Two new members of the rhodesite mero-plesiotype series close to delhayelite and hydrodelhayelite: synthesis and crystal structure

Two new members of the mero-plesiotype rhodesite series (KCa₂Na[Si₈O₁₉]·5H₂O, abbreviated as TR03; a 6.5850, b 23.776, c 7.0250 Å; Pn2₁m; and KNa₃Sr[Si₈O₁₉] ·4.3H₂O, TR04; a 6.5699, b 23.7225, c 7.0225 Å, β 91.81°; P2₁/m) have been hydrothermally synthesized in Teflon-lined autoclaves at 200–230 °C and structurally characterized using X-ray diffraction single-crystal data (Bruker-AXS Smart Apex diffractometer equipped with a CCD area detector, MoKα radiation). The crystal structures were solved by direct methods and refined to R = 0.090 [TR03; 2634 reflections with I_o > 2σ(I_o); {010} twinning by merohedry] and R = 0.062 [TR04; 4535 reflections with I_o > 2s(I_o)]. Both structures are based on a rhodesite-type porous heteropolyhedral framework, where two types of channels (effective width about 3.5 Å in the widest channel) cross the double silicate layer. They show specific “octahedral” O layers that are discussed with particular reference to delhayelite and hydrodelhayelite. TR03 can be considered a Na-bearing hydrodelhayelite; this extra cation resides in the O layer and connects two edge-sharing rows of Ca-octahedra that, instead, are isolated in hydrodelhayelite. A further increase of the number of cations pfu leads to a fully occupied O layer, as found in delhayelite and TR04. The H₂O molecules are hosted both in the channels and in the O layer; bond lengths and Raman spectra show that they are loosely hydrogen bonded. The structural features that support the wide range of chemical compositions found in the rhodesite series are discussed.