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The crystal structure of ruizite, ideally Ca2Mn3+2[Si4O11(OH)2](OH)2?2H2O [dicalcium dimanganese(III) tetrasilicate tetrahydroxide dihydrate] was first determined in space group A2 with an isotropic displacement parameter model (R = 5.6%) [Hawthorne (1984). Tschermaks Mineral. Petrogr. Mitt. 33, 135-146]. A subsequent refinement in space group C2/m with anisotropic displacement parameters for non-H atoms converged with R = 8.4% [Moore et al. (1985). Am. Mineral. 70, 171-181]. The current study reports a redetermination of the ruizite structure by means of single-crystal X-ray diffraction data of a natural sample from the Wessels mine, Kalahari Manganese Field, Northern Cape Province, South Africa. Our data (R1 = 3.0%) confirm that the space group of ruizite is that of the first study rather than C2/m. This work improves upon the structure reported by Hawthorne (1984) in that all non-H atoms were refined with anisotropic displacement parameters and all hydrogen atoms were located. The crystal structure consists of [010] chains of edge-sharing MnO6 octahedra flanked by finite [Si4O11(OH)2] chains. The Ca2+ cations are situated in the cavities of this arrangement and exhibit a coordination number of seven. [ABSTRACT FROM AUTHOR]