On Li-bearing micas: estimating Li from electron microprobe analyses and an improved diagram for graphical representation

Abstract Lithium may constitute an essential element in micas, yet it cannot be detected by the electron microprobe. Since Li is critical for correctly classifying micas and properly calculating their formulae, several methods have been proposed to overcome this analytical deficiency. We offer empirical relationships between Li 2 O and SiO 2 , MgO, F, and Rb in trioctahedral micas, and between Li 2 O and F as well as Rb in dioctahedral 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 diagram with the axis variables [Mg-Li] and [Fe tot + Mn + Ti-Al VI ] for 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 composition and octahedral site occupancy, and it also allows compositional relationships between Li-bearing and Li-free mica varieties as well as between trioctahedral and dioctahedral micas to be displayed on a single, two-dimensional diagram.