Abstract

Two chemically and structurally related new minerals were discovered in fumarolic sublimates at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia: wulffite, K3NaCu4O2(SO4)4, in the Arsenatnaya fumarole with euchlorine, aphthitalite, tenorite, hematite, lammerite, johillerite, fluoborite, etc., and parawulffite, K5Na3Cu8O4(SO4)8, in the Yadovitaya fumarole with euchlorine, fedotovite, hematite, langbeinite, steklite, etc. Later, parawulffite was found in sublimates of fumaroles related to the 2012–2013 Tolbachik Fissure Eruption. Both minerals form prismatic crystals (wulffite: up to 1.2 × 2 mm; parawulffite: up to 0.3 × 1 mm) in clusters and crusts. They are transparent with vitreous luster, brittle, with Mohs’ hardness ca 2½. Both minerals are dark green or deep emerald-green. D(meas.) is 3.23(2) and 3.35(2), and D(calc.) is 3.19 and 3.32 g cm–3 for wulffite and parawulffite, respectively. Both minerals are optically biaxial (+); wulffite: α 1.582(3), β 1.610(3), γ 1.715(3) and parawulffite: α 1.585(3), γ 1.717(4). Chemical data (wt.%, electron-microprobe; first value is for wulffite, second one for parawulffite) are: Na2O 4.11, 5.61; K2O 16.46, 13.74; Rb2O 0.95, 1.42; Cs2O 0.65, 1.21; CuO 38.88, 38.79; ZnO 0.15, 0.18; SO3 39.11, 39.26; total 100.31, 100.21. The empirical formulae are: wulffite (based on 18 O apfu): Na1.08(K2.85Rb0.08Cs0.04)Σ2.97(Cu3.99Zn0.02)Σ4.01S3.99O18; parawulffite (based on 36 O apfu): Na2.95(K4.75Rb0.25Cs0.14)Σ5.14(Cu7.95Zn0.04)Σ7.99S7.99O36. Wulffite is orthorhombic, Pn21a, a 14.2810(6), b 4.9478(2), c 24.1127(11) Å, V 1703.79(13) Å3, Z = 4. Parawulffite is monoclinic, P2/c, a 13.9043(10), b 4.9765(3), c 23.5855(17) Å, β 90.209(6)°, V 1631.98(19) Å3, Z = 2. The strongest reflections of their X-ray powder patterns [d,Å(I)(hkl)] are: wulffite: 9.27(100)(102), 7.16(22)(200), 3.125(16)(313, 215), 2.882(16)(411), 2.780(33)(117), 2.725(14)(413), 2.472(20)(020); parawulffite: 9.06(100)(-102, 102), 7.00(23)(200), 3.096(31)(313, -215), 2.736(33)(-117, 117), 2.492(24)(020), 2.321(26)(019, 513). The crystal structures of both minerals, solved from single-crystal X-ray diffraction data [R = 0.0721 (wulffite) and 0.0985 (parawulffite)], are closely related with respect to parts of the heteropolyhedral quasi-framework formed by the Cu–O–S chains (built up of Cu2+-centered square pyramids and SO4 tetrahedra) and the chains of NaO6 octahedra. The new minerals differ from one another in ratios of alkali cations and their arrangement in cavities of the quasi-framework and in the configuration of Cu–O–S chains. Wulffite is named in honor of the Russian crystallographer Georgiy Viktorovich Wulff (1863–1925), who suggested the model of X-ray interference in crystals (1913). The name of parawulffite reflects its similarity to wulffite.

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