Hydrothermal synthesis and structural study on Rb<sub>1.71</sub>Ca<sub>4</sub>{Si<sub>6</sub>O<sub>15</sub>[O<sub>0.855</sub>(OH)<sub>0.145</sub>]<sub>2</sub>}2H<s

The hydrothermal treatment of a glass with a molar ratio of Rb2O : CaO : SiO2 = 1 : 4 : 6 at 400 C and 500 bar resulted in the formation of single-crystals of Rb1.71Ca4[Si6O15(O0.855(OH)0.145)2]2H2Oor [Ca4Si6O15(O0.855(OH)0.145)2](Rb1.712H2O).The basic crystallographic data of this phase at room conditions are as follows: space group P 21/m, a = 6.7766(4), b = 22.3162(10), c = 6.7782(4), = 114.056(7),V = 936.02(8) 3 , Z = 2.A striking feature of the crystals is a polysynthetic twinning, clearly observable under a petrographic microscope.The diffraction patterns of all investigated samples can be explained as a superposition of two reciprocal lattices, with a two-fold axis parallel to [101] being the twin element.Synchrotron radiation was used to determine the crystal structure from a data set collected at the X06DA beamline of the Swiss Light Source, Paul Scherrer Institute.Leastsquares refinements resulted in a residual of R(|F|)=0.073for 1712 reflections and 163 parameters.According to Liebau's crystal chemical classification, the compound can be described as an unbranched dreier double chain silicate.The bands run parallel to [101] and are formed by the condensation of two wollastonite-type single-chains.The two calcium cations within the asymmetric unit are coordinated by seven ligands each.The [CaX7]-groups (X: O 2-, OH -, H2O) are linked into layer-like units parallel to (010) by sharing common edges.Adjacent layers are connected by the silicate ribbons, resulting in a negatively charged heteropolyhedral network enclosing tunnel-like cavities.The rubidium atoms are distributed among a total of four partially occupied and mutually exclusive extra-framework positions within these channels.Each of the monovalent cation positions is coordinated by eight ligands, providing additional direct linkage between the network-forming polyhedra.The crystal structure of Rb1.71Ca4[Si6O15(O0.855(OH)0.145)2]2H2O is closely related to natural paratobermorite, a hydrous calcium silicate of the tobermorite supergroup of minerals.