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

Abstract

A large number of phlogopites K(Mg3− x Al x )(Al1+ x Si3− x O10)(OH) y F2− 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 x nom = 0.0 to 1.6 and y = y nom = 0.0 to 2.0, i.e . from hydroxyl-phlogopites to OH-free compositions and from trisilicic starting mixtures to extremely Al-rich 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 x est . 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 x est = 0.82 and further addition of Al to the initial oxide mixture did not yield more Al-rich phlogopite.