Aluminum ordering and clustering in Al-rich synthetic phlogopite: The influence of fluorine investigated by {19F/1H} 29Si CPMAS NMR spectroscopy

The influence of fluorine on cationic and anionic ordering in the mica mineral phlogopite has been investigated using 29Si, 1H, and 19F MAS as well as {1H}/{19F} 29Si CPMAS and CP-depolarization NMR spectroscopies. It can be shown that the mere presence of fluorine achieves a tremendous loss of capability to incorporate aluminum into the phlogopite structure. Fluorine is usually located in Mg-rich octahedral and Si-rich tetrahedral clusters of the phlogopite structure while hydroxyl groups are located in Al-rich octahedral and tetrahedral clusters as derived from {1H}/{19F} 29Si CPMAS NMR spectroscopies. The ordering effect in these two basic structural clusters can also be proven by a smaller 29Si linewidth in the {19F} 29Si CPMAS NMR experiments compared to the usual 29Si MAS NMR experiment showing a stronger ordering of Si environments near the two different anion types fluorine and hydroxyl. Intensities of the {1H}/{19F} 29Si CPMAS NMR signals as function of the contact-time show a deviation from the classical I-S model and can be attributed to the I-I*-S model. Time constants like the proton/fluorine spin diffusion time (Tdf), the spin-spin relaxation time (T2), the parameter (), and the proton/fluorine spin-lattice time in the rotating frame (T1) were extracted to give information about the local structure.