Raman and NMR spectroscopic characterization of high-pressure K-cymrite (KAlSi₃O₈^.H₂O) and its anhydrous form (kokchetavite)

Abstract

To facilitate identification of high-pressure K-cymrite (KAlSi₃O₈·H₂O) phase and its anhydrous form (kokchetavite) in natural rocks, we have synthesized both phases and have characterized them by micro-Raman and NMR spectroscopy. K-cymrite was synthesized at 5 GPa and 800 °C. Kokchetavite was obtained by dehydrating K-cymrite at ambient pressure and 550 °C. The ¹H MAS and ¹H-²⁹Si CP MAS NMR spectra of K-cymrite are consistent with the reported crystal structure that contains H₂O molecules and has a disordered Si-Al distribution. The Raman spectra obtained under ambient conditions for K-cymrite (and kokchetavite) contain major peaks at 114.0 (109.1), 380.2 (390.0) and 832.5 (835.8) cm^-1. For K-cymrite, OH stretching vibration is also observed at 3541 cm^-1 with a shoulder at 3623 cm^-1. The Raman spectrum for kokchetavite is consistent with that previously reported for a natural sample found as inclusions in clinopyroxenes and garnets in a garnet-pyroxene rock. However, the data for K-cymrite are inconsistent with the Raman features of a previously reported “relict K-cymrite in K-feldspar” from an eclogite. Pressure- and temperature-dependencies of the Raman shifts for the strongest peak of both phases are also reported.