Hydrothermal synthesis and structural study on Rb_1.71Ca₄{Si₆O₁₅[O_0.855(OH)_0.145]₂}2H₂O, a tobermorite related compound

Abstract

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 Rb_1.71Ca₄{Si₆O₁₅[O_0.855(OH)_0.145]₂}2H₂O or {Ca₄Si₆O₁₅[O_0.855(OH)_0.145]₂}·(Rb_1.71·2H₂O). 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) Å, β = 114.056(7)°, V = 936.02(8) ų, 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. Least-squares refinements resulted in a residual of R(|F|) = 0.073 for 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: O²⁻, 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 Rb_1.71Ca₄{Si₆O₁₅[O_0.855(OH)_0.145]₂}2H₂O is closely related to natural paratobermorite, a hydrous calcium silicate of the tobermorite supergroup of minerals. The paper will cover the common features of both phases and the differences between the compounds, including aspects of OD-theory.