Estimation of lithium contents in trioctahedral micas using microprobe data; application to micas from granitic rocks

An empirical relationship between Li (sub 2) O and SiO (sub 2) contents in lithium-bearing trioctahedral micas is utilised as the basis of a procedure to estimate Li (sub 2) O contents from electron microprobe-derived analyses of such micas. The relationship has been defined by fitting a least squares regression line to a dataset of over 250 quality-screened published analyses. As a result it is proposed that approximate lithium contents may be calculated for a wide range of lithium-bearing trioctahedral micas from the following relationship (expressed in wt.% oxide): Li (sub 2) O = (0.287 X SiO (sub 2) ) - 9.552. Evaluation of the method with a dataset of over 500 microprobe analyses of micas from British granites indicates that it is generally applicable to many trioctahedral micas (e.g. siderophyllite, zinnwaldite, lepidolite). However, it is not applicable to those with high MgO (> 8.0 wt.%; thus excluding phlogopite and eastonite), neither is it applicable to dioctahedral micas (e.g. muscovite). The microprobe technique, which is the most universal method used today to analyse minerals and which in other respects is ideal for analysis of micas, cannot detect lithium. Therefore the estimation of lithium contents is potentially of considerable value to igneous petrologists and mineralogists working on lithium micas as it allows a more realistic calculation of structural formulae for microprobe analyses.