Structural evolution of Li-exchaned natrolite at pressure-induced over-hydration: An X-ray diffraction study

The behavior of Li-exchanged natrolite Li_1.92Na_0.10[Al_2.02Si_2.98O₁₀]∙2H₂O at compression in penetrating (water-containing) medium was studied by in situ synchrotron powder diffraction in diamond anvil cell up to 2.5 GPa. Within 0-1.3 GPa the compression is almost isotropic, and upon the further pressure increase the sample undergoes additional hydration, leading to abrupt volume expansion by 22%, a record value for natrolite. In the proposed model for the high-pressure phase Li₂[Al₂Si₃O₁₀]∙6H₂O the Li⁺ cations have no contact with the framework O-atoms and are surrounded by “water-jacket” in the form of semi-octahedron (tetragonal pyramid) composed of five H₂O molecules. Such polyhedra, lining up along the channel axis, are joined through their edges and create a “water” column expanding the structure.