The new mineral cryobostryxite, KZnCl3·2H2O, is found in the Northern fumarole field at the First scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Cryobostryxite is a secondary mineral formed in the upper, moderately hot (30–80 °C) zone of active fumaroles, probably as a product of the interactions between high-temperature volcanic sublimates and meteoric water. It is associated with gypsum, ralstonite and opal; earlier, primary minerals of the assemblage are sellaite, fluorite, halite, anhydrite, cotunnite, sofiite, flinteite, chubarovite, anglesite, challacolloite, zincomenite, saltonseaite, hollandite, hematite, jakobssonite, leonardsenite and olsacherite. Cryobostryxite occurs as anthodites (up to 0.5 × 2 mm), their aggregates (up to 4 × 5 mm), granular crusts (up to 2 × 2 mm) and, rarely, coarse prismatic to acicular crystals (up to 0.2 × 1 mm). The mineral is transparent, colourless, with vitreous lustre. It is brittle, cleavage is not observed. The Mohs hardness is ca. 2. Dmeas = 2.30(2), Dcalc = 2.300 g cm−3. Cryobostryxite is optically biaxial (+), α = 1.522(2), β = 1.530(2), γ = 1.576(2) and 2Vmeas = 30(15)°. The chemical composition (wt.%, electron-microprobe data, H2O calculated for 2 molecules per formula unit, pfu) is: K 14.85, Tl 4.08, Zn 25.82, Cl 41.70, H2O(calc.) 14.19, total 100.64. The empirical formula calculated on the basis of K + Tl + Zn + Cl = 5 apfu is (K0.96Tl+0.05)∑1.01Zn1.00Cl2.99·2H2O. Cryobostryxite is monoclinic, P21/c, a = 6.2795(3), b = 10.1397(3), c = 12.0829(7) Å, β = 107.732(5)°, V = 732.79(6) Å3 and Z = 4. The strongest reflections of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 7.62 (30) (011), 5.986 (43) (100), 5.766=(35) (002), 3.907 (33) (−121), 3.466 (20) (121), 3.062 (100) (−202, 023), 2.996 (24) (−211, 200) and 2.853 (27) (−114). The crystal structure, solved from single-crystal X-ray diffraction data (R = 0.0654), contains isolated Zn-centred tetrahedra ZnCl3(H2O) connected via eight-coordinated K-centred polyhedra KCl7(H2O) to form a pseudo-framework. Both Zn- and K-centred polyhedra involve only O(1) atoms of the H2O(1) molecules, whereas H2O(2) molecules are located in holes of the K-Zn-Cl-H2O(1) polyhedral pseudo-framework. The mineral name is based on two Greek words, κρύος, cold or ice, and βóστρυξ, curl, reflecting the typical appearance: visually, anthodites of the mineral are very similar to ice curls. A novel chemical family of minerals including the potassium zinc chlorides flinteite K2ZnCl4, mellizinkalite K3Zn2Cl7 and cryobostryxite is discussed in the light of crystal-chemical data and formation conditions.

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