New zinc and potassium chlorides from fumaroles of the Tolbachik volcano, Kamchatka, Russia: mineral data and crystal chemistry. I. Mellizinkalite, K3Zn2Cl7

The new mineral mellizinkalite, K3Zn2Cl7, is found in the Glavnaya Tenoritovaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Associated minerals are belloite, avdoninite, eriochalcite, sylvite, halite, carnallite, mitscherlichite, sanguite, chrysothallite, romanorlovite, gypsum, chlorothionite, kainite and earlier hematite, tenorite and chalcocyanite. Mellizinkalite occurs as irregularly shaped grains up to 0.5 mm across or crude elongated crystals up to 0.25 x 1.3 mm, their clusters and crusts up to 2 x 2 mm in area and up to 0.5 mm thick. The mineral is yellow-brown to reddish brown, transparent, with vitreous lustre. It is moderately brittle, slightly plastic. The Mohs' hardness is ca. 2. Cleavage is not observed, the fracture is uneven. D-meas = 2.46(2) and D-calc = 2.49 g cm(-3). Mellizinkalite is optically biaxial (-), alpha = 1.556(5), beta = 1.612(5), gamma = 1.663(5) and 2V(meas) = 85(5)degrees. The Raman spectrum is reported. The chemical composition (wt.%, electron-microprobe data) is: K 23.5, Rb 0.52, Mg 0.47, Cu 1.77, Zn 24.4, Cl 50.0, total 100.7. The empirical formula calculated on the basis of 12 atoms pfu is: (K2.95Rb0.03)(Sigma 2.98)(Zn1.84Cu0.14Mg0.09)(Sigma 2.07)Cl-6.95. Mellizinkalite is triclinic, P (1) over bar, a = 6.7737(4), b = 10.5715(13), c = 11.0730(9) angstrom, alpha = 117.930(10), beta = 106.909(5), gamma = 90.389(8)degrees, V = 660.61(10) angstrom(3) and Z = 2. The strongest reflections of the X-ray powder pattern [d,angstrom (I) (hkl)] are: 9.20(69)(001, 010, 0-11), 6.40(100)(100), 5.712(47)(-110, -1-11), 4.608(92)(002, 020), 3.499(55)(012), 3.473(73)(0-13, 0-31, 0-23), 3.393(66)(-201) and 3.075(49)(003). The crystal structure, solved from single-crystal X-ray diffraction data (R = 0.065), is unique. It consists of alternating layers of different ZnCl4 polyhedra. The Zn(1) cations are located in flat squares which are connected to each other via common Cl-Cl edges to form Zn2Cl6 dimers, whereas Zn(2) cations occupy isolated tetrahedra. Potassium cations occupy sites between the layers of Zn-centred polyhedra. The mineral (IMA2014-010) is named from three Latin words, mellis-honey, zincum and kalium, alluding to its colour and species-defining cations, zinc and potassium.