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A new mineral, dioskouriite, CaCu4Cl6(OH)4∙4H2O, represented by two polytypes, monoclinic (2M) and orthorhombic (2O), which occur together, was found in moderately hot zones of two active fumaroles, Glavnaya Tenoritovaya and Arsenatnaya, at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Dioskouriite seems to be a product of the interactions involving high-temperature sublimate minerals, fumarolic gas and atmospheric water vapor at temperatures not higher than 150 °C. It is associated with avdoninite, belloite, chlorothionite, eriochalcite, sylvite, halite, carnallite, mitscherlichite, chrysothallite, sanguite, romanorlovite, feodosiyite, mellizinkalite, flinteite, kainite, gypsum, sellaite and earlier hematite, tenorite and chalcocyanite in Glavnaya Tenoritovaya and with avdoninite and earlier hematite, tenorite, fluorophlogopite, diopside, clinoenstatite, sanidine, halite, aphthitalite-group sulfates, anhydrite, pseudobrookite, powellite and baryte in Arsenatnaya. Dioskouriite forms tabular, lamellar or flattened prismatic, typically sword-like crystals up to 0.01 mm × 0.04 mm × 0.1 mm combined in groups or crusts up to 1 × 2 mm2 in area. The mineral is transparent, bright green with vitreous luster. It is brittle; cleavage is distinct. The Mohs hardness is ca. 3. Dmeas is 2.75(1) and Dcalc is 2.765 for dioskouriite-2O and 2.820 g cm−3 for dioskouriite-2M. Dioskouriite-2O is optically biaxial (+), α = 1.695(4), β = 1.715(8), γ = 1.750(6) and 2Vmeas. = 70(10)°. The Raman spectrum is reported. The chemical composition (wt%, electron microprobe data, H2O calculated by total difference; dioskouriite-2O/dioskouriite-2M) is: K2O 0.03/0.21; MgO 0.08/0.47; CaO 8.99/8.60; CuO 49.24/49.06; Cl 32.53/32.66; H2O(calc.) 16.48/16.38; -O=Cl −7.35/−7.38; total 100/100. The empirical formulae based on 14 O + Cl apfu are: dioskouriite-2O: Ca1.04(Cu4.02Mg0.01)Σ4.03[Cl5.96(OH)3.90O0.14]Σ10∙4H2O; dioskouriite-2M: (Ca1.00K0.03)Σ4.03(Cu4.01Mg0.08)Σ4.09[Cl5.99(OH)3.83O0.18]Σ10∙4H2O. Dioskouriite-2M has the space group P21/c, a = 7.2792(8), b = 10.3000(7), c = 20.758(2) Å, β = 100.238(11)°, V = 1531.6(2) Å3 and Z = 4; dioskouriite-2O: P212121, a = 7.3193(7), b = 10.3710(10), c = 20.560(3) Å, V = 1560.6(3) Å3 and Z = 4. The crystal structure (solved from single-crystal XRD data, R = 0.104 and 0.081 for dioskouriite-2M and -2O, respectively) is unique. The structures of both polytypes are based upon identical BAB layers parallel to (001) and composed from Cu2+-centered polyhedra. The core of each layer is formed by a sheet A of edge-sharing mixed-ligand octahedra centered by Cu(1), Cu(2), Cu(3), Cu(5) and Cu(6) atoms, whereas distorted Cu(4)(OH)2Cl3 tetragonal pyramids are attached to the A sheet on both sides, along with the Ca(OH)2(H2O)4Cl2 eight-cornered polyhedra, which provide the linkage of the two adjacent layers via long Ca−Cl bonds. The Cu(4) and Ca polyhedra form the B sheet. The difference between the 2M and 2O polytypes arises as a result of different stacking of layers along the c axis. The cation array of the layer corresponds to the capped kagomé lattice that is also observed in several other natural Cu hydroxychlorides: atacamite, clinoatacamite, bobkingite and avdoninite. The mineral is named after Dioskouri, the famous inseparable twin brothers of ancient Greek mythology, Castor and Polydeuces, the same in face but different in exercises and achievements; the name is given in allusion to the existence of two polytypes that are indistinguishable in appearance but different in symmetry, unit cell configuration and XRD pattern. [ABSTRACT FROM AUTHOR]

A new pumpellyite-group mineral shuiskite-(Cr), ideally Ca2CrCr2[SiO4][Si2O6(OH)](OH)2O, was found at the Rudnaya mine, Glavnoe Saranovskoe deposit, Middle Urals, Russia. It occurs on the walls of 0.5 to 1 cm thick fractures in chromitite, filled with calcite, Cr-bearing clinochlore, and uvarovite. Shuiskite-(Cr) forms long prismatic to acicular crystals up to 0.1 × 0.5 × 7 mm elongated along [010] and slightly flattened on [100]. The crystals are commonly combined into radial, sheaf-like aggregates. Most observed crystals are simple twins with a (001) composition plane. Shuiskite-(Cr) is greenish-black under daylight or purplish-black under incandescent light. It is optically biaxial (–), α = 1.757(5), β = 1.788(6), γ = 1.794(6), 2V (meas.) = 45(10)°, 2V (calc.) = 46° (589 nm). The Dcalc is 3.432 g/cm3. The IR spectrum is reported. The chemical composition (wt.%) is CaO 21.33, MgO 3.17, Al2O3 5.41, Cr2O3 28.50, TiO2 0.18, SiO2 33.86, H2O 5.82, total 98.27. The empirical formula calculated based on the sum of eight metal cations and Si atoms per formula unit is Ca2.02Mg0.42Cr3+1.99Al0.56Ti0.01Si3.00O10.57(OH)3.43. The simplified formula is Ca2(Cr,Mg)(Cr,Al)2[SiO4][Si2O6(OH,O)](OH,O)(OH)2. Shuiskite-(Cr) is monoclinic, C2/m, a = 19.2436(6), b = 5.9999(2), c = 8.8316(3) Å, β = 97.833(3)°, V = 1010.17(6) Å3, and Z = 4. The crystal structure, solved from single-crystal X-ray diffraction data (R = 0.0469), is based on a pair of chains of edge-sharing Cr-centred octahedra running along the b axis, linked together via the [SiO4] and [Si2O6(OH)] groups and Ca-centred polyhedra. The mineral species shuiskite, ideally Ca2MgCr2[SiO4][Si2O6(OH)](OH)3, was renamed to shuiskite-(Mg) by the decision of the IMA CNMNC. The shuiskite solid solution series with the general formula Ca2XCr2[SiO4][Si2O6(OH,O)](OH)2(OH,O), which includes shuiskite-(Mg) and shuiskite-(Cr) with X = Mg and Cr3+, respectively, appeared in the pumpellyite group. [ABSTRACT FROM AUTHOR]