Ordering of tetrahedral and octahedral layers in synthetic lepidolites and Li-muscovites: two-dimensional { ¹ H} →  ²⁹ Si ∕  ²⁷ Al CPMAS HETCOR and { ¹⁹ F} →  ²⁹ Si ∕  ²⁷ Al CPMAS HETC

Abstract. Cationic ordering in the octahedral and tetrahedral sheets of synthetic lepidolites with composition K(LixAl3−x)[Si2xAl4−2xO10](OH)yF2−y and Li-muscovites with K(LixAl2-x/3□1-2x/3)[Si3AlO10](OH)yF2−y has been investigated using two-dimensional (2-D) {1H/19F}→29Si/27Al cross-polarisation magic-angle-spinning (CPMAS) heteronuclear correlation (HETCOR) solid-state nuclear magnetic resonance (NMR) spectroscopic experiments. The nominal composition ranged between (x=1.5–2.0; y=0.0–2.0) and (x=0.0–1.5; y=0.0–2.0), respectively. {1H}→29Si CPMAS HETCOR solid-state NMR spectra show a clear preferred neighbourhood of the octahedral Li2Al-OH environments close to tetrahedral Si(OSi)3. On the other hand, the LiAl2-OH group shows a preference to be close to Si(OSi)3 and Si(OAl)(OSi)2 environments in the tetrahedral sheet. {19F}→29Si CPMAS HETCOR solid-state NMR spectra show similar results of the Li2Al-F environments close to Si(OSi)3 and LiAl2-F groups to be close to the Si(OSi)3 and Si(OAl)(OSi)2 environments. However, each 19F NMR signal is split into two signals. One component of the Li2Al-F signal shows a slightly stronger correlation to the Si(OAl)2(OSi) groups. Signals resulting from the LiAl2-F groups show much lower signal-to-noise ratio, as F prefers to be coordinated by Li2Al. {1H}→27Al 2-D CPMAS HETCOR NMR spectroscopy shows a direct neighbourhood of OH groups to both octahedral and tetrahedral Al but only for OH coordinated by LiAl2. For Li2Al coordinations, only a correlation to octahedral Al is visible. In the {19F}→27Al CPMAS HETCOR experiments, the splitting of the Li2Al-F signal can be also observed here with a stronger correlation to the [4]Al sites. All results show an ordering of the tetrahedral sheets depending on the octahedral composition. The OH-/F- distribution shows a tendency of an incorporation of F− next to Li-rich environments and of OH− anions next to Al-rich environments.