The compressional behaviour of microporous materials (in particular, zeolites and feldspathoids) compressed in a fluid can be substantially governed by the potential crystal-fluid interaction, due to the selective sorption of new molecular species (or solvated ions) through the structural cavities in response to the applied (hydrostatic) pressure.

When no crystal-fluid interaction takes place, the experimental findings and computational modelling performed so far show that the effects of the applied pressure, at the atomic scale, are mainly accommodated by the tilting of the (quasi-rigid) (Si,Al,P)O₄ tetrahedra, around the bridging oxygen atoms that act as hinges between tetrahedra [1]. Tilting of tetrahedra was proved to be the dominant mechanism at low-mid P-regime, then followed by distortion and compression of these polyhedra, which become dominant at the mid-high P-regime (i.e., once titling is not sufficient anymore to accommodate the deformation energy) [2]. Specific mechanisms of deformation at the atomic scale, in response to compression, are controlled by the topology of the framework of tetrahedra. For example, the continuous increase of channels ellipticity, with increasing pressure, is one of the most common deformation mechanisms in zeolitic frameworks, but inversion of ellipticity occurs only in response to a phase transition, with a drastic structure rearrangement (e.g., reconstructive in character). On the other hand, the compressibility of the cavities (in the form of channels or cages) is governed by the so-called extraframework population (made by ions and small molecules), leading to different bulk compressibility in isotypic structures [3]. The elastic parameters available for zeolites (natural or synthetic) show that microporosity does not necessarily imply high compressibility, and most of the zeolites appear to be less compressible than many other Crustal minerals [1,3]. A high compressibility is somehow expected for porous framework structures due to the tetrahedral tilting, but the bonding configuration between the framework of tetrahedra and the stuffed species affects the overall compressional behaviour, making this class of host-guest structures less compressible than other rock-forming silicates.

When compressed in penetrating fluids, some zeolites experience a P-induced intrusion of new monoatomic species or molecules from the fluids themselves. Materials having well-stuffed cavities at room P-T conditions tend to hinder the penetration of new species. Crystal-fluid interactions in zeolites have observed using pressure-fluids made by: monoatomic species (e.g., He, Ar, Kr, Xe), small (e.g., H₂O, CO₂) or more complex molecules (e.g., C₂H₂, C₂H₄, C₂H₆O, C₂H₆O₂, BNH₆, electrolytic MgCl₂·21H₂O solution), with potential geological and technological implications [4,5]. Diverse variables govern the P-mediated sorption phenomena: the “free diameters” of the framework cavities, nature and bonding configuration of the extraframework population, kinetic diameter of the potentially-penetrating molecules, rate of P-increase, temperature at which the experiment is conducted and surface/volume ratio of the crystallites under investigations.