Influence of F content on the composition of Al-rich synthetic phlogopite: Part I. New information on structure and phase-formation from 29Si, 1H, and 19F MAS NMR spectroscopies

摘要

The influence of F content on the formation and stability of Al-rich phlogopite has been investigated. Samples with varying OH/F-ratios and nominal gel compositions of K(Mg_3–xAl_x)(Al_1+xSi_3–xO₁₀)(OH)_y(F)_2–y (0.0 x 0.8 and 0.5 y 1.8) were studied using 29Si, 1H, and 19F MAS NMR spectroscopies, powder X-ray diffraction, electron-probe microanalysis, and scanning electron microscopy. The synthetic phlogopites were synthesized from sol-gels in cold-seal pressure vessels at 1073 K, 2 kbar. The main phase (phlogopite) and three other impurity phases [corundum (-Al₂O₃), kalsilite (KAlSiO₄), and potassium aluminum hexafluoride (K₃AlF₆·0.5H₂O)] were clearly identified by powder X-ray diffraction and electron-probe microanalysis. For phlogopite, the unit-cell parameters a₀ and b₀ decrease whereas c₀ increases with increasing Al-content (x). The average crystal size of phlogopite is about 1–2 µm. The 29Si MAS NMR spectra show up to four resonances at approximately –91, –87, –83, and –80 ppm, which can be assigned as Q3(n Al) signals with n = 0–3. The ideal Si/IVAl ratio calculated from the initial composition is always lower than that derived experimentally. Hydroxyl-rich compositions indicate an increased Al-content in the tetrahedral sheets, suggesting a stabilizing effect on the formation of Al-rich phlogopite. These conclusions are supported by 1H and 19F MAS NMR spectra. The 1H MAS NMR spectra show a water signal at 4.7 ppm, a signal due to Mg₂AlOH at 1.8 ppm, and a signal due to Mg₃OH at 0.7 ppm. The 19F MAS NMR spectra exhibit a signal for Mg₂AlF at –150 ppm, one due to AlF₆ (K₃AlF₆·0.5H₂O) at –157 ppm, and one for Mg₃F at –174 ppm. Comparison of the 1H and 19F MAS NMR spectra for different Al- and F-contents reveals a non-statistical distribution of F- and OH-groups at the crystallographic sites in the octahedral sheets where F prefers sites coordinated by three Mg, and OH sites with Al in the next-nearest coordination sphere.