The crystal structure of nishanbaevite (Fig. 3) is unique. It is based on the complex heteropolyhedral (100) double layers formed by zig-zag [001] chains of Al-centred polyhedra and isolated from each other tetrahedra AsO4 and SO4. Within these layers two symmetrically equivalent single sheets are connected by the gliding plane b. There are two crystallographically non-equivalent Al sites which centre alternating Al(1)O5 trigonal bipyramids and Al(2)O6 octahedra sharing edges to form the chains (Fig. 3a). Adjacent chains are connected via AsO4 tetrahedra sharing common vertices with Al-centred polyhedra and thus forming double-layered core of the heteropolyhedral sheet. Each tetrahedron shares three vertices with the chains of one layer of the core (Fig. 4) and one vertex with the chains of the second one. SO4 tetrahedra are linked to the external parts of the double-layered core from both sides sharing two common vertices with Al(2)O6 octahedra. The large-cation A site with disordered distribution of K and subordinate Na is located in the interlayer space between SO4 tetrahedra.
The topological analysis aimed with the search of the related compounds was performed with the program package ToposPro (Blatov et al. 2014). The ‘Junction atoms’ algorithm (Shevchenko and Blatov 2021) was used to simplify the nishanbaevite structure with the oxygen atoms as junctions; potassium/sodium atoms were excluded from the consideration at this step. As a result, the topological representation of nishanbaevite is consisted of centers of AlO6, AlO5, AsO4 and SO4 groups. The simplified double periodic layer is represented by a 6,6-coordinated net with AlO6, AlO5, AsO4 groups as nodes and bridging SO4 groups as links. The term “6,6-coordinated” means that the net contains two types of topologically different nodes with the number of contacts (coordination) equal to six. One type of nodes is represented by the AlO6 groups, while AlO5 and AsO4 groups are topologically equivalent, i.e. they are equally connected to other groups, and they represent the other type of nodes. The specific feature of the node formed by AlO6 is that it is bounded to all types of building units, namely AlO6, AlO5, AsO4 and SO4 and formally its coordination is equal to 8 (Fig. 5a). However, within the simplified net SO4 groups are considered as edges and thus in such approach AlO6 node is surrounded by 6 neighboring nodes. In contrast with it the coordination of AlO5 and AsO4 nodes is formed only by AlO6, AlO5 and AsO4 groups located within the net. The difference between AlO5 and AsO4 is in the method of connecting to the six other groups: AsO4 uses for connection only vertices (oxygen atoms), while AlO5 additionally uses two edges (Figs. 5b and c). The net has the unique topology, which has never occurred in crystal structures; we have deposited this topology in the TopCryst system (Shevchenko et al. 2022) under the name dyp1. The search in the databases of the TopCryst system revealed five compounds, namely, Cu3Mo2O9, Zn3Mo2O9, [Cu(1−x)Znx]3Mo2O9 and Zn3O(SO4)2 as well as glikinite, a natural analogue of synthetic Zn3O(SO4)2 (Nazarchuk et al. 2020) which contain the nishanbaevite-like 6,6-coordinated layers as parts of the 3D framework structure. These layers are topologically close to the half of the double-layered core of the heteropolyhedral sheet in the nishanbaevite structure but differ from it in the ratio of tetrahedra and non-tetrahedral polyhedra and the presence of additional non-tetrahedral (five-fold or octahedral) polyhedron in zig-zag chains. Besides glikinite, two minerals demonstrate some structural similarities with nishanbaevite, namely vergasovaite Cu3O(MoO4)(SO4) (Berlepsch et al. 1999) and cupromolybdite Cu3O(MoO4)2 (Zelenski et al. 2012) isostructural to one another (Table 6). Their structural relationship with nishanbaevite (Figs. 3 and 4) is illustrated in Fig. 6. Noteworthy, all these minerals originate from fumaroles of the Tolbachik volcano.
