The crystal structures of two Cr-rich pumpellyite-group minerals, shuiskite with 22.6 wt% Cr2O3 from the Biserskoe deposit and chromian pumpellyite-(Mg) with 16.4 wt% Cr2O3 from the Glavnoe Saranovskoe deposit (both belong to the Saranovskaya group of chromite deposits, Middle Urals, Russia) were studied through single-crystal X-ray diffraction. The structures (space group C2/m) of shuiskite with a = 19.2156(7), b = 5.9779(2), c = 8.8268(3) Å, β = 97.785(3)° and chromian pumpellyite-(Mg) with a = 19.1776(8), b = 5.9537(3), c = 8.8282(4) Å, β = 97.668(4)° were refined to R1 = 3.01 and 3.12%, respectively; the structure of shuiskite was determined for the first time. The formulae (Z = 4) obtained from the structural refinements and taking into account electron-microprobe data are: WCa2.00X(Mg0.46Cr0.36Al0.18)Σ1.00Y(Cr1.04Al0.96)Σ2.00ZSi3.00O10.54OH3.46 for shuiskite and WCa2.00X(Mg0.50Cr0.30Al0.20)Σ1.00Y(Al1.30Cr0.70)Σ2.00ZSi3.00O10.50OH3.50 for chromian pumpellyite-(Mg). Molecular water is absent in both minerals. The presence of silanol groups Si–OH is confirmed by Infrared (IR) spectroscopy. The existence of shuiskite as an individual mineral in its initial sense, i.e., with Mg and Cr3+ dominant at the X and Y sites, respectively, is confirmed. The simplified formula of shuiskite has been refined: Ca2(Mg,Cr)(Cr,Al)2[SiO4][Si2O6(OH,O)](OH,O)(OH)2. The simplified formula of chromian pumpellyite-(Mg) is Ca2(Mg,Cr)(Al,Cr)2[SiO4][Si2O6(OH,O)](OH,O)(OH)2. In high-chromium pumpellyite-group minerals the preference of Cr for the Y site increases with increasing total Cr content: the distribution coefficient of Cr3+ between the X and Y sites, KD = (Cr/Al)X/(Cr/Al)Y, is 2.79 in chromian pumpellyite-(Mg) and 1.85 in shuiskite.

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