Solid-state nuclear magnetic resonance investigation of synthetic phlogopite and lepidolite samples

In the present work, our aim is to decipher the cationic ordering in the octahedral and tetrahedral sheets of two Al-rich synthetic materials, namely, phlogopites of nominal composition K(Mg_3-x Al_x )[Al_1+x Si_3-x O₁₀ ](OH)_y F_2-y and lepidolites in the system trilithionite-polylithionite with composition K (Li_x Al_3-x )[Al_4-2x Si_2x O₁₀ ](OH)_y F_2-y , by directly probing the aluminium distribution through ²⁷ Al and ¹⁷ O magic-angle spinning, multiple-quantum magic-angle spinning, and ²⁷ Al-²⁷ Al double-quantum single-quantum nuclear magnetic resonance (NMR) experiments. Notably, ²⁷ Al-²⁷ Al double-quantum single-quantum magic-angle spinning NMR spectra, recorded at 9.34 and/or 20.00 T, show the spatial proximity or avoidance of the Al species inside or between the sheets. In both studied minerals, the ensemble of NMR data suggests a preference for ^[4] Al in the tetrahedral sheet to occupy position close to the ^[6] Al of the octahedral sheets.