Table 6
The framework Cu- and Zn-oxomolybdate and/or oxosulfate compounds with polyhedral units similar to the single sheets in the double layers of nishanbaevite
Compound / Mineral | Space group | Unit-cell parameters (Å,°) | V, Å3 | Reference |
Cu3Mo2O9 | Pnma | 7.629(1) 6.876(1) 14.573(1) | 764.5 | Hase et al. 2015 |
[Cu(1−x)Znx]3Mo2O9 x = 0.09 x = 0.18 x = 0.33 x = 0.48 x = 0.62 x = 0.76 | Pnma | 7.6925(8) 6.9014(6) 14.5910(18) 7.7022(5) 6.9160(6) 14.5485(11) 7.7060(6) 6.9480(6) 14.4715(11) 7.7337(5) 6.9860(6) 14.4536(11) 7.7334(4) 7.0229(5) 14.4651(12) 7.7493(5) 7.0615(6) 14.5020(10) | 774.6(1) 775.0(1) 774.8(1) 780.9(1) 785.6(1) 793.6(1) | Reichelt et al. 2005 |
Cu3O[(Mo1.94S0.06)O4]2 cupromolybdite | Pnma | 7.66380(10) 6.86700(10) 14.5554(2) | 766.012(18) | Zelenski et al. 2012 |
Cu3O((Mo,S)O4SO4) vergasovaite | Pnma | 7.421(2) 6.754(3) 13.624(5) | 682.9 | Berlepsch et al. 1999 |
Zn3Mo2O9 | P21/m | 7.7573(12) 7.1319(13) 8.370(2) 117.397(7) | 411.1 | Söhnel et al. 1996 |
Zn3S2O9 | P21/m | 7.937(2) 6.690(2) 7.851(2) 124.39(1) | 344.0 | Bald and Gruehn 1981 |
(Zn,Cu)3S2O9 glikinite (Zn,Cu)3O(SO4)2 | P21/m | 7.298(17) 6.588(11) 7.840(12) 117.14(3) 7.3156(6) 6.6004(5) 7.8941(7) 117.424(5) | 335.4(11) 338.34(5) | Nazarchuk et al. 2020 Nekrasova et al. 2021 |
The cationic array {Al, As, S, K} in nishanbaevite is unique; no similar arrays or subarrays were found using the TopCryst system. It is typical for the compounds with large alkali and alkali-earth elements which usually distort cationic arrays. Consequently, it is an additional indication on the layer character of the nishanbaevite structure.
Arsenate and sulfate tetrahedral anions are ordered in nishanbaevite. The As site contains admixed lighter constituents, S or/and Si and minor P (assumed based on electron microprobe data: see above). During structure refinement, they were formally considered together and designated as P' in Tables 2 and 3; scattering curve of P was used for P'. The sizes of AsO4 tetrahedra with < As-O > 1.665 Å versus SO4 tetrahedra with < S-O > 1.461 Å also confirm the As/S segregation in nishanbaevite. The similar As/S ordering in tetrahedral sites was also revealed in vasilseverginite Cu9O4(AsO4)2(SO4)2, another H-free sulfate-arsenate from the Arsenatnaya fumarole (Pekov et al. 2021).
The As/S ordering and the edge-sharing Al-polyhedra approach nishanbaevite to arsentsumebite Pb2Cu(AsO4)(SO4)(OH), a member of the brackebuschite supergroup (Zubkova et al. 2002). However, there is a partial As/S-ordering in arsentsumebite and the electronic contents in its tetrahedral sites correspond to the occupation of T1 = As0.63S0.37 and T2 = As0.37S0.63. In common with arsentsumebite the double walls of nishanbaevite contain M = M-T chains, where M = M means edge-sharing between M-polyhedra and M-T represents corner sharing between M-polyhedra and TO4 tetrahedra (Eby et al. 1993).
The heteropolyhedral chains Cu(AsO4)(SO4)OH with the similar role of chemically different AsO4 and SO4 tetrahedra are the basic features of arsentsumebite. In common with it in gallobeudantite PbGa3(AsO4)(SO4)(OH)6 distorted Ga-octahedra with shared vertices form the layers topologically related to those in so-called hexagonal bronzes. The octahedra occur at the vertices of a 63 kagome plane net, forming six-membered rings. At their junction there is a three-membered ring. Ordered AsO4 and SO4 tetrahedra are linked to these layers below and above in such a way that they are located on the same level respectively and are oriented along anti parallel directions (Jambor et al. 1996).
