The hydrogen-bond system of pumpellyite, simplified formula WCa2XY2ZSi4O14-n(OH)n (Z = 4), from Oberhalbstein, Grisons, Switzerland (OHS) and Cr-rich pumpellyite from Sarany, Ural, Russia (SAR) was studied using electron-microprobe analysis (EMPA), attenuated total reflection Fourier transformed infrared spectroscopy (ATR-FTIR), and single-crystal X-ray diffraction. Structure refinements converged at R1 factors of 2.91 % for OHS and 2.46 % for SAR. The site occupancies at X and Y are X(Mg0.48Al0.494(3)Fe0.026(3))Y(Al1.0) for OHS, and X(Mg0.49Cr0.285(5)Al0.225(5))Y(Al0.744(4)Cr0.256(4)) for SAR, respectively. Both bond-valence calculations and located hydrogen sites indicated that the O5, O7, O10, and O11 positions host hydroxyl groups. One additional new hydrogen position was found in this study. Thus, two alternate hydrogen sites are linked to O5. The ATR-FTIR spectrum in the region of the OH stretching vibrations is mainly characterized by two intense OH bands at 3522 and 3379 cm−1 and an additional broad band around 3100 cm−1. The exact number of hydroxyl groups depends on the concentration of divalent and trivalent cations at the X site. With only divalent cations at X, pumpellyite has four hydroxyl groups pfu (1) bifurcated O5–H5···O1/O5–H5···O5, (2) bifurcated O7–H7···O3/O7–H7···O7, (3) O10–H10···O5, and (4) O11–H11···O7. If there are only trivalent cations at X, only three OH groups pfu, (1) O5–H5′···O10, (2) bifurcated O7–H7···O3/O7–H7···O7, and (3) O11–H11···O7, are necessary for charge balance. The latter system with 3OH groups is very similar to that in sursassite.

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