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. D_calc = 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: (K_0.57Na_0.41Ca_0.01)_Σ0.99(Al_1.99Fe³⁺_0.02Cu_0.01)_Σ2.02(As_0.95S_0.95Si_0.09P_0.03)_Σ2.02O₉. Nishanbaevite is orthorhombic, Pbcm, a = 15.487(3), b = 7.2582(16), c = 6.6014(17) Å, V = 742.1(3) ų 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).