New arsenate minerals from the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. XIX. Axelite, Na14Cu7(AsO4)8F2Cl2

Abstract

AbstractThe new mineral axelite, ideally Na14Cu7(AsO4)8F2Cl2, 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 sylvite, halite, arsmirandite, bradaczekite, johillerite, tilasite, ericlaxmanite, lammerite, hematite, tenorite, cassiterite, pseudobrookite, aphthitalite-group sulfates, anhydrite, fluoborite, sanidine and fluorophlogopite. Axelite occurs as tabular, quadratic, rectangular or stronger distorted crystals up to 0.02 × 0.1 × 0.1 mm, sometimes combined in interrupted crusts up to 0.4 mm across overgrowing sylvite. It is transparent, sky-blue, with vitreous lustre. Cleavage was not observed. Dcalc is 3.662 g cm–3. Axelite is optically uniaxial (–), ɛ = 1.650(4) and ω = 1.678(4). Chemical composition (wt.%, electron microprobe data) is: Na2O 22.54, K2O 0.08, CaO 0.04, MgO 0.05, CuO 26.69, P2O5 1.75, V2O5 0.15, As2O5 44.14, SO3 0.04, F 1.57, Cl 3.60, –O=(F,Cl) –1.47, total 99.18. The empirical formula based on O+F+Cl=36 apfu is Na14.37K0.03Ca0.01Mg0.02Cu6.63P0.49V0.03As7.59S0.01O32.36F1.63Cl2.01. Axelite is tetragonal, P4bm, a = 14.5957(2), c = 8.34370(18) Å, V = 1777.51(6) Å3 and Z = 2. The strongest reflections of the powder X-ray diffraction (XRD) pattern [d,Å(I)(hkl)] are: 8.32(44)(001), 5.156(47)(220), 4.168(21)(002), 3.246(34)(222), 3.180(61)(331), 2.747(100)(402), 2.709(36)(511) and 2.580(29)(440). The crystal structure, solved from single-crystal XRD data (R = 4.50%), is unique. It is based on the heteropolyhedral chains built by clusters formed by CuO4Cl square pyramids connected with AsO4 tetrahedra. Adjacent chains are connected via common vertices of AsO4 tetrahedra with CuO4Cl pyramids to form a heteropolyhedral pseudo-framework. Axelite is remotely related, in both structural and chemical aspects, to lavendulan-like minerals and synthetic compounds. The mineral is named in honour of the outstanding Finnish–Russian crystallographer, mineralogist and material scientist Axel Gadolin (1828–1892).