Abstract

Two new minerals shchurovskyite, ideally K2CaCu6O2(AsO4)4, and dmisokolovite, ideally K3Cu5AlO2(AsO4)4, are 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 Peninsula, Russia. They are associated with one another and with johillerite, bradaczekite, tilasite, melanarsite, tenorite, hematite, aphthitalite, langbeinite, orthoclase, etc. Shchurovskyite occurs as coarse tabular or prismatic crystals up to 0.15 mm in size or anhedral grains forming parallel aggregates and crusts up to 1.5 cm × 2 cm across. Dmisokolovite forms tabular, prismatic or dipyramidal crystals up to 0.2 mm in size, commonly combined in clusters or crusts up to 0.7 cm × 1.5 cm across. Both minerals are transparent with a vitreous lustre. They are brittle, with Mohs' hardness ≈ 3. Shchurovskyite is olive-green or olive drab. Dmisokolovite is bright emerald-green to light green. Dcalc = 4.28 (shchurovskyite) and 4.26 (dmisokolovite) g cm−3. Both are optically biaxial; shchurovskyite: (+), α = 1.795(5), β = 1.800(5), γ = 1.810(6), 2Vmeas = 70(15)°; dmisokolovite: (−), α = 1.758(7), β = 1.782(7), γ = 1.805(8), 2Vmeas = 85(5)°. The Raman spectra are given. Chemical data (wt.%, electron-microprobe; first value is for shchurovskyite, second for dmisokolovite): Na2O 0.00, 0.83; K2O 8.85, 10.71; Rb2O 0.11, 0.00; MgO 0.00, 0.35; CaO 4.94, 0.21; CuO 43.19, 38.67; ZnO 0.42, 0.20; Al2O3 0.04, 4.68; Fe2O3 0.00, 0.36; P2O5 0.59, 0.78; V2O5 0.01, 0.04; As2O5 40.72, 43.01; SO3 0.35, 0.00; total 99.22, 99.84. The empirical formulae, based on 18 O a.p.f.u., are shchurovskyite: K2.05Rb0.01Ca0.96Cu5.92Zn0.06Al0.01P0.09S0.05As3.86O18; dmisokolovite: Na0.28K2.36Mg0.09Ca0.04Cu5.04Zn0.04 Al0.95Fe0.053+P0.11As3.88O18. The strongest reflections of X-ray powder patterns [d, Å(I)(hkl)] are shchurovskyite: 8.61(100)(200, 001), 5.400(32)(110), 2.974(32)(3İ12, 510), 2.842(47)(003, 020), 2.757(63)(6İ01, 511), 2.373(36)(512, 420) and 2.297(31)(4İ21, 2İ22, 313); dmisokolovite: 8.34(95)(002), 5.433(84)(110), 2.921(66)(510, 3İ14), 2.853(58)(511, 020) and 2.733(100)(006, 512, 6İ02). Shchurovskyite is monoclinic, C2, a = 17.2856(9), b = 5.6705(4), c = 8.5734(6) Å, β = 92.953(6)°, V = 839.24(9) Å3 and Z = 2. Dmisokolovite is monoclinic, C2/c, a = 17.0848(12), b = 5.7188(4), c = 16.5332(12) Å, β = 91.716(6)°, V = 1614.7(2) Å3 and Z = 4. Their crystal structures [single-crystal X-ray diffraction data, R = 0.0746 (shchurovskyite) and 0.1345 (dmisokolovite: model)] are closely related in the topology of the main building units. They are based on a quasi-framework consisting of AsO4 tetrahedra and polyhedra centred by Cu in shchurovskyite or by Cu and Al in dmisokolovite. K and Ca are located in channels of the quasi-framework. The minerals are named in honour of outstanding Russian geologists and mineralogists Grigory Efimovich Shchurovsky (1803–1884) and Dmitry Ivanovich Sokolov (1788–1852).

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