Lorenzo Pasetti

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 (Fe ²⁺ ), which led to a misrepresentation of the Fe ³⁺ content in certain samples. The model has been thus implemented in this work by using μXANES, enabling the accurate quantification of Fe ²⁺ and Fe ³⁺ in dravite–schorl minerals, hence refining Mg/(Mg + Fe ²⁺ ) 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 Fe ²⁺ and Fe ³⁺ 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 + Fe ²⁺ ) ratio. The observed linear correlations for P ₂ peak position, P ₁ / P ₂ relative intensities, and ^W OH(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 Fe ²⁺ contents.

Due to their appealing colors, gem quality tourmalines, particularly the blue to green Cu- and Mn-bearing Li-tourmalines known as the Paraíba type, have been of significant interest since their discovery at the end of 1980s. At the same time, the demand of other similar colored tourmalines increased. Most Paraíba-type tourmalines belong to the elbaite species; however, liddicoatite gems can also be found. Recognizing and classifying various tourmaline species, especially these valued Paraíba-type tourmalines, are important for geologists, mineralogists, mineral collectors, and gemologists. This study explores the application of Raman spectroscopy in random crystal orientations to distinguish between the elbaite and liddicoatite tourmaline species. Raman spectra were collected from faceted blue to green Li-tourmalines alongside chemical analysis using EDXRF (Energy Dispersive X-ray Fluorescence), UV-Vis-NIR (Ultraviolet-Visible-Near InfraRed Spectroscopy), and PL (Photoluminescence spectroscopy) to provide comprehensive characterization. The results show that Raman spectroscopy, particularly in the OH stretching region, is a useful tool for differentiating elbaite from liddicoatite, and this identification remains consistent regardless of crystal orientation. The fingerprint region in the Raman spectra, on the other hand, is orientation-dependent and can only differentiate the two species when detected in specific orientations. Furthermore, Paraíba-type tourmalines can be identified by visible-near infrared (Vis-NIR) spectroscopy, although not by Raman spectroscopy.

Thanks to their high chemical and mechanical stability, their diffusion in all types of detrital sediments and their complex structural formula (XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W), tourmalines have attracted strong interest in provenance studies since, from their chemical composition, it is possible to reconstruct the source rocks in ancient sediments. Dravite and schorl, belonging to the alkali subgroup 1, are the most abundant tourmaline species, and they have different Y‐site compositions in the unit cell: dravite has more magnesium, while schorl has a higher iron content. For this reason, it is important to measure the Mg–Fe relative content in order to classify the analysed tourmalines in the dravite–schorl series. Raman spectroscopy is a suitable technique as it allows quick and easy measurements that provide chemical and structural information on tourmalines with a high spatial resolution, thus allowing analysis of small grains that could be found in sediments. In this work, we correlated the relative Mg–Fe content (x = Mg/(Mg + Fe)) in different tourmaline samples from the dravite–schorl series ( ) with variations in Raman spectrum parameters in order to find a model for quick tourmaline identification useful for provenance studies. The chemical compositions of the analysed tourmalines are obtained by scanning electron microscope coupled with energy‐dispersive spectroscopy (SEM‐EDS). Raman measurements with a portable spectrometer have also been performed in order to evaluate our results for in situ applications.