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X-ray photoelectron spectroscopy characterization of natural chromite from Mercedita Mine (Eastern Cuba): quantification of the Fe³⁺/Fe²⁺ ratio

¹ Cristalografía y Mineralogía, Dpto. Ciencias de la Tierra, Universidad de Zaragoza, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain
² Departmento de Mineralogíay y Petrología and Instituto Andaluz de Ciencias de la Tierra (CSIC-Universidad de Granada), Avda. Fuentenueva s/n, 18002 Granada, Spain
³ Instituto de Nanociencias de Aragon (INA), Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain

^* Corresponding author, e-mail: fanlo{at}unizar.es

The quantitative estimation of the Fe³⁺/Fe²⁺ ratio has a crucial interest to know the upper-mantle oxidation state. The relative proportions of Fe³⁺ and Fe²⁺ are usually estimated from electron microprobe analysis (EPMA) assuming chromite stoichiometry. In order to test a new technique to estimate the Fe³⁺/Fe²⁺ ratio and to avoid EPMA systematic errors, X-ray photoelectron spectroscopy (XPS) has been used to determine the Fe³⁺/Fe²⁺ ratio in four chromite samples from Cuba and the results compared with those obtained by EPMA. Due to the high surface sensitivity of XPS the samples were etched by removing the outermost layers by bombarding with an Ar⁺ ion gun. Therefore, the surface, as well as 8, 16 and 24 nm in-depth levels, were analyzed, revealing that the spectra change significantly with depth. Thus, from the surface down to 24 nm deep, Fe³⁺ concentrations decrease from values up to 10 at.% to values below the detection limit of XPS (0.1 at.%). By contrast, a systematic Fe³⁺ detection (between 1.32 and 4.04 at.%) was observed by EPMA. Consequently, XPS allows us to calculate ferric iron contents, a key datum for the calculation of the real structural formula of chromite. XPS can therefore be considered a useful tool in evaluating mantle oxidation state.

Key-words: chromite, Fe³⁺/Fe²⁺, XPS, EPMA.