Dravertite, CuMg(SO4)2, a new mineral species from the Tolbachik volcano, Kamchatka, Russia

The new mineral dravertite, ideally CuMg(SO₄)₂, was found in sublimates of two active fumaroles at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. In the Arsenatnaya fumarole, dravertite (holotype) is closely associated with dolerophanite, euchlorine, tenorite, hematite, langbeinite, steklite, fedotovite, wulffite, anhydrite and anglesite. In the Yadovitaya fumarole, it is associated with euchlorine, chalcocyanite, steklite, alumoklyuchevskite, piypite, parawulffite, cryptochalcite, dolerophanite, hematite, tenorite, vergasovaite, cupromolybdite, yaroshevskite and ziesite. Dravertite occurs as crude equant crystals up to 0.08 mm across forming compact clusters or spherulitic crusts up to 5 × 5 cm² in area and up to 1-cm thick on basalt scoria. Dravertite is transparent in small grains and translucent in aggregates, with a vitreous lustre. It is light blue to colourless (Arsenatnaya) or light brown (Yadovitaya). The mineral is brittle, with Mohs’ hardness ca. 3½. Cleavage is imperfect, the fracture is uneven. D_calc = 3.508 g cm⁻³ (based on the empirical formula). Dravertite is optically biaxial (−), α = 1.624(3), β = 1.661(3), γ = 1.663(3) and 2V_meas = 35(10)°. The infrared spectrum is reported. The chemical composition (holotype, average of 7 analyses) is: MgO 11.00, MnO 0.16, CuO 31.16, ZnO 2.62, SO₃ 54.76, total 99.72 wt.%. The empirical formula based on 8 O apfu is: Mg_0.79Mn_0.01Cu_1.14Zn_0.09S_1.99O₈. Dravertite is monoclinic, P2₁/n, a = 4.8141(3), b = 8.4443(5), c = 6.7731(4) Å, β = 94.598(5)°, V = 274.45(3) ų and Z = 2. The strongest reflections of the powder X-ray diffraction (XRD) pattern [d, Å(I)(h k l)] are: 4.175(68)(1 1 0), 3.666(64)(−1 1 1), 3.579(63)(0 2 1), 3.443(59)(1 1 1), 2.719(41) (−1 1 2), 2.637(100)(0 2 2), 2.430(68)(1 3 0) and 1.791(24)(0 4 2). The crystal structure, solved from single-crystal XRD data (R = 0.0591), contains chains of alternating edge-sharing octahedra centred by Cu²⁺ and Mg. The CuO₆ octahedra show significant Jahn–Teller distortion while MgO₆ octahedra are much more regular. The octahedral chains are connected via isolated SO₄ tetrahedra to form a pseudo-framework. In terms of structure, dravertite can be considered as a monoclinically distorted, cation-ordered derivative of orthorhombic (Pnma) chalcocyanite CuSO₄. The mineral is named in honour of the Russian mineralogist and geologist Petr Lyudovikovich Dravert (1879–1945).