The correlation between lithium and magnesium in trioctahedral micas: Improved equations for Li2O estimation from MgO data

A major disadvantage of the electron microprobe is its inability to determi ne Li, which may make up an essential component in micaceous minerals. Corr ect classification of micas and proper calculation of their formulae requir e alternative methods. One of these is the indirect estimation of the lithi um concentration by empirical approaches based on element correlations. Rel ationships between Li2O and SiO2 have long been used for this purpose, howe ver, they are valid only for a limited range of mica compositions (i.e. Li- rich, Mg-poor varieties). In this paper me report the results from a renewed study of the correlation between Li2O and MgO contents in trioctahedral micas, It is demonstrated t hat the relations between both oxides are strongly dependent upon the geolo gical environment in which the mica was formed. It is necessary to distingu ish a 'normal group', which comprises micas from 'common' S- and I-type mag matic rocks and most metamorphic rocks, a 'low Li-Mg group', to which belon g Fe-rich micas from non-alkaline rocks of A-type affinity, and a 'high Li- Mg group' which includes micas from either peraluminous or peralkaline igne ous rocks and Mg-enriched wall-rocks. These wall Pocks supplied additional magnesium to the mica-forming environment. We offer empirical expressions r elating Li2O and MgO contents for these three types which: in many cases, a llow a sufficiently good estimation of the lithium content in trioctahedral micas of widely varying compositions.