QUANTITATIVE DETERMINATION OF THE OXIDATION STATE OF IRON IN BIOTITE USING X-RAY PHOTOELECTRON SPECTROSCOPY: I. CALIBRATION

Abstract

X-ray photoelectron spectroscopy (XPS) analyses of Fe(III)ΣFe were calibrated with nine single crystals of biotite of known Fe(III)ΣFe content. Peak shape parameters for the component Fe2+ and Fe3+ Fe 3p peaks were obtained by a constrained least squares fitting method that minimized the difference between Fe(III)ΣFe determined by XPS and wet chemistry/electron microprobe (WCEM) analyses. Fe2+ and Fe3+ peak separation was estimated from the separation of minima in the second derivative of Fe 3p spectra. The single set of derived peak parameters yielded a good linear correlation (r = 0.87) between XPS and WCEM values over the sample range of 2-20% Fe(III)ΣFe. Target transformation factor analysis of a time series of spectra for a sample displaying progressive oxidation during XPS analysis resulted in Fe2+ and Fe3+ component peak shapes largely consistent with the constrained least squares fitting method. Peak asymmetry for the Fe3+ component was the only parameter that differed for the two methods. Beam damage, which appeared to be restricted to three single crystals with low ΣFe, low FeMg, and high Fe(III)ΣFe, caused increases in Fe(III)ΣFe that were proportional to the duration of sample exposure.