Nishanbaevite, KAl2O(AsO4)(SO4), a new As/S-ordered arsenate-sulfate mineral of fumarolic origin

The new mineral nishanbaevite, ideally KAl2O(AsO4)(SO4), was found in sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with euchlorine, alumoklyuchevskite, langbeinite, urusovite, lammerite, lammerite-β, ericlaxmanite, kozyrevskite, and hematite. Nishanbaevite occurs as long-prismatic or lamellar crystals up to 0.03 mm typically combined in brush-like aggregates and crusts up to 1.5 mm across. It is transparent, colourless, with vitreous lustre. Dcalc = 3.012 g cm− 3. Nishanbaevite is optically biaxial (–), α = 1.552, β ≈ γ = 1.567. The chemical composition (average of seven analyses) is: Na2O 3.79, K2O 8.01, CaO 0.10, CuO 0.21, Al2O3 30.08, Fe2O3 0.50, SiO2 1.62, P2O5 0.66, As2O5 32.23, SO3 22.59, total 99.79 wt%. The empirical formula calculated based on 9 O apfu is: (K0.57Na0.41Ca0.01)Σ0.99(Al1.99Fe3+0.02Cu0.01)Σ2.02(As0.95S0.95Si0.09P0.03)Σ2.02O9. Nishanbaevite is orthorhombic, Pbcm, a = 15.487(3), b = 7.2582(16), c = 6.6014(17) Å, V = 742.1(3) Å3 and Z = 4. The strongest reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 15.49(100)(100), 6.56(30)(110), 4.653(29)(111), 3.881(54)(400), 3.298(52)(002), 3.113(29)(121), and 3.038(51)(202, 411). The crystal structure, solved from single-crystal XRD data (R = 7.58%), is unique. It is based on the complex heteropolyhedral sheets formed by zig-zag chains of Al-centred polyhedra (alternating trigonal bipyramids AlO5 and octahedra AlO6 sharing edges) and isolated tetrahedra AsO4 and SO4. Adjacent chains of Al polyhedra are connected via AsO4 tetrahedra to form a heteropolyhedral double-layer. Its topological peculiarity is considered and compared with those in structurally related compounds. The (K,Na) site is located in the interlayer space between SO4 tetrahedra. The position of nishanbaevite among the arsenate-sulfates and their specific structural features are discussed. The mineral is named in honour of the Russian mineralogist Tursun Prnazorovich Nishanbaev (1955–2017).


Introduction
Natural sulfates and arsenates are numerous: altogether, about 800 valid mineral species belonging to these chemical classes are known. Sulfate and arsenate minerals are especially common and diverse in the oxidation zone of ore deposits where they occur in significant amounts and form close associations with each other. At the same time, arsenate-sulfates, the mixed oxysalts with both SO 4 and AsO 4 as species-defining anions are few: twenty-one mineral species. Among them, only beudantite PbFe 3 + 3 (AsO 4 )(SO 4 ) 2 (OH) 6 can be considered as a relatively common mineral. A brief review of these minerals, including data on As-S order/disorder in their structures was reported by Pekov et al. (2021). Nineteen arsenate-sulfate minerals contain OH groups or/and H 2 O molecules and have supergene or low-temperature hydrothermal origin whilst two species are H-free, namely vasilseverginite Cu 9 O 4 (AsO 4 ) 2 (SO 4 ) 2 and as described in this paper nishanbaevite KAl 2 O(AsO 4 )(SO 4 ). Both these minerals, as well as a single natural H-free phosphatesulfate, karlditmarite Cu 9 O 4 (PO 4 ) 2 (SO 4 ) 2 , are found only in high-temperature sublimates of the Arsenatnaya fumarole at the Tolbachik volcano, Kamchatka, Russia.
The new mineral nishanbaevite (Cyrillic: нишанбаевит) is named in honour of the Russian mineralogist Tursun Prnazorovich Nishanbaev , a Head of the Natural History Museum of the Ilmen Natural Reserve, Miass, Russia. Dr. Nishanbaev made a significant contribution to the mineralogy of anthropogene counterparts of volcanic fumaroles which originate on burning dumps of coal mines.
Both the new mineral and its name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification (IMA2019-012). The type specimen is deposited in the systematic collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow with the catalogue number 96592.

