Improved calibrations for Raman-spectroscopic determinations of CO2 in cordierite using three excitation wavelengths (488, 515 and 633 nm)

Abstract

Ten natural cordierite single crystals with X Fe in the range 0.044-0.76 and 0.12 to 1.97 wt% CO 2 were investigated with Raman microspectroscopy using three different wavelengths. Improved calibration diagrams for CO 2 determinations in single crystals and thin sections for the 488, 515 and 633 nm laser excitation wavelengths show significantly lower standard deviations for CO 2 determinations (AE0.04 to 0.07 wt%) compared to an earlier study done solely at 633 nm. The diagrams are based on the linear relationship between CO 2 in wt.% and the ratios of the Raman peak of CO 2 to two Si-O-stretching vibrations. The consideration of orientation dependencies and the improvement of background corrections and spectra fitting procedures also contributed to the significantly higher precision. Because Fe-incorporation in cordierite leads to structural changes that result in Raman peak shifts and peak broadening, including peaks considered in this investigation, a correction was introduced for precise CO 2 determination on Fe-bearing samples. This improved CO 2 determination method was then applied on a 2D spatial scale to two cordierite grains in a thin section of a granulite-facies metapelite from Kösseldorf in the Sauwald area (southern Bohemian Massif, Upper Austria), which yielded CO 2 contents of 0.27-0.31 AE0.07 wt%.