Study of Mg-Fe content in tourmalines from the dravite-schorl series 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 (NaMgxFe1-x3Al6Si6O18BO33OH3OH$$ Na{\left({Mg}_x{Fe}_{1-x}\right)}_3{Al}_6\left({Si}_6{O}_{18}\right){\left({BO}_3\right)}_3{(OH)}_3 OH $$) 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. Tourmalines belonging to the dravite-schorl series are analysed, correlating variations in magnesium and iron relative content [X = Mg/(Mg + Fe)] with changes in Raman peaks parameters in both the fingerprint and OH stretching regions. Changes in P2 peak position, P1-P2 peaks relative intensities and WOH3 peak position are detected depending on the Mg-Fe ratio, allowing us to distinguish between dravite and schorl samples and obtain information on their chemical composition.image