Occurrence
Nishanbaevite was detected in the single specimen collected by us in July 2015 from the Arsenatnaya fumarole located at the apical part of the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption (GTFE), Tolbachik volcano, Kamchatka Peninsula, Far-Eastern Region, Russia (55°41'N 160°14'E, 1200 m asl). This scoria cone is a monogenetic volcano about 300 m high and approximately 0.1 km 3 in volume. It was formed in 1975 during the first phase of the GTFE (Fedotov and Markhinin 1983). Fumarole fields at the Second scoria cone are still active; gas vents with temperatures up to 500 °C are numerous at the summit of the cone. The active, hot Arsenatnaya fumarole, first uncovered by us in July 2012, belongs to oxidizing-type fumaroles and is one of the world's most prolific mineralogical occurrences of the volcanic exhalation origin: for this decade, two hundred mineral species, mainly sulfates and arsenates, are found here, including 64 IMA-approved new minerals. The Arsenatnaya fumarole and its mineralogical features, including zonation in the distribution of mineral associations, were characterized by Pekov et al. (2014Pekov et al. ( , 2018 and Shchipalkina et al. (2020).
Nishanbaevite was found in a pocket about 1.3 m below the day surface, within the upper part the so-called polymineralic zone of the fumarole (zone IV: Shchipalkina et al. 2020). The temperatures measured by us using a chromelalumel thermocouple in this area during sampling were 380-400 °C. Walls of the pocket were covered by sublimate incrustations with dominating euchlorine. Other minerals associated with nishanbaevite are alumoklyuchevskite, langbeinite, urusovite, lammerite, lammerite-β, ericlaxmanite, kozyrevskite and hematite.
Nishanbaevite could be deposited directly from the gas phase as a volcanic sublimate or, more probably, it was formed as a result of the interaction between fumarolic gas and basalt scoria at the temperatures not lower than 400 °C. The latter could be a source of Al which has low volatility in such post-volcanic systems at temperatures up to 400-500 °C (Symonds and Reed 1993).

General appearance, physical properties and optical data
Nishanbaevite occurs as long-prismatic, typically lath-like crystals up to 0.01 × 0.01 × 0.05 mm 3 , or lamellar crystals up to 0.02 mm × 0.03 mm and less than 1 μm thick. Some crystals observed under the scanning electron microscope (SEM) demonstrate signs of X-shaped interpenetration twins ( Fig. 1), however, the twin law was not determined. The crystals are combined in clusters which form brush-like, openwork aggregates (Fig. 1) up to 0.5 mm across and crusts up to 1.5 mm across (Fig. 2). They overgrow incrustations of langbeinite, alumoklyuchevskite, or hematite or occur on the surface of basalt scoria altered by fumarolic gas.
The mineral is transparent, colourless in individuals and snow-white in aggregates (Fig. 2), with a white streak and a vitreous lustre. It is non-fluorescent under ultraviolet light or an electron beam. Nishanbaevite is brittle, cleavage or parting was not observed and the fracture is uneven (observed under the microscope). Density calculated using the empirical formula and unit-cell volume found from single-crystal X-ray diffraction data is 3.012 g cm − 3 .

Chemical composition
The chemical data for nishanbaevite were obtained using electron probe micro-analysis (EPMA). A Jeol JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer was used, with an acceleration voltage of 20 kV, a beam current of 20 nA, and a 3 μm beam diameter. The following natural and synthetic reference materials and the following analytical lines were used: jadeite (Na Kα, Al Kα, Si Kα), KTiOPO 4 (K Kα, P Kα), CaSiO 3 (Ca Kα), CuO (Cu Kα), FeS 2 (Fe Kα), GaAs (As Lα), and ZnS (S Kα). Peak and background counting times are 20 and (10 + 10) s, respectively. Contents of other elements with atomic numbers > 6 are below their detection limits.
Single-crystal XRD studies of nishanbaevite were carried out using an Xcalibur S diffractometer equipped with a CCD (charge-coupled device) detector. More than a hemisphere of three-dimensional data was collected. Data reduction was performed using CrysAlisPro Version 1.171.37.34 (Agilent Technologies 2014). The data were corrected for Lorentz factor and polarization effects. The crystal structure was solved by direct methods and refined using the SHELX software package (Sheldrick 2015) to R = 0.0758 for 547 unique reflections with I > 2σ(I). Crystal data, data collection information and structure refinement details are given in Table 2, coordinates and thermal displacement parameters of atoms in Table 3, selected interatomic distances in Table 4 and bond valence calculations in Table 5.
Unfortunately, even the best of tested crystals of nishanbaevite was not perfect and very small that caused rather low-quality diffraction data. However, the reasonable values of interatomic distances (Table 4) and bond valence sums (Table 5), as well as very good agreement between the measured and calculated powder XRD patterns (Table 1) show that the structure is determined correctly.

