Improving the Raman Model for Dravite and Schorl Tourmalines by μXANES Analysis of Iron Oxidation States

Tourmalines, a complex borosilicate mineral supergroup, are significant in geological studies due to their chemical and mechanical stability across various temperature and pressure conditions, making them useful as source rock indicators. A major issue in the characterization of tourmaline composition is the consideration of the iron oxidation state, which can significantly influence the distribution of elements at each site. This study reports the enhancement of a previous model that correlates Raman spectral parameters of dravite–schorl tourmalines with their composition, taking into account the Fe valence state in Y and Z sites measured through micro-x-ray absorption near edge structure (μXANES) spectroscopy. Raman spectroscopy was employed in a prior study on these two tourmaline species by correlating peak positions and intensities with differences in the magnesium–iron ratio. However, it was assumed that all iron was in the ferrous oxidation state (Fe2+), which led to a misrepresentation of the Fe3+ content in certain samples. The model has been thus implemented in this work by using μXANES, enabling the accurate quantification of Fe2+ and Fe3+ in dravite–schorl minerals, hence refining Mg/(Mg + Fe2+) ratios for Raman spectral analysis. Results demonstrate the validity of the correlation between Raman peaks in both the fingerprint and OH stretching regions and the magnesium–ferrous iron ratio. Our research confirms that Raman spectrum analysis is an effective method for recognizing tourmalines from the dravite–schorl series and evaluating their composition, including now the evaluation of the Fe2+ and Fe3+ occupancy. By integrating μRaman and μXANES techniques, one can acquire insights into the oxidation state of iron in tourmalines from the dravite–schorl series, thereby enhancing the accuracy of the Mg/(Mg + Fe2+) ratio. The observed linear correlations for P2 peak position, P1/P2 relative intensities, and WOH(3) peak position in the Raman spectrum enable the rapid identification of dravite and schorl tourmalines, as well as the retrieval of relative Mg and Fe2+ contents.