Can lithium contents in mica be correctly calculated? Tischendorf's proposal

[Marco E. Ciriotti]

Prossima pubblicazione.

Referenza:
▪ Breiter, K., Vašinová Galiová, M., Korbelová, Z., Hložková, M. (2023): Can lithium contents in mica be correctly calculated? Tischendorf's proposal (Mineralogical Magazine 61/1997) 25 years after. Mineralogical Magazine, 87, (in press).

Abstract:
Micas are the most common hosts of lithium in acid igneous rocks. Unfortunately, Li contents cannot be determined by the electron-probe microanalysis (EPMA), the most common method of mineral analysis. In an effort to avoid the use of other, technically more complex and expensive methods, several methods of Li-contents estimation from the EPMA data have been developed. The methods proposed by Tischendorf (Mineralogical Magazine 1997) have found the widest application. After 25 years of common usage of this method in mineralogical practice, we subjected these methods to a test by a direct Li determination using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). About 3000 spot analyses of Li in micas from eight areas worldwide performed by LA-ICP-MS were compared with the values yielded by the methods of Tischendorf. We concluded that no lithium estimation method can fully make up for a real local analysis by LA-ICP-MS or SIMS. Generally, SiO2-based estimation for trioctahedral micas provides a better match to the analyzed values than F-based estimation for dioctahedral micas. The Rb-based estimation for dioctahedral micas does not provide applicable results. The usage of averaged Si- and F-based estimations can be accepted in common petrological studies for a general characterization of mica species. Large errors of individual spot estimations preclude their usage in detailed mineralogical studies.