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

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(Mg3-xAlx)(Al1+xSi3-xO10)(OH)(y) (F)(2-y) (0.0 less than or equal to x less than or equal to 0.8 and 0.5 less than or equal to y less than or equal to 1.8) were studied using Si-29, H-1, and F-19 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 (alpha-Al2O3), kalsilite (KAlSiO4), and potassium aluminum hexafluoride (K3AlF6.0.5H(2)O)] were clearly identified by powder X-ray diffraction and electron-probe microanalysis. For phlogopite, the unit-cell parameters a(0) and b(0) decrease whereas c(0) increases with increasing Al-content (x). The average crystal size of phlogopite is about 1-2 mum. The Si-29 MAS NMR spectra show up to four resonances at approximately -91,-87, -83, and -80ppm, which can be assigned as Q(3)(nAl) signals with n = 0 - 3. The ideal Si/Al-IV 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 H-1 and F-19 MAS NMR spectra. The H-1 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 F-19 MAS NMR spectra exhibit a signal for Mg2AlF at -150 ppm, one due toAlF(6) (K3AlF6.0.5H(2)O) at -157 ppm, and one for Mg3F at -174 ppm. Comparison of the H-1 and F-19 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.