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

The high-pressure (HP) behavior of natural stellerite |Ca4.00Na0.16 (H2O)(32)| [Al8.16Si27.84O72] 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 |Ca4.00Na1.47 (H2O)(30)| [Al9.47Si26.53O72]. 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.