Discussion
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 AsO 4 and SO 4 tetrahedra. 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)O 5 trigonal bipyramids and Al(2)O 6 octahedra sharing edges to form the chains (Fig. 3a). Adjacent chains are connected via AsO 4 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. SO 4 tetrahedra are linked to the external parts of the double-layered core from both sides sharing two common vertices with Al(2)O 6 octahedra. The large-cation A site with disordered distribution of K and subordinate Na is located in the interlayer space between SO 4 tetrahedra. A 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 AlO 6 , AlO 5 , AsO 4 and SO 4 groups. The simplified double periodic layer is represented by a 6,6-coordinated net with AlO 6 , AlO 5 , AsO 4 groups as nodes and bridging SO 4 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 node is represented by the AlO 6 groups, while AlO 5 and AsO 4 groups are topologically equivalent, i.e. they are equally connected to other groups, and they represent the other type of node. The specific feature of the node formed by AlO 6 is that it is bounded to all types of building units, namely AlO 6 , AlO 5 , AsO 4 and SO 4 and formally its coordination is equal to 8 (Fig. 5a). However, within the simplified net SO 4 groups are considered as edges and thus in such approach AlO 6 node is surrounded by 6 neighboring nodes. In contrast with it the coordination of AlO 5 and AsO 4 nodes is formed only by AlO 6 , AlO 5 and AsO 4 groups located within the net. The difference between AlO 5 and AsO 4 is in the method of connecting to the six other   (Söhnel et al. 1996) and Zn 3 O(SO 4 ) 2 (Bald and Gruehn 1981) [the unit cell of the latter can be transformed to one close to that chosen for monoclinic Zn 3 O(MoO 4 ) 2 using the matrix − 1 0-1 / 0-1 0 / 0 0 1)] and glikinite (Zn,Cu) 3 O(SO 4 ) 2 from the same Arsenatnaya fumarole (Nazarchuk et al. 2020), a natural Cu-bearing analogue of synthetic Zn 3 O(SO 4 ) 2 (Bald and Gruehn 1981). All these sulfates and molybdates 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) polyhedra in zigzag chains. Besides glikinite, two minerals demonstrate some structural similarities with nishanbaevite, namely vergasovaite Cu 3 O(MoO 4 )(SO 4 ) (Berlepsch et al. 1999) and cupromolybdite Cu 3 O(MoO 4 ) 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. Notably, all these minerals originate from fumaroles of the Tolbachik volcano. 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 EPMA 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 AsO 4 tetrahedra with < As-O > 1.665 Å versus SO 4 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 Cu 9 O 4 (AsO 4 ) 2 (SO 4 ) 2 , another H-free sulfate-arsenate from the Arsenatnaya fumarole .
The As/S ordering and the edge-sharing Al-polyhedra in nishanbaevite is similar to that in arsentsumebite Pb 2 Cu(AsO 4 ) (SO 4 ) (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     The crystal chemical features of the AsO 4 -SO 4 -ordered members of the alunite supergroup can be considered on the example of gallobeudantite PbGa 3 (AsO 4 )(SO 4 )(OH) 6 . In its structure distorted GaO 6 octahedra with shared vertices form the layers topologically related to those in so-called hexagonal bronzes. The octahedra occur at the vertices of a 6 3 kagome plane net, forming six-membered rings. At their junction there is a three-membered ring. Ordered AsO 4 and SO 4 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 PbFe 3 + 3 (AsO 4 )(SO 4 )(OH) 6 with centrosymmetric structure (R-3 m). Both structures closely follow the the alunite-jarosite model (Szymański 1988;Giuseppetti and Tadini 1989). Oberwolfachite SrFe 3 + 3 (AsO 4 )(SO 4 )(OH) 6 with disordered As/S distribution is a new mineral of the beudantite group within the alunite supergroup which contain 10 and 53 mineral species, respectively (Chukanov et al. 2021). The heteropolyhedral layer in the structure of sarmientite Fe 3 + 2 (AsO 4 )(SO 4 )(OH)·5H 2 O consists of the pairs of octahedral-tetrahedral (Fe,As)O n -chains in which AsO 4 tetrahedra share all four O vertices with two nonequivalent Fe octahedra. The monodentate sulfate groups [SO 4 ] 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 SO 4 tetrahedra sharing vertices with Al(2)O 6 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 Fe 2 (AsO 4 )(SO 4 )(OH)·9H 2 O (Majzlan et al. 2012). The dominant feature of the structure of bukovskýite is the chains composed of Fe 3+ octahedra and arsenate tetrahedra with the overall composition Fe 2 (AsO 4 )(H 2 O) 6 (OH). Sulfate tetrahedra are located in the space between these chains and are linked to them by a network of H-bonds.
The dominant structural feature of chalcophyllite Cu 9 Al(AsO 4 ) 2 (SO 4 ) 15 (OH) 12 ·18H 2 O 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 Cu 9 Al(HSiO 4 ) 2 [(SO 4 ) (HAsO 4 ) 0.5 ](OH) 12 ·8H 2 O contains the sheets topologically identical to chalcophyllite. They are also formed by Cu-and Al-centred polyhedra whereas AsO 4 tetrahedra in chalcophyllite are replaced by acidic SiO 3 (OH) tetrahedra in Fig. 4 A half of a double-layer core formed by Al-centred polyhedra and AsO 4 tetrahedra (a) and the double-layer core of the complicate sheet (b) in the structure of nishanbaevite barrotite. It is suggested that sulfate and arsenate tetrahedra in barrotite must occupy the interlayer space.