Bubnovaite, K2Na8Ca(SO4)6, a new mineral species with modular structure from the Tolbachik volcano, Kamchatka peninsula, Russia

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) ų, 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 Na_7.65K_2.05 (Ca_0.67Mg_0.11Cu_0.13Zn_0.09Pb_0.06)_∑1.06S_6.03O₂₄. 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.