The influence of extraframework cations on the behavior of K-exchanged gonnardite at high pressure

The behavior of K-exchanged gonnardite K_2.18Na_0.04Ca_0.02(H₂O)_2.2[Al_2.26Si_2.74O₁₀] at compression in penetrating water-containing medium was studied by synchrotron powder diffraction in diamond anvil cell. In contrast to the initial gonnardite, which exhibits the transition to high-hydrated paranatrolite at elevated humidity, its K-exchanged form is not capable of over-hydration even at high pressure. Within the whole studied pressure range (up to 4.8 GPa) the sample experiences the compression. At 3.5-4 GPa the compression anomaly is observed, which is interpreted as a phase transition accompanied by the symmetry lowering from tetragonal to monoclinic. The high-pressure structure evolution of K-exchanged forms of gonnardite and natrolite is compared; the difference in their behavior is explained by distinct configuration of the framework water-cation subsystem.