A new mineral borisenkoite, Cu3[(V,As)O4]2, and the isomorphous series borisenkoite–lammerite-β in fumarolic exhalations of the Tolbachik volcano, Kamchatka, Russia

Abstract

The new mineral borisenkoite Cu3[(V,As)O4]2 was found in sublimates of the Yadovitaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Associated minerals are sanidine, hematite, lammerite, lammerite-β, bradaczekite, zincobradaczekite, mcbirneyite, pseudolyonsite, lyonsite, starovaite, tenorite, rutile, tripuhyite, pseudobrookite, piypite, langbeinite, calciolangbeinite, aphthitalite, alumoklyuchevskite, palmierite, cupromolybdite, and corundum. Borisenkoite forms (1) prismatic crystals (up to 0.04 × 0.04 × 0.10 mm3) usually combined in clusters up to 0.4 mm and (2) rims up to 0.05 mm in width around lammerite. Borisenkoite is red-brown, golden-brown or brown, with strong greasy to adamantine lustre. Dcalc is 4.69 g·cm−3. Chemical composition (wt.%, electron-microprobe) is: CuO 53.25, ZnO 1.13, Fe2O3 0.16, P2O5 0.05, V2O5 25.06, As2O5 20.44, total 100.07. The empirical formula, based on 8 O apfu, is: (Cu2.94Zn0.06Fe0.01)Σ3.01(V1.21As0.78)Σ1.99O8. Borisenkoite is monoclinic, P21/c, a 6.3779(7), b 8.6021(9), c 11.3597(11) A, β 92.013(8)o, V 622.84(11) A3 and Z = 4. The strongest reflections in the powder XRD pattern [d,A(I)(hkl)] are: 4.309(48)(− 102, 020), 3.424(40)(022, − 121), 2.994(48)(113, 210), 2.917(50)(− 211), 2.830(100)(004), 2.782(92)(031) and 2.568(38)(123). The crystal structure was solved from single-crystal XRD data and refined to R = 0.0654. Borisenkoite is isotypic with lammerite-β. They form a limited solid-solution series extended from Cu3(AsO4)2 to Cu3(V1.5As0.5)O8, with gap between Cu3(As1.75V0.25)O8 and Cu3(As1.25V0.75)O8. The assumption that As5+ can stabilize borisenkoite structure is the reason to propose the simplified formula Cu3[(V,As)O4]2 avoiding the formal end-member formula Cu3(VO4)2. Borisenkoite is named in honour of the Russian geochemist, mineralogist, and geologist Leonid Fedorovich Borisenko (1922–2000).