Kalithallite, K3Tl3+Cl6⋅2H2O, a new mineral with trivalent thallium from the Tolbachik volcano, Kamchatka, Russia

Abstract

Abstract A new mineral kalithallite, K3Tl3+Cl6⋅2H2O, was found in an active fumarole belonging to the Northern fumarole field at the First scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Kalithallite is a product of the relatively low-temperature (70–150°C) interactions involving high-temperature sublimate minerals, volcanic gas and atmospheric water vapour. The associated minerals are cryobostryxite, KZnCl3⋅2H2O, halite, sylvite, opal and gypsum. Kalithallite forms lamellar to tabular crystals up to 5 × 30 × 40 μm combined in open-work aggregates up to 1 mm across. It is transparent, colourless in individuals and white to pale cream coloured or pale beige in aggregates, with vitreous lustre. Dcalc = 3.01 g cm–3. Kalithallite is optically uniaxial (–), ɛ = 1.656(3) and ω = 1.662(3). The chemical composition (wt.%, electron-microprobe data, H2O calculated by stoichiometry) is: K 17.72, Zn 0.85, Tl 38.76, Cl 35.91, H2Ocalc 5.99, total 99.23. The empirical formula calculated on the basis of K+Zn+Tl+Cl = 10 apfu is K2.72Zn0.06Tl1.14Cl6.08⋅2H2O. Kalithallite is tetragonal, I4/mmm, a = 15.9333(5), c = 18.1088(7) Å, V = 4595.2(4) Å3 and Z = 14. The strongest reflections of the powder X-ray diffraction (XRD) pattern [d,Å(I)(hkl)] are: 5.98(100)(202); 5.64(36)(220); 3.984(20)(400); 3.528(30)(224); 3.315(22)(422); 2.890(15)(334); and 2.817(24)(206, 440). Kalithallite is isotypical to synthetic K3Tl3+Cl6⋅2H2O. The crystal structure was refined from the powder XRD data using the Rietveld method, RBragg = 0.55%, Rp = 0.56%, and Rwp = 0.75%. The structure contains Tl3+Cl6 octahedra and K-centred polyhedra of three types: KCl8, KCl8(H2O) and KCl7(H2O)2. The mineral is named as a kalium–thallium ordered compound.