However, in most of arsenate-sulfates with As/S ordered structures the sulfate and arsenate tetrahedral groups perform a different function. In contrast with the acentric gallobeudantite (R3m), the disordered As/S distribution within the same tetrahedra was revealed in chemically related beudantite PbFe3 + 3(AsO4)(SO4)(OH)6 with centrosymmetric structure (R-3m). Both structures closely follow the the alunite-jarosite model (Szymanski 1988). Oberwolfachite SrFe3 + 3(AsO4)(SO4)(OH)6 with disordered As/S distribution is a new mineral of the beudantite group within the alunite supergroup which actually contain 10 and 52 mineral species, respectively (Chukanov et al. 2021).
The heteropolyhedral layer in the structure of sarmientite Fe3 + 2(AsO4)(SO4)(OH)·5H2O consists of the pairs of octahedral-tetrahedral (Fe,As)-chains in which AsO4 tetrahedra share all four O vertices with two nonequivalent Fe octahedra. The monodentate sulfate groups [SO4] play the role of the branches in these chains (Colombo et al. 2014). In nishanbaevite the heteropoyhedral (Al,As)-chains form the double layers. The SO4 tetrahedra sharing vertices with Al(2) octahedra are suspended to the external parts of the layers and obviously play a subordinate role in these complexes.
The similar function of sulfate groups clearly appeared in bukovskýite Fe2(AsO4)(SO4)(OH)·9H2O (Majzlan et al. 2012). The dominant feature of the structure of bukovskýite are the chains composed of Fe3+ octahedra and arsenate tetrahedra with the overall composition Fe2(AsO4)(H2O)6(OH). Sulfate tetrahedra are located in the space between these chains and are linked to them by a network of H-bonds.
The heteropolyhedral layers formed by alternating corner-linked Al–O octahedra and acid-arsenate tetrahedra are the characteristic feature of juansilvaite Na5Al3[AsO3(OH)]4[AsO2(OH)2]2(SO4)2·4H2O (Kampf et al. 2017). Isolated SO4 tetrahedra are located in the interlayer region between these layers.
The dominant structural feature of chalcophyllite Cu9Al(AsO4)2(SO4)15(OH)12·18H2O is the arrangement of Cu and Al polyhedra into complex sheets. The As-centred tetrahedra are attached above and below these sheets by three vertices. The sheets are connected to each other by the hydrogen bonding system, where the sulfate groups are located in between (Sabelli 1980). According to (Sarp et al. 2014), the structure model of barrotite Cu9Al(HSiO4)2[(SO4)(HAsO4)0.5](OH)12·8H2O contains the sheets topologically identical to chalcophyllite. They are also formed by Cu- and Al-centred polyhedra whereas AsO4 tetrahedra in chalcophyllite are replaced by acid SiO3(OH) tetrahedra in barrotite. It is suggested that sulfate and arsenate tetrahedra in barrotite must occupy the interlayer space.
Leogangite Cu10(AsO4)4(SO4)(OH)6 ·8H2O (Lengauer et al. 2004) is another example which exhibits the topological difference between AsO4 and SO4 tetrahedra within the same structure. It contains the thick heteropolyhedral layers which are formed by the groups of five CuO5 polyhedra (four distorted square pyramids + one distorted trigonal dipyramid) and AsO4 tetrahedra. The sulfate tetrahedra link these layers to a loose framework.
Thus a large group of arsenate-sulfates exhibits the participation of AsO4 tetrahedra in combination with cationic polyhedra in the heteropolyhedral complexes whereas SO4 tetrahedra play a subordinate role in their structures. The different function of both AsO4 and SO4 tetrahedral groups is related with the closer values of thermochemical electronegativities between metal cations and As5+ vs cations and S6+. In particular, in nishanbaevite the difference of these values (Δ) between Al and As is 0.63 (Al 2.52 and As 3.15) whereas Δ between Al and S is 0.92 (Al 2.52 and S 3.44) (Tantardini and Oganov 2021).