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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 D_calc 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 Ca_2.02Mg_0.42Cr³⁺_1.99Al_0.56Ti_0.01Si_3.00O_10.57(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) ų, 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. Full article