ON LI-BEARING MICAS - ESTIMATING LI FROM ELECTRON-MICROPROBE ANALYSESAND AN IMPROVED DIAGRAM FOR GRAPHICAL REPRESENTATION

Lithium may constitute an essential element in micas, yet it cannot be detected by the electron microprobe. Since Li is critical for correct ly classifying micas and properly calculating their formulae, several methods have been proposed to overcome this analytical deficiency. We offer empirical relationships between Li2O and SiO2, MgO, F, and Rb in trioctahedral micas, and between Li2O and F as well as Rb in dioctahe dral micas. The resultant regression equations enable lithium contents to be sufficiently well estimated from EPM analyses within the range of validity discussed. Secondly, we introduce an easy to handle, new d iagram with the axis variables [Mg-Li] and [Fe-tot + Mn + Ti-Al-VI] fo r graphical representation and discuss its scientific rationale. Being based on absolute abundances of cations in the octahedral layer, the diagram provides a simple means to classify micas in terms of composit ion and octahedral site occupancy, and it also allows compositional re lationships between Li-bearing and Li-free mica varieties as well as b etween trioctahedral and dioctahedral micas to be displayed on a singl e, two-dimensional diagram.