Vasilseverginite, Cu9O4(AsO4)2(SO4)2, a new fumarolic mineral with a hybrid structure containing novel anion-centered tetrahedral structural units.

The new mineral vasilseverginite, ideally Cu₉O₄(AsO₄)₂(SO₄)₂, was found in 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 tenorite, lammerite, stranskiite, lammerite-β, langbeinite, dolerophanite, sanidine, hematite, and gahnite. Vasilseverginite occurs as prismatic crystals up to 0.02 × 0.02 × 0.06 mm³ combined in groups or interrupted crusts up to 1 × 2 cm² in area and up to 0.1 mm thick. It is transparent, bright green, with vitreous luster. D_calc is 4.41 g⋅cm^–3. Vasilseverginite is optically biaxial (–), α 1.816(5), β 1.870(5), γ 1.897(5), estimated 2V is 30(15)°. Chemical composition (wt%, electron-microprobe) is: CuO 64.03, ZnO 0.79, Fe₂O₃ 0.25, P₂O₅ 0.05, As₂O₅ 20.83, SO₃ 14.92, total 100.87. The empirical formula calculated on O = 20 apfu is C u 8.78 Z n 0.11 F e 0.03 3 + Σ 8.92 A s 1.98 P 0.01 S 2.03 O 20 $\left(\mathrm{Cu}_{8.78} \mathrm{Zn}_{0.11} \mathrm{Fe}_{0.03}^{3+}\right)_{\Sigma 8.92} \mathrm{As}_{1.98} \mathrm{P}_{0.01} \mathrm{~S}_{2.03} \mathrm{O}_{20}$. Vasilseverginite is monoclinic, P21/n, a = 8.1131(4), b = 9.9182(4), c = 11.0225(5) Å, β = 110.855(2)°, V = 828.84(6) ų, and Z = 2. The strongest reflections in the powder XRD pattern [d,Å(I)(hkl)] are: 7.13(41)(101̅), 5.99(70)(110, 111̅), 5.260(100)(101), 4.642(46)(111), 3.140(31)(031̅), 2.821(35)(023̅), 2.784(38)(132̅, 032̅), 2.597(35)(204̅), and 2.556(50) (231̅, 212). The crystal structure, solved using single-crystal X‑ray diffraction data, R₁ = 0.025, is based upon complex [O₄Cu₉]¹⁰⁺ layers parallel to (101̅) that are composed of edge- and corner-sharing (OCu₄) tetrahedra. The topology is unprecedented in inorganic structural chemistry. The crystal structure can be considered a hybrid of the structures of popovite Cu₅O₂(AsO₄)₂ and dolerophanite Cu₂O(SO₄) according to the scheme Cu₉O₄(AsO₄)₂(SO₄)₂ = Cu₅O₂(AsO₄)₂ + 2Cu₂O(SO₄). The chemical hybridization does not result in a significant increase in chemical complexity of vasilseverginite compared to the sum of those of popovite and dolerophanite, whereas the structural hybridization leads to the doubling of structural information per unit cell. The mineral is named in memory of the outstanding Russian mineralogist, geologist, and chemist Vasiliy Mikhailovich Severgin (1765–1826). [ABSTRACT FROM AUTHOR]