Kaliochalcite, KCu2(SO4)2[(OH)(H2O)], a new tsumcorite-group mineral from the Tolbachik volcano, Kamchatka, Russia

Abstract

The new tsumcorite-group mineral kaliochalcite, KCu 2 (SO 4 ) 2 [(OH)(H 2 O)] (IMA 2013–037), is found in several fumaroles at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Two fumaroles, Yadovitaya and Arsenatnaya, are considered as its type localities. The mineral seems to be a product of the interactions involving the high-temperature, sublimate KCu-sulphates and atmospheric water vapour at temperatures not higher than 100–150°C. Kaliochalcite mostly occurs in polymineralic crusts (up to several dozens cm 2 in area and up to 0.5 cm thick), where it is typically the major component. The mineral forms fine-grained pseudomorphs after anhydrous KCu-sulphates, mainly euchlorine, fedotovite or piypite, usually with their relics. Other associated minerals are hematite, tenorite, langbeinite, aphthitalite, steklite, lammerite, chlorothionite, gypsum, etc . Pseudo-rhombohedral or more complicated crystals of kaliochalcite (commonly up to 0.02 × 0.04 mm, rarely up to 0.03 × 0.1 mm) are observed in cavities. The mineral is light green, bright grass-green or almost colourless. Kaliochalcite is transparent in individuals and translucent in aggregates, with vitreous lustre. It is brittle, the Mohs’ hardness is 4. No cleavage was observed, the fracture is uneven. The caculated density, D calc , is 3.49 g cm −3 . Kaliochalcite is optically biaxial (+), α 1.630(3), β 1.650(3), γ 1.714(3), 2 V meas 55(10)°. The IR spectrum is given. The chemical composition (wt%, electron-microprobe data, H 2 O by selective sorption from the gaseous products of heating) is: Na 2 O 0.04, K 2 O 11.01, CaO 0.27, FeO 0.15, CuO 40.28, ZnO 0.39, SO 3 40.97, H 2 O 5.84, total 98.95. The empirical formula, calculated on the basis of 10 O apfu , is: (K 0.94 Ca 0.02 Na 0.01 ) Σ0.97 (Cu 2.03 Zn 0.02 Fe 0.01 ) Σ2.06 S 2.05 O 8.20 (OH) 1.01 (H 2 O) 0.79 . Kaliochalcite is monoclinic, space group C 2/ m, a 8.935(2), b 6.252(2), c 7.602(2) A, β 117.318(5)°, V 377.3(2) A 3 and Z = 2. The strongest reflections of the X-ray powder pattern [ d ,A( I )( hkl )] are: 6.78(100)(001), 3.484(70)(20–2), 3.249(63)(11–2), 2.892(77)(201), 2.852(83)(02–l), 2.554(72)(31–2, 22–1), 2.326(44)(22–2) and 1.693(37)(42–3, 22–4). The crystal structure was solved from single-crystal X-ray diffraction data, R = 0.100. The structure is built up from chains of edge-sharing Cu 2+ O 6 Jahn-Teller distorted octahedra connected by SO 4 tetrahedra and H-bonds to form {Cu 2 (SO 4 ) 2 [(OH)(H 2 O]} − layers. The linkage between these layers is provided by K + cations and H-bonds. Kaliochalcite is named as the potassium analogue of natrochalcite.