High-pressure behaviour of stellerite: single-crystal X-ray diffraction study

The high-pressure (HP) behavior of natural stellerite |Ca_4.00Na_0.16 (H₂O)₃₂| [Al_8.16Si_27.84O₇₂] has been studied by single-crystal X-ray diffraction using a diamond-anvil cell under pressures up to 4.5 GPa, with a 4:1 ethanol:water mixture and paraffin as pressure-transmitting media. The changes in the structure of stellerite at high pressures, especially the STI framework deformation, are similar to those in Na-rich stilbite |Ca_4.00Na_1.47 (H₂O)₃₀| [Al_9.47Si_26.53O₇₂]. Both stilbite and stellerite udergo pressure-induced hydration, in which H₂O molecules first occupy partly vacant sites and then the initially vacant positions. Some difference in the behavior of the two minerals is due to the presence of Na⁺ cations in stilbite. Sodium occupies positions in the 10-membered ring and prevents H₂O molecules from penetrating near the ring center. Meanwhile, both stellerite and stilbite can fill the initially vacant sites in the 8-membered ring at high pressures. The pressure-induced changes, including the reduction of H₂O sites in the cation coordination and a total number of H₂O molecules, are less significant in Na-bearing stilbite than in stellerite.