New arsenate minerals from the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. XIII. Pansnerite, K 3 Na 3 Fe 3+ 6 (AsO 4 ) 8

The new mineral pansnerite, ideally K 3 Na 3 Fe 3+ 6 (AsO 4 ) 8 , 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 aphthitalite, hematite, sanidine, badalovite, khrenovite, achyrophanite, arsenatrotitanite, ozerovaite, tilasite, calciojohillerite, johillerite, nickenichite, svabite, katiarsite, yurmarinite, anhydrite, rutile, cassiterite and pseudobrookite. Pansnerite forms tabular to lamellar (flattened on {010}), usually pseudo-hexagonal crystals up to 0.2 mm × 0.7 mm × 1 mm and crystal clusters up to 2 mm across. It is transparent to translucent, light green, pale greenish, yellowish–greenish or yellowish, with vitreous lustre. The mineral is brittle, with perfect {010} cleavage. The Mohs’ hardness is ca 3. D calc is 3.596 g cm –3 . Pansnerite is optically biaxial (–), α = 1.702(4), β = 1.713(4), γ = 1.717(4), 2V meas = 45(10)° and 2V calc = 62°. Chemical composition (holotype, wt.%, electron microprobe data) is: Na 2 O 6.39, K 2 O 8.52, CaO 0.08, MgO 0.08, MnO 0.02, NiO 0.02, CuO 1.35, ZnO 0.34, Al 2 O 3 7.35, Cr 2 O 3 0.04, Fe 2 O 3 16.72, SiO 2 0.16, P 2 O 5 0.22, V 2 O 5 0.09, As 2 O 5 57.76, SO 3 0.04, total 99.20. The empirical formula based on 32 O apfu is K 2.86 Na 3.26 Ca 0.02 (Fe 3+ 3.31 Al 2.28 Cu 0.27 Zn 0.07 Mg 0.03 Cr 0.01 ) Σ5.97 (As 7.95 P 0.05 Si 0.04 V 0.02 S 0.01 ) Σ8.06 O 32 . Pansnerite is orthorhombic, Cmce , a = 10.7372(3), b = 20.8367(8), c = 6.47335(15) Å, V = 1448.27(7) Å 3 and Z = 2. The strongest reflections of the X-ray powder diffraction pattern [ d ,Å( I )( hkl )] are: 10.49(100)(020), 5.380(88)(111), 4.793(65)(220), 3.105(46)(311, 002), 3.079(32)(112, 061), 2.932(35)(260), 2.783(65)(202) and 2.694(52)(400, 222). The crystal structure was solved from single-crystal X-ray diffraction data, R 1 = 2.82%. The structure is based on heteropolyhedral layers formed by M O 6 octahedra ( M = Fe 3+ and Al) sharing common vertices and connected by AsO 4 tetrahedra. Na + and K + cations are located in the interlayer space. The mineral is named in honour of the German–Russian mineralogist and geographer Lavrentiy Ivanovich Pansner (1777–1851). Pansnerite forms a solid-solution series with the isotypic mineral ozerovaite, ideally KNa 2 Al 3 (AsO 4 ) 4 .