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

X-ray photoelectron spectroscopy (XPS) analyses of Fe(III)/Sigma Fe we re calibrated with nine single crystals of biotite of known Fr(III)/Si gma Fe content. Peak shape parameters for the component Fe2+ and Fe3Fe 3p peaks were obtained by a constrained least squares fitting metho d that minimized the difference between Fe(III)/Sigma Fe determined by XPS and wet chemistry/electron microprobe (WCEM) analyses. Fe2+ and F e3+ peak separation was estimated from the separation of minima in the second derivative of Fe 3p spectra. The single set of derived peak pa rameters yielded a good linear correlation (r = 0.87) between XPS and WCEM values over the sample range of 2-20% Fe(III)/Sigma Fe. Target tr ansformation factor analysis of a time series of spectra for a sample displaying progressive oxidation during XPS analysis resulted in Fe2and Fe3+ component peak shapes largely consistent with the constrained least squares fitting method. Peak asymmetry for the Fe3+ component w as the only parameter that differed for the two methods. Beam damage, which appeared to be restricted to three single crystals with low Sigm a Fe, low Fe/Mg, and high Fe(II)/Sigma Fe, caused increases in Fe(III) /Sigma Fe that were proportional to the duration of sample exposure. C opyright (C) 1997 Elsevier Science Ltd.