COMPARATIVE HIGH-PRESSURE STUDY OF ORTHORHOMBIC AND TETRAGONAL EDINGTONITE: EQUATIONS OF STATE AND CRYSTAL STRUCTURES

Edingtonite (Ba₂Al₄Si₆O₂₀·8H₂O) is a fibrous zeolite. The crystal framework is based on a 4=1 Secondary Building Unit (SBU) consisting of four-member tetrahedral rings joined by a fifth tetrahedron to form [001]-chains. The framework is characterized by two systems of channels: 8-ring channels along [001] and 8-ring channels along [110]. The difference between the orthorhombic (O) and tetragonal (T) edingtonite is due to Si/Al-ordering into the tetrahedral sites. In both symmetries, the extra-framework content lies in the [001]-channels. The structural behaviour of T- and O-edingt. from Ice River (Canada) was investigated at HP by in situ single-crystal X-ray diffraction. A BGI-DAC was used for the high-pressure study. At room condition, the Ba-site is split into two sites ~0.34 Å apart. Most of Ba cations (~90%) are occupying the Ba1 site and a minor amount (~10%) the Ba2 site. Similar behaviour was observed for O- and T-specimen. To analyse the effects of the lattice microporosity on compressibility, the HP-experiments were conducted using different pressure transmitting media: a) non-penetrating anhydrous media, in order to avoid superhydration of the zeolite samples; b) hydrous media, to force water molecules to fill the channels. Glycerol, as a non-penetrating medium, was used for the T-edingt.. The lattice parameters were determined up to 5.5 GPa. Cell volume data were fitted using the BM-EoS. For the T-edingt. the results are: V₀=601.6(3) ų, K_T0=59(2) GPa and K’=3.4(8). Methanol:ethanol:water 16:3:1 mixture, as a penetrating medium, was used for both T and O-specimens. The experiment is in progress, but at least up to 4.5 GPa no overhydration effect has been observed. Fitting the volume data of O-edingt. with a BM-EoS we obtain: V₀=598.71(7)ų, K_T0=59(1) GPa and K’=4.1(6). The EoS of T-edingt. appears to be the same in the two different pressure media. The evolution of the crystal structure was studied by comparing the results of the refinement of data collected at different pressures. The relevant structural variations are produced by cooperative rotation of the SBUs along [001]. Extra-framework content shows an interesting behaviour at HP conditions: the occupancy of the split Ba2 site decreased with pressure and at P > 2.5 GPa the Ba2 site is completely empty, only the position Ba1 is occupied.