P-induced crystal-fluid interactions in natural zeolites with ERI and OFF topologies

Open-framework minerals, such as zeolites, have extensively proved their ability to adsorb and host new molecules or ions, also as a response to a pressure stimulus. This peculiar property might be exploited, among the others, for the production of new multifunctional materials or to enhance industrial catalytic processes involving zeolites. Furthermore, from a geological point of view, a comprehensive understanding of this property might unveil the role played by zeolites as fluids (e.g., H2O) carriers and moderators in the upper crust, e.g. during the early subduction of altered basalts or oceanic sediments. In this study, we have investigated the high-pressure behavior and the P-induced crystal-fluid interaction, of two different natural zeolites belonging to the ABC-6 family: erionite (AABAAC) and offretite (AAB) (ERI and OFF framework topologies), which, due to structural similarities, are commonly associated in nature. Experiments were performed by in-situ high-P single-crystal synchrotron XRD, using diamond anvil cells (DAC). Different P-transmitting media have been used: i) non-penetrating silicone oil and ii) potentially penetrating alcohols:water mixture (16:3:1) and pure H2O. The unit–cell P-V patterns unambiguously suggest the occurrence of a P-induced intrusion of molecules from the penetrating fluids, which induces a decrease in compressibility both in erionite and offretite. The magnitude of this phenomenon in natural erionite appears to be comparable to that observed in other synthetic samples (e.g., AlPO4-5, Si-FER), which, however, are characterized by almost empty structural nanocavities.