The new mineral elasmochloite, Na3Cu6BiO4(SO4)5, 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 tenorite, hematite, langbeinite, aphthitalite, krasheninnikovite and johillerite. Elasmochloite occurs as lamellar crystals flattened on {001}, up to 0.005 × 0.07 × 0.1 mm in size, separate or combined into open-work clusters up to 0.3 mm across. It is transparent, green, with vitreous lustre. The calculated density (Dcalc) is 3.844 g cm−3. Elasmochloite is optically uniaxial or pseudo-unixial (−), α = 1.611(2), β = γ = 1.698(2), 2 V ≈ 0°. Pleochroism is strong, Z ≈ Y (grass-green) > X (turquoise-blue). The chemical composition obtained by electron-microprobe analysis is (in wt%): Na2O 6.67, K2O 0.82, CuO 38.77, ZnO 0.25, PbO 3.17, Bi2O3 17.66, SO3 32.81, total 100.15. The empirical formula based on 24 O atoms per formula unit (apfu) is Na2.63K0.21Cu5.96Zn0.04Pb0.17Bi0.93S5.01O24. Elasmochloite is monoclinic, P21/n, a 10.1273(9), b 10.1193(8), c 21.1120(16) Å, β 102.272(8)°, V 2114.1(3) Å3 and Z = 4. The strongest reflections of the powder X-ray diffraction (XRD) pattern [d, Å(I)(hkl)] are: 10.33(100)(002), 7.04(18)(110, 1¯11), 6.33(14)(111, 1¯12), 3.576(24)(2¯21), 2.920(14)(2¯25), 2.529(14)(4¯02, 040) and 2.460(14)(2¯27). The crystal-structure model was obtained from single-crystal XRD data, R1 = 20.6%. It contains two types of alternating polyhedral layers: (1) “copper-bismuth slabs” composed by [BiO4O2] polyhedra, [CuO5] square pyramids and [CuO4] squares and (2) “sodium slabs” consisting of [NaO5] and [NaO6] polyhedra. Corner-sharing [SO4] tetrahedra integrate cationic polyhedra into the whole structure. In an anion-centred approach, the structure can be expressed as a stacking of perforated layers composed of [Cu8BiO4] “half-cube” clusters interleaved with [SO4] tetrahedra and Na cations. Elasmochloite belongs to a novel structure type but has some common structural features with nabokoite KCu7Te4+O4(SO4)5Cl and favreauite PbCu6BiO4(Se4+O3)4(OH)·H2O. The mineral name is based on the Greek words έλασμα, lamella, and χλόη, the green shoot, in allusion to the green colour and lamellar crystal habit.

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