Semi-quantitative determination of the Fe/Mg ratio in synthetic cordierite using Raman spectroscopy‡

Investigations of H2O-bearing synthetic well-ordered Mg-Fe-cordierites (XFe = 0–1) with micro-Raman spectroscopy revealed a linear correlation between the Fe/Mg ratio and the position of certain Raman peaks. In the range between 100 and 1250 cm−1, all peaks except for three peaks shift toward lower wavenumbers with increasing XFe as a consequence of the substitution of the lighter Mg by the heavier Fe atom on the octahedral sites and the associated structural changes. Selected medium and strong peaks show a shift of 5 to 13 cm−1, respectively. Based on recent quantum-mechanical calculations (Kaindl et al. 2011) these shifts can be attributed to specific vibrational modes in the cordierite structure, thus showing that the Mg-Fe exchange affects the vibrational modes of tetrahedral, octahedral, and mixed sites. The peaks (wavenumber Mg-/Fe-cordierite) at 122/111, 262/257, 430/418, 579/571, 974/967, and 1012/1007 cm−1 were selected for detailed deconvolution analysis. The shifts of these peaks were then plotted vs. XFe and regression of the data lead to the formulation of a set of linear equations. Assuming ideal Fe-Mg mixing of the end-members and using linear peak shifts then allows the development of calibration diagrams for the semi-quantitative determination of the Fe-Mg contents of cordierite samples with Raman spectroscopy. In addition, the effect of different H2O contents and the degree of Al-Si ordering on the Fe/Mg determination were also investigated. Testing the calibration against data from six well-characterized natural cordierite samples yielded excellent agreement.