Aluminium ordering and clustering in synthetic phlogopite: OH/F influence on the Al-content of phlogopite studied by NMR spectroscopy

A large number of phlogopites K(Mg3-xAlx)(Al1+xSi3-xO10)(OH)yF2-y were synthesised in hydrothermal experiments at a temperature of 873 K and a pressure of 2 kbar using sol-gel mixed oxides. The nominal composition ranged from xnom = 0.0 to 1.6 and y = ynom = 0.0 to 2.0, i.e. from hydroxyl-phlogopites to OH-free compositions and from trisilicic starting mixtures to extremely Alrich ones. The samples have been analysed by 1H, 29Si and 19F MAS NMR spectroscopy as well as 27Al MAS and 3Q-MAS NMR spectroscopy and X-ray diffraction in order to investigate the influence of the F-content on the ability to incorporate Al into the phlogopite structure. Especially, analysis of the 29Si MAS NMR signal areas has been used to estimate the Al-content of the tetrahedral sheets and thus the real Al-content of the phlogopites xest. We observed that the difference in Al-content between F-free and F-bearing compositions is much larger than between samples of mixed F-contents. The presence of F in the structure lowers drastically the phlogopite ability to take up Al, while the exact amount of that element is not that important. A saturation effect has been found: the highest Al-content of pure hydroxyl-phlogopite samples was reached at xest = 0.82 and further addition of Al to the initial oxide mixture did not yield more Al-rich phlogopite.