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

The new mineral borisenkoite Cu 3 [(V,As)O 4 ] 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 mm 3 ) 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. D calc is 4.69 g·cm −3 . Chemical composition (wt.%, electron-microprobe) is: CuO 53.25, ZnO 1.13, Fe 2 O 3 0.16, P 2 O 5 0.05, V 2 O 5 25.06, As 2 O 5 20.44, total 100.07. The empirical formula, based on 8 O apfu , is: (Cu 2.94 Zn 0.06 Fe 0.01 ) Σ3.01 (V 1.21 As 0.78 ) Σ1.99 O 8 . Borisenkoite is monoclinic, P 2 1 / c , a 6.3779(7), b 8.6021(9), c 11.3597(11) Å, β 92.013(8)º, V 622.84(11) Å 3 and Z = 4. The strongest reflections in the powder XRD pattern [ d ,Å( 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 Cu 3 (AsO 4 ) 2 to Cu 3 (V 1.5 As 0.5 )O 8 , with gap between Cu 3 (As 1.75 V 0.25 )O 8 and Cu 3 (As 1.25 V 0.75 )O 8 . The assumption that As 5+ can stabilize borisenkoite structure is the reason to propose the simplified formula Cu 3 [(V,As)O 4 ] 2 avoiding the formal end-member formula Cu 3 (VO 4 ) 2 . Borisenkoite is named in honour of the Russian geochemist, mineralogist, and geologist Leonid Fedorovich Borisenko (1922–2000).