Itelmenite, ideally Na2CuMg2(SO4)4, was found in a fumarole of the Naboko scoria cone of the Tolbachik volcano Fissure Eruption (2012–2013), Kamchatka Peninsula, Russia. Itelmenite occurs as irregularly shaped grains as well as microcrystalline masses associated with anhydrite, saranchinaite, hermannjahnite, euchlorine, thénardite, aphthitalite and hematite. Itelmenite is orthorhombic, Pbca, a = 9.568(2) Å, b = 8.790(2) Å, c = 28.715(8) Å, V = 2415.0(11) Å3 and Z = 4 (from single-crystal diffraction data). The nine strongest lines of the powder X-ray diffraction pattern are [d(I)(hkl)]: 7.9614(41)(102), 7.1803(32)(004), 5.9122(64)(112), 3.8455(87)(122), 3.6292(52)(214), 3.3931(62)(215), 3.0003(44)(027), 2.9388(100)(312) and 2.4975(56)(230). The chemical composition determined by the electron-microprobe analysis is (wt.%): Na2O 10.77, K2O 0.20, MgO 11.10, CuO 15.38, ZnO 5.61, SO3 56.42, total 99.48. The empirical formula based on O = 32 apfu is (Na3.93K0.05)Σ3.98Mg3.12(Cu2.19Zn0.78)Σ2.97S7.97O32. The simplified formula is Na2CuMg2(SO4)4 taking into account structural data. The crystal structure was solved by direct methods and refined to an agreement index R1 = 0.034 on the basis of 1855 independent observed reflections. The structure of itelmenite is based on a unique type of [A2+3(SO4)4]2– (A = Mg, Cu and Zn) heteropolyhedral framework with voids filled by Na+ cations.

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