Incommensurately modulated high-pressure modification of K-cymrite: Role of guest H2O molecules

K-cymrite (KAlSi3O8·H2O), a high-pressure phase implicated in volatile and large-ion lithophile elements (LILE) transport during subduction, exhibits a complex structural evolution under high pressure. This study investigates the incommensurate modulated high-pressure modification of K-cymrite, revealing a phase transition from the hexagonal P6/mmm structure to a monoclinic (3+1)-dimensional structure with the superspace group C2/m(0β0)s0 above 8.5 GPa. Single-crystal X-ray diffraction data, collected up to 20.2 GPa, demonstrate the emergence of satellite reflections at 7.3–8.5 GPa, indicative of an incommensurate modulation that resembles a lock-in mechanism with transition to commensurate 3b phase at ∼15 GPa, before reverting to incommensurability above 16.2 GPa. The modulation is characterized by wave-like deformation of double tetrahedral layers and ditrigonal-like distortion of six-membered rings, driven by the ordering of guest H2O molecules interacting with interlayer K+ cations. These findings underscore the need to incorporate modulation effects into thermodynamic models of crust subduction and volatile cycling.