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 - x Al x )(Al 1 + x Si 3 - x O 1 0 )(OH) y (F) 2 - y (0.0 ≤ x ≤ 0.8 and 0.5 ≤ y ≤ 1.8) were studied using 2 9 Si, 1 H, and 1 9 F 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 (α-Al 2 O 3 ), kalsilite (KAlSiO 4 ), and potassium aluminum hexafluoride (K 3 AlF 6 .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 μm. The 2 9 Si MAS NMR spectra show up to four resonances at approximately -91, -87, -83, and -80 ppm, which can be assigned as Q 3 (n All signals with n = 0-3. The ideal Si/ I V Al 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 1 H and 1 9 F MAS NMR spectra. The 1 H MAS NMR spectra show a water signal at 4.7 ppm, a signal due to Mg 2 AlOH at 1.8 ppm. and a signal due to Mg 3 OH at 0.7 ppm. The 1 9 F MAS NMR spectra exhibit a signal for Mg 2 AlF at -150 ppm. one due to AlF 6 (K 3 AlF 6 .0.5H 2 O) at -157 ppm, and one for Mg 3 F at -174 ppm. Comparison of the 1 H and 1 9 F 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.