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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 10 )(OH) y  (F) 2-y  (0.0 &lt; x &lt; 0.8 and 0.5 &lt; y &lt; 1.8) were studied using  29 Si,  1 H, and  19 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 0)] 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 pm. The  29 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 Al) signals with n = 0 - 3. The ideal Si/ iv 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  19 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  19 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 0) at -157 ppm, and one for Mg 3 F at -174 ppm. Comparison of the  1 H and  19 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.

american mineralogist

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