Ferric/ferrous iron ratios in sodic amphiboles: Mossbauer analysis, stoichiometry-based model calculations and the high-resolution microanalytical flank method

Although the electron microprobe has become the standard microanalytical tool in modern geosciences, conventional electron microprobe analysis does not allow determination of the valence states of elements such as Fe. The correct classification of minerals and interpretation of reaction microfabrics and grain zonation require high-quality information on ferric/ferrous ratios on a scale of micrometers. The flank method developed by Hofer et al. (1994, Eur J Mineral 6:407-418) has revived new interest in electron-induced X-ray-spectroscopy to resolve oxidation states in minerals with high spatial resolution. We have recharacterized well-documented sodic amphiboles of the glaucophane-ferroglaucophane-riebeckite-magnesioriebeckite series by electron probe microanalysis and combined the microanalytical data with ferric/ferrous ratios from Mossbauer spectroscopy, Li data from bulk ICP-AES analysis and H2O data from bulk Karl-Fischer titration. The combination of microanalysis and high-quality analysis on the bulk materials results in a data set that allows comparison of model-based stoichiometric calculations and the calibration of the high-resolution flank method. The calibration obtained allows ferric/ferrous ratios to be determined within an error of +/- 5%. We have found it necessary to apply an empirical correction for absorption phenomena. The advantages of the method must be weighed against the complex calibration procedures necessary and thus the flank method will probably not find use as a routine method. However, in cases where high-resolution data in terms of valence state are needed, the flank method will provide useful data on ferric/ferrous ratios down to minimum FeOtotal content of 6-8 wt%.