Synthetic lepidolites and Li-muscovites were characterised by nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction. Both Li and F / OH content influence the occurrence of the impurity phases. A solid solution series exists for lepidolites with polylithionite and trilithionite as endmembers but does not between trilithionite and muscovite. NMR investigations indicate there is a preference for incorporating fluorine and OH groups near Li-rich and Al-rich environments, respectively.