7](SeO
3)
2} groups are linked via Cu–O bonds into an open three-dimensional framework (
Figure 9a) with large cavities occupied by Cd
2+, K
+, and Cl
− ions (
Figure 9b).
Table 6 provides a list of bcps in the crystal structure of burnsite. It can be seen that most of the interatomic interactions can be described as belonging to the intermediate type with ∇
2ρ(r
c) > 0 and
H(r
c) < 0, except for the Se–Cl interactions that are of the closed-shell type (∇
2ρ(r
c) > 0 and
H(r
c) > 0).
Figure 8c shows the atomic configuration inside the cavity occupied by two (SeO
3) groups and three Cl
− ions.
Each (SeO3) group forms participates in three Se–Cl interactions, so that the full coordination of Se4+ ions can be viewed as trigonal prismatic, similar to that observed in the crystal structures of the Zn2(SeO3)Cl2 polymorphs (see above). Each Cl2 atom participates in two Cl–Se closed-shell interactions.