Abstract

Bubnovaite, ideally K2Na8Ca(SO4)6, has been found in volcanic fumaroles of the 2012–2013 Tolbachik fissure eruption, Kamchatka peninsula, on the new cinder Naboko cone. Bubnovaite occurs as aggregates of light-blue needle-like or tabular crystals associated with aphthitalite and thénardite. The calculated density is 2.655 g/cm3. The mineral is uniaxial, optically negative, ω = 1.492(2), ε = 1.489(2). No pleochroism has been observed. Bubnovaite is trigonal, space group P31c, a = 10.804(3), c = 22.011(6) Å, V = 2225(2) Å3, Z = 4 (from single-crystal diffraction data). The eight strongest lines of the X-ray powder diffraction pattern are (I–d(Å)-hkl): 80–3.943-023, 35–2.894-026, 62–2.868-033, 91–2.718-034, 100–2.707-220, 10–2.647-018, 6–2.231-135, 21–1.970-046. The chemical composition determined by electron-microprobe analysis is (wt.%): Na2O 26.99, K2O 10.99, CaO 4.27, MgO 0.51, CuO 1.21, ZnO 0.81, PbO 1.58, SO3 54.93, total 101.30. The empirical formula based on 24 O atoms is Na7.65K2.05 (Ca0.67Mg0.11Cu0.13Zn0.09Pb0.06)∑1.06S6.03O24. The simplified formula is K2Na8Ca(SO4)6, which requires Na2O 28.22, K2O 10.72, CaO 6.38, SO3 54.68, total 100 wt.%. The crystal structure of bubnovaite (R1 = 0.078) can be described as based upon an ABACABACABAC twelve-layer superstructure of K, Na and Ca cations with partially disordered SO4 tetrahedra. Bubnovaite is closely related to α-K2SO4; ‘metathénardite’, Na2SO4; aphthitalite, K3Na(SO4)2; and hanksite, Na22K(SO4)9(CO3)2Cl. The mineral is named in honour of Professor Rimma Sergeevna Bubnova, Institute of Silicate Chemistry, Russian Academy of Sciences, Saint-Petersburg State University, St. Petersburg, Russia, for her important contributions to the crystal chemistry of vanadates, silicates, borates, borosilicates and other inorganic oxysalts.

You do not currently have access to this article.