The new mineral chubarovite, KZn2(BO3)Cl2, was found in the sublimates of active fumaroles at the Second and First scoria cones of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. At the Second scoria cone it occurs in the Arsenatnaya fumarole (the holotype) with fluoborite, krasheninnikovite, sylvite, halite, langbeinite, aphthitalite, orthoclase, fluorophlogopite, hematite, and tenorite. At the First scoria cone, chubarovite is closely associated with sellaite, fluorite, anhydrite, halite, cotunnite, challacolloite, sofiite, and flinteite. Chubarovite forms hexagonal or trigonal lamellar to tabular crystals up to 1.5 mm across and up to 0.5 mm thick, with aggregates and crystal crusts up to 1 cm across. The major crystal form is {001}, and lateral faces are {101}, {102}, {103}, {100}, and {110}; twins of two types are observed. Chubarovite is transparent, colorless, with vitreous luster. It is flexible but not elastic. The Mohs hardness is ca. 2. Cleavage is (001) perfect, mica-like. D(meas.) is 2.68(2), D(calc.) is 2.716 g cm–3. Chubarovite is optically uniaxial (–), with ω 1.541(2), ε 1.539(2). The infrared spectrum is given. Chemical data (wt.%, determined by electron-microprobe, boron by ICP OES) are: K2O 16.48, Rb2O 0.46, ZnO 53.96, B2O3 10.98, Cl 24.48, –O=Cl2 –5.53, total 100.83. The empirical formula, based on 5 (O+Cl) apfu, is: (K1.05Rb0.01)Σ1.06Zn2.00B0.95O2.92Cl2.08. Chubarovite is trigonal, R32, a 4.9429(4), c 26.348(2) Å, V 557.50(8) Å3, and Z = 3. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 8.79(100)(003), 4.394(43)(006), 4.225(25)(101), 4.074(91)(012), 3.590(90)(104), 3.324(30)(015), 2.470(67)(110), and 2.245(25)(1.0.10). Chubarovite has a novel structure type. Its crystal structure, solved from single-crystal X-ray diffraction data (R = 0.020), is composed of layers of two types with an alternation along [001]. The anionic {Zn2(BO3)Cl2} layer consists of flat triangular BO3 groups sharing all O vertices with bases of ZnO3Cl tetrahedra. Each Cl atom is shared between one Zn-centered tetrahedron and three edge-connected KCl6 octahedra belonging to the cationic layer formed by K+ cations. The mineral is named in honor of the Russian mineralogist and physicist Valeriy M. Chubarov (born 1948).

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