P-mediated crystal-fluid interaction in the ABC-6 zeolite group: the case of ERI, OFF and EAB topologies

The Pressure-mediated intrusion of molecules and solvated ions into the nano-cavities of microporous (or layered) materials is an efficent method to facilitate the mass transfer from fluids to crystalline solids. This phenomenon, which can be observed in both synthetic and natural zeolites, for example, could expand their industrial applications, develop new functional materials, and improve catalytic performance of these open-framework compounds. In addition, from a geological perspective, a comprehensive understanding of the crystal-fluid interaction induced by pressure may lead to a reassessment of the role played by zeolites as carriers of fluids during the early stages of subduction. It is worth noting that this class of open-framework silicates can contain up to 20 wt% of H2O. In this study, we have investigated the crystal-fluid interaction, driven by pressure, in three different natural zeolites belonging to the “ABC-6 group”: erionite (ERI framework, with a 6-membered ring sequence of AABAAC), offretite (OFF, sequence of AAB), and bellbergite (EAB, sequence of AABCCB). The aims of this study were twofold: 1) to determine the potential role of erionite as a fluid carrier during subduction, as one of the alteration minerals occurring in oceanic floor basalts, and 2) to compare the mechanisms adopted by structurally similar frameworks in accommodating bulk compression and adsorbing new molecules, as well as to determine the magnitude of the crystal-fluid interaction in structurally similar frameworks. Synchrotron XRD experiments were conducted on single crystals of natural erionite, offretite and bellbergite using a diamond anvil cell, both with potentially penetrating and non-penetrating pressure-transmitting fluids. The latters served as a reference for evaluating the crystal-fluid interaction, as the adsorption of new molecules reduces the bulk compressibility for the "pillar" effect exerted by the intruded guest species in the structural voids. The results revealed that erionite shows the largest adsorption capacity among the three zeolites. Moreover, the occurrence and magnitude of the crystal-fluid interaction phenomena were found to be strongly influenced by the H2O content in the hydrous pressure-transmitting fluids used in our experiments. In the case of offretite, liquid neon acted as penetrating fluid, with intruded atoms forming weak Van der Waals interactions with the extra-framework population. Natural bellbergite, on the other hand, shows limited intrusion of guest molecules from the pressure-transmitting fluids, highlighting the significant role of "secondary factors," such as the initial extra-framework content of the mineral, on crystal-fluid interaction phenomena.