Aluminum ordering and clustering in Al-rich synthetic phlogopite: The influence of fluorine investigated by 29Si CPMAS NMR spectroscopy

Abstract

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 ( T df ), the spin-spin relaxation time ( T 2), the λ parameter (λ), and the proton/fluorine spin-lattice time in the rotating frame ( T 1ρ) were extracted to give information about the local structure.