High pressure behavior of K-cymrite (KAlSi₃O₈·H₂O) crystal structure

Compressibility and structural evolution of K-cymrite, hexagonal high-pressure KAlSi₃O₈·H₂O, has been studied up to 18 GPa using synchrotron single crystal X-ray diffraction in Ne pressure medium. K-cymrite retains its original symmetry P6/mmm up to a pressure of 7.3 GPa. As the pressure increases from 7.3 to 8.5 GPa, the weak satellite reflections appear on diffraction patterns and remain up to maximum applied pressure of 18 GPa indicating incommensurate modulation. However, main reflections can be still indexed in hexagonal cell and structure successfully solved in initial P6/mmm group. After pressure release, K-cymrite reverts to initial non-modulated single-crystal state. The parameters of third-order Birch-Murnaghan equation of state for K-cymrite are V₀ = 190.45(12) ų, K₀ = 56.5(7) GPa and K₀’ = 3.2(12), with bulk modulus notably deviating from earlier result (K₀ = 45(2) GPa and K₀’ = 1.3(10)) obtained in vaseline media.