Mesolite, |Na2Ca2(H2O)8|[Al6Si9O30]: Crystal structure reinvestigation and pressure-mediated crystal-fluid interaction

Abstract

The crystal structure and the pressure-mediated crystal-fluid interaction of mesolite have been re-investigated by a multi-methodological approach, based on single-crystal neutron diffraction and by in-situ single-crystal synchrotron X-ray diffraction, using a diamond anvil cell. The structure refinement based on neutron intensity data collected at 20 K confirms the general model previously reported for mesolite, but largely improves the description of the hydrogen-bond network (with accurate sites location, their libration regime and interactions). Twelve out of the nineteen oxygen sites in the crystal structure of mesolite are involved in H-bonds as donors or as acceptors, reflecting the complex configuration of the H-bonding network. In the high-pressure investigations, four different pressure-transmitting fluids have been employed: the non-penetrating Daphne oil 7575 and the potentially penetrating methanol:ethanol:H₂O (1:1:1) mixture, distilled H₂O and liquid Ne. The Daphne oil 7575 experiment provided insight into the intrinsic compressional behaviour of mesolite, without any pressure-induced crystal-fluid interaction, yielding an isothermal bulk modulus K_V0 = 55.9(7) GPa (β_V0 = 0.0179(2) GPa⁻¹). In the aqueous mixtures, H₂O molecules have been observed to continuously penetrate into the structural cavities, firstly in the natrolite- and then in the scolecite-type sheets, in the pressure range 0.8–1.9 GPa. By comparing the results of this study to the literature data, there is an apparent correlation between the pressure at which the adsorption process occurs and the H₂O concentration of the pressure-transmitting medium: a higher H₂O fraction allows the over-hydration of the scolecite-type sheets at lower pressures. When compressed in liquid Ne, atoms of neon appear to be able to penetrate into the natrolite-type sheets, interacting with the extra-framework population via weak van der Waals forces.