The high-pressure behaviour of mesolite: structural evolution and H₂O rearrangement under compression

1. INTRODUCTION Mesolite (Na₂Ca₂[Si₉Al₆O₃₀]·8H₂O) [1] is a fibrous zeolite hosting three extra-framework cation sites and multiple H₂O groups occupying channels of different size (8-membered rings). Despite its importance within the natrolite group, its response to compression has remained poorly characterized. Using a multimethodological approach, based on single-crystal X-ray and neutron diffraction, and EMPA analysis, we investigated the structural evolution of natural mesolite to determine the role of pressure on H₂O molecules rearrangement and channel deformation. 2. RESULTS AND DISCUSSION The neutron data collection at 20 K and structure refinement allowed a precise determination of the positions of all the hydrogen sites (Fig.2), mainly consistent with the previous X-ray data [1]. When compressed in non-aqueous fluids (such as daphne oil), mesolite displays a continuous volume decrease up to ~4.5 GPa (fig. 2). Compression is strongly anisotropic, with the a-axis showing the highest compressibility, due to the orientation and geometry of the channel systems. In contrast, experiments performed in aqueous PTM (distilled water or a methanol–ethanol–water 1:1:1 mixture) show a marked unit-cell volume increase in mesolite starting at ~0.8 GPa (see Fig. 2). This over-hydration, consistent with the behaviour of other NAT-topology zeolites, results from the pressure-driven incorporation of additional H₂O molecules into the framework channels. In these conditions, the ψ deformation parameter decreases with pressure, indicating an expansion along the shorter diameter of the channel (thus reducing its ellipticity). A high-pressure experiment in Ne did not provide definitive evidence of Ne- uptake within the cavities, although a weak positive residual in the natrolite-like channel and a discontinuity in the ψ deformation parameter suggest minor Ne incorporation (see Fig. 2). 3. EXPERIMENTAL Neutron single-crystal diffraction was carried out at 20 K on the D19 diffractometer at the Institut Laue-Langevin (ILL, Grenoble). A 4.1 × 1.3 × 1.3 mm³ mesolite crystal was mounted on a vanadium pin and cooled using a closed-cycle helium cryostat. High-pressure single-crystal X-ray diffraction experiments were carried out using Boehler–Almax diamond anvil-cell with 600 μm culets. A series of pressure-transmitting fluids have been used, and listed in Fig. 1. Further information on the experimental set-up is provided in [2]. 4. CONCLUSIONS Neutron data collection allowed all eight independent hydrogen sites positions to be located and refined, enabling accurate characterization of their libration regime, along with the hydrogen-bonding network geometry. At high pressure, mesolite undergoes anisotropic compression controlled by the deformation of its channel systems. Pressure induces selective H₂O migration and the expulsion of H₂O groups from the smallest channel, whereas H₂O in the large rings remains stable up to collapse. References [1] G. Artioli, J.V. Smith, J.J. Pluth (1986). Acta Crystallographica. 937–942. [2] T. Poreba, D. Comboni, M. Mezouar, G. Garbarino, M. Hanfland (2022). Journal of Physics: Condensed Matter, 35, 5