Can lithium contents in mica be correctly calculated? Tischendorf́s proposal ( Mineralogical Magazine 61/1997) 25 years after

Abstract

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, SiO 2 -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.