New arsenate minerals from the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. XVII. Paraberzeliite, NaCaCaMg2(AsO4)3, an alluaudite-group member dimorphous with berzeliite

The new alluaudite-group mineral paraberzeliite was found in the Arsenatnaya fumarole, Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. In the deepest zone of Arsenatnaya, paraberzeliite (holotype) is associated with anhydrite, diopside, hematite, svabite, berzeliite, schaferite, calciojohillerite, magnesioferrite, ludwigite, fluorapatite, powellite, baryte, and rhabdoborite-group and aphthitalite-group members. In the middle zone of the fumarole, paraberzeliite occurs with hematite, calciojohillerite, badalovite, johillerite, nickenichite, tilasite, svabite, fluorophlogopite, sanidine, cassiterite, anhydrite, metathenardite and belomarinaite. Paraberzeliite forms prismatic crystals up to 0.2 mm x 0.2 mm x 1 mm often occurring in open-work aggregates. It is transparent, brown (from light to dark brown, sometimes with purple or red hue) or green (from pale greenish to yellow-green). The mineral is brittle, cleavage was not observed. The Mohs hardness is 31/2. D-calc is 3.811 g cm(-3). Paraberzeliite is optically biaxial (+), alpha = 1.718(4), beta = 1.728(4), gamma = 1.742(4) and 2V(meas.) = 85(5)degrees. The chemical composition (wt.%, electron-microprobe; holotype) is: Na2O 6.43, CaO 16.65, MgO 11.64, MnO 1.65, CuO 0.06, Fe2O3 2.45, V2O5 1.10, As2O5 59.46, total 99.44. The calculated empirical formula based on 12 O atoms per formula unit is (Na1.20Ca1.71Mg1.66Mn0.13Fe0.183+)(sigma 4.88)(As2.98V0.07)(sigma 3.05)O-12. Paraberzeliite is monoclinic, C2/c, a = 12.3143(7), b = 13.0679(5), c = 6.7717(4) angstrom, beta = 113.657(7)degrees, V = 998.14(10) angstrom(3) and Z = 4. The crystal structure was solved from single-crystal X-ray diffraction data, R = 0.0349. Paraberzeliite is isostructural with other alluaudite-group minerals. Its simplified crystal chemical formula is (CaNaCaMg2)-Ca-A-Ca-(1)-Na-A-Na-(2)'-Ca-M-Ca-(1)-Mg-M-Mg-(2)((AsO4)-As-T)(3). The idealised formula is NaCa2Mg2(AsO4)(3), or, according to the actual nomenclature of alluaudite-group arsenates, NaCaCaMg2(AsO4)(3). The name paraberzeliite reflects the dimorphism of this alluaudite-group mineral with the arsenate garnet berzeliite, ideally (Ca2Na)Mg-2(AsO4)(3).