QUANTITATIVE DETERMINATION OF THE OXIDATION STATE OF IRON IN BIOTITE USING X-RAY PHOTOELECTRON SPECTROSCOPY: II. IN SITU ANALYSES

Abstract

X-ray photoelectron spectroscopy (XPS) was used to determine Fe(III)ΣFe in individual biotite crystals in thin sections of ten metapelites and one syenite. The in situ XPS analyses of Fe(III)ΣFe in biotite crystals in the metapelites were compared with published Fe(III)ΣFe values determined by Mossbauer spectroscopy (MS) for mineral separates from the same hand samples. The difference between Fe(III)ΣFe by the two techniques was greatest for samples with the lowest Fe(III)ΣFe (by MS). For eight metamorphic biotites with Fe(III)ΣFe = 9-27% comparison of the two techniques yielded a linear correlation of r = 0.94 and a statistically acceptable fit of [Fe(III)ΣFe]xps = [Fe(III)ΣFe]ms. The difference between Fe(III)ΣFe by the two techniques was greater for two samples with Fe(III)ΣFe =< 6% (by MS). For biotite in the syenite sample, Fe(III)ΣFe determined by both in situ XPS and bulk wet chemistry/electron probe microanalysis were similar. This contribution demonstrates that XPS can be used to analyze bulk Fe(III)ΣFe in minerals in thin sections when appropriate precautions are taken to avoid oxidation of the near-surface during preparation of samples.