Crystal chemistry of shuiskite and chromian pumpellyite-(Mg)

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: ^WCa_2.00^X(Mg_0.46Cr_0.36Al_0.18)_Σ1.00^Y(Cr_1.04Al_0.96)_Σ2.00^ZSi_3.00O_10.54OH_3.46 for shuiskite and ^WCa_2.00^X(Mg_0.50Cr_0.30Al_0.20)_Σ1.00^Y(Al_1.30Cr_0.70)_Σ2.00^ZSi_3.00O_10.50OH_3.50 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, K_D = (Cr/Al)_X/(Cr/Al)_Y, is 2.79 in chromian pumpellyite-(Mg) and 1.85 in shuiskite.