The crystal structure of ruizite, ideally Ca2Mn(3+) 2[Si4O11(OH)2](OH)2·2H2O [dicalcium dimanganese(III) tetra-silicate tetra-hydroxide 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 (R 1 = 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 octa-hedra flanked by finite [Si4O11(OH)2] chains. The Ca(2+) cations are situated in the cavities of this arrangement and exhibit a coordination number of seven.
Keywords: crystal structure; finite silicate chain; redetermination; ruizite.