Quantification of ferric iron content in minerals via the STEM-EELS-mapping method†

Abstract

The oxidation state of iron (Fe) in minerals is crucial for Earth and planetary sciences. Accurate determination of the oxidation state of Fe aids in the estimation of redox conditions during mineral formation and thus the reconstruction of the evolutionary history of Earth and other solid celestial bodies. Compared to conventional methods that provide the average bulk Fe³⁺/ΣFe ratio in minerals, the recently developed scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) technique enables the accurate determination of Fe oxidation states at sub-micrometer scales. However, the focused electron beam in STEM mode can induce the generation of Fe³⁺ and consequently introduces uncertainty in the quantification of Fe³⁺ in minerals. In this study, a calibration relationship was established between the relative ratios of the Fe-L₃ and Fe-L₂ peak areas in the EELS spectra and the Fe³⁺/ΣFe ratio based on the characterization of a series of pyroxene reference materials with varying Fe³⁺ contents. The EELS spectra were collected in the STEM-EELS-mapping mode using focused ion beam (FIB)-derived sections. The impacts of sample preparation, data acquisition, and data processing procedures on the determination of the Fe³⁺ contents in minerals were examined and discussed to gain insights into the constraints of parameters for accurate quantification of Fe³⁺ in minerals. The obtained calibration relationship was further validated for the accurate determination of Fe³⁺ content in other Fe-bearing minerals, suggesting its promising applicability in the quantification of Fe³⁺ in precious nanoscale terrestrial and extraterrestrial materials.