Shuvalovite, K2(Ca2Na)(SO4)3F, a new mineral from the Tolbachik volcano, Kamchatka, Russia

Abstract

The new mineral shuvalovite, ideally K2(Ca2Na)(SO4)3F, 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 closely associated with calciolangbeinite, tenorite, hematite, orthoclase, fluorophlogopite and fluorite. Shuvalovite occurs as coarse lamellar to tabular (flattened on [100]), rectangular, octagonal or irregular crystals up to 0.05 × 0.7 × 0.9 mm combined in open-work groups or crusts up to 1 × 1 cm on basalt scoria. Shuvalovite is transparent, colourless, and has a vitreous lustre. The mineral is brittle, with Mohs’ hardness ca. 3. Cleavage was not observed, the fracture is uneven. D calc = 2.64 g cm−3. Shuvalovite is optically biaxial (–), α = 1.493(1), β = 1.498(1), γ = 1.498(1) and 2 V meas ≤ 20°. The IR spectrum is reported. The chemical composition (average of 22 analyses) is: Na2O 7.37, K2O 19.33, CaO 21.39, SO3 49.49, F 3.78, O =F −1.59, total 99.77 wt%. The empirical formula calculated on the basis of 13 (O + F) apfu is: Na1.16K2.01Ca1.86S3.02O12.03F0.97. Shuvalovite is orthorhombic, Pnma , a = 13.2383(4), b = 10.3023(3), c = 8.9909(4) A, V = 1226.22(7) A3 and Z = 4. The strongest reflections of the powder X-ray diffraction pattern [ d ,A( I )( hkl )] are: 4.245(45)(102, 121), 3.963(62)(301), 3.281(100)(122), 3.210(30)(031), 3.144(84)(302, 321), 3.112(67)(131, 401), 3.016(78)(222) and 2.785(52)(420). The crystal structure, solved from single-crystal X-ray diffraction data ( R = 0.067), contains two crystallographically inequivalent isolated SO4 tetrahedra. For the S(2) site, a disordered arrangement of coordinating O atoms is found that defines two possible orientations of the S(2)O4 tetrahedron. The Ca(1) cations occupy CaO7F polyhedra whereas Ca(2) cations occupy CaO5F or CaO6F polyhedra, depending on the presence or absence of the half-occupied O(6) site split around the mirror plane. The K(1) and K(2) cations are nine-fold coordinated. All Ca and K sites contain admixed Na, the majority of which is located in the Ca(2) site. The comparative crystal chemistry of structurally different sulfates with the general formula M 5(SO4)3 X (shuvalovite, krasheninnikovite and apatite-type compounds) is discussed. Shuvalovite is named in honour of the Russian nobleman and statesman Ivan Ivanovich Shuvalov (1727–1797), an enthusiastic patron of the sciences, arts and literature, one of the founders of the Moscow University in 1755.