Micas are the most common hosts of lithium in granitoid igneous rocks. Unfortunately, their Li contents cannot be determined by electron-probe microanalysis (EPMA) which is the most common method of mineral analysis. In an effort to avoid the use of other, technically more complex and expensive methods, several empirical schemes for the estimation of Li-contents from EPMA data have been developed. The methods proposed by Tischendorf (Mineralogical Magazine, 1997) have found the widest application. After 25 years of common usage, we have evaluated these methods by direct Li determination using laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS). Approximately 3000 spot analyses of Li in micas from eight areas worldwide obtained by LA–ICP–MS were compared with the values yielded by the methods of Tischendorf. We conclude that none of the lithium estimation methods can compensate fully for a real local analysis by LA–ICP–MS or secondary-ion mass spectrometry (SIMS). Generally, SiO2-based estimation for trioctahedral micas provides a better match to the analysed values than F-based estimation for dioctahedral micas. The Rb-based estimation for dioctahedral micas does not provide acceptable results. The usage of averaged Si- and F-based estimations can be accepted in common petrological studies for a general characterisation of mica species. Large errors of individual spot estimations preclude their usage in detailed mineralogical studies.

This paper presents the results of an experimental study of the miscibility gap between trioctahedral and dioctahedral micas in the system K2O Li2O-MgO-FeO-Al2O3-SiO2-H2O-HF at 600°C under 2 kbar PH2O. The existence of this miscibility gap is known from previous experimental studies. The gap is large in the lithium-free system; its width reduces progressively with increasing Li content; for sufficient Li contents (Li > 0.6 atom per formula unit, based on 11 oxygens), a single Li-mica phase is obtained, intermediate between trioctahedral and dioctahedral micas. Any bulk composition located inside the miscibility gap gives an assemblage of two micas, one of the biotite-type and one of the muscovite-type. All the compositions located outside the gap, and, in particular, those belonging to the joins phlogopite-trilithionite and muscovite-zinnwaldite (or its magnesian equivalent) give a single mica phase, provided that the fluorine content is sufficient. The ratio Li/F ≈ 1 is a convenient suitable value. The types of micas and the evolutions of their compositions are well characterized by their interplanar distance d060. These experimental results allow the interpretation of most compositions of naturally occurring lithium micas, in the range 0 ⩽ Li ⩽ 1 a./f.u. Natural micas of biotite-type and muscovite-type are located on both sides of the miscibility gap and their compositions get closer with increasing Li content.