The crystal structure of the mineral görgeyite, K 2 S0 4 • 5 CaS0 4 · H 2 0, has been determined from visually estimated photographic intensities using the direct methods program MULTAN, and refined by block-diagonal least-squares structure factors to an R value of 0.096. The mineral is monoclinic and contains four formula units in a cell of dimensions a = 17.51(1), b = 6.822(5), c = 18.21(l)Ä,/3 = 113.3° and space &oupC2/c.Two independent calcium atoms, one of which sits on the diad axis, are nine-fold coordinated by oxygen atoms in a closely similar arrangement, with average Ca-Ο distances of 2.52 Ä in the range 2.40 -2.68 A. A third calcium atom is eight-fold coordinated with a mean Ca-Ο distance of 2.46 Ä in a range of 2.37-2.64 A.The potassium atom is also eight-fold coordinated. The three independent sulphate ions are approximately regular with mean S-Ο distances of 1.472(8), 1.477(27), and 1.470(7) A respectively but their behaviour in providing oxygen coordination is different for each. The hydrogen atom of the water molecule has not been positively located though it is likely that a hydrogen bond is present. Abstract.A three-dimensional X-ray diffraction analysis of BaS 2 0 3 · H 2 0 was based on 523 symmetry-independent reflexions. The structure was refined by full-matrix least-squares to an R (excluding zero-weight data) of 0.065 for 492 reflexions. The structural details of the earlier studies are confirmed. An analysis of the thermal motions of the individual nonhydrogen atoms was made on the basis of thermal vibration ellipsoids and molecular geometry.