The new mineral kononovite, NaMg(SO4)F, the first sulphate member of the durangite (tilasite) group, 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 closely associated with langbeinite, hematite, anglesite and euchlorine. Uklonskovite is a product of the supergene alteration of kononovite. The new mineral occurs as prismatic to thick tabular crystals up to 0.04 × 0.06 × 0.1 mm, isolated or, more typically, forming clusters or interrupted crusts up to several cm2 in area and up to 0.05 mm thick overgrowing basalt scoria. Kononovite is white, transparent in tiny grains and translucent in blocky crystals, with vitreous lustre. It is brittle but with signs of weak plasticity; one direction of imperfect cleavage is observed. The Mohs’ hardness is ca. 3. Dmeas = 2.91(1), Dcalc = 2.945 g cm–3. Kononovite is optically biaxial (+), α = 1.488(2), β = 1.491(2), γ = 1.496(2), 2Vmeas = 75(5)°. The IR spectrum is reported. The chemical composition (wt%, electron-microprobe data) is: Na2O 18.68, K2O 0.14, MgO 24.77, ZnO 0.28, PbO 0.10, SO3 48.44, F 11.82, Cl 0.12, O = (F,Cl) −5.00, total 99.35. The empirical formula calculated on the basis of 5 (O + F + Cl) anions pfu is: Na0.99K0.01Mg1.01Zn0.01S0.99O3.97F1.02Cl0.01. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 4.766(38)(−111), 3.567(33)(021), 3.233(82)(−112), 3.210(55)(002), 3.041(100)(200), 2.589(53)(130), 2.571(38)(022) and 2.269(33)(131). Kononovite is monoclinic, space group C2/c [by analogy with synthetic NaMg(SO4)F which is practically identical to the mineral in its powder XRD pattern], a = 6.662(2), b = 8.584(3), c = 7.035(2) Å, β = 114.06(3)°, V = 367.4(1) Å3 and Z = 4. The mineral is named in honour of the Russian mineralogist Oleg V. Kononov (born 1932), Moscow State University. Two types of fluorine mineralization in deposits of the Tolbachik fumaroles are discussed.

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