The new mineral flinteite, ideally K2ZnCl4, was discovered in active fumaroles at two scoria cones of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. In the Northern fumarole field at the First scoria cone (locality of the holotype), flinteite is a common mineral associated with halite, sellaite, fluorite, saltonseaite, chubarovite and hollandite. In the Arsenatnaya fumarole at the Second scoria cone, flinteite occurs with langbeinite, aphthitalite, fluoborite, sylvite, halite, tenorite, hematite, zincite, chubarovite, krasheninnikovite, vanthoffite, etc. In the Glavnaya Tenoritovaya fumarole (Second scoria cone), flinteite is associated with mellizinkalite, belloite, avdoninite, eriochalcite, sylvite, halite, mitscher-lichite, sanguite, chrysothallite, romanorlovite, gypsum, chlorothionite, kainite, etc. Flinteite typically forms prismatic crystals up to 0.2 × 0.3 × 1.2 mm, their groups, granular aggregates or crusts up to 0.5 × 5 × 5 mm. The mineral is light green, light yellow to bright greenish-yellow or colourless. It is transparent, with vitreous lustre. Flinteite is brittle, one direction of distinct cleavage was observed. The Mohs hardness is ca. 2. Dcalc = 2.49 g cm−3. Flinteite is optically biaxial (+), α = 1.573(1), β = 1.574(1), γ = 1.576(1), 2Vmeas = 40(25)°. The chemical composition of theholotype (wt%, electron-microprobe data) is: K 24.97, Tl5.82, Co 0.07, Zn 22.23, Cl 46.95, total 100.04. The empirical formula calculated based on the sum of all atoms = 7 pfu is: (K1.91Tl0.09)Σ2.00Zn1.04Cl3.96. Some samples show the following substitutions for K (wt%): up to 27.7 Tl, 0.6-2.4 Rb, 0.5–2.2 Cs. Flinteite is orthorhombic, Pna21, a = 26.8090(10), b = 12.4085(6), c = 7.2512(3) A, V = 2412.18(18) Ǻ3 and Z = 12. The strongest reflections of the powder X-ray diffraction pattern [d, Ǻ(I)(hkl)] are: 6.23(27)(011, 020), 5.123(88)(311, 320), 3.629(98)(611, 002), 3.599(100)(031), 3.133(35)(022), 3.039(26)(630), 2.897(35)(910) and 2.688(46)(911, 920). The crystal structure, solved from singlecrystal X-ray diffraction data (R = 0.0686), contains three Zn sites centring isolated ZnCl4 tetrahedra and six independent K sites. Flinteite is the natural analogue of a well-known synthetic room-temperature modification of K2ZnCl4, a ferroelectric material. The mineral is named in honour of the Russian crystallographer Evgeniy E. Flint (1887–1975).

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