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 alltypes of detrital sediments and their complex structural formula (XY3Z6(T6O18)(BO3)3V3W), tourmalines have attracted strong interest in provenance studiessince, from their chemical composition, it is possible to reconstruct the sourcerocks in ancient sediments. Dravite and schorl, belonging to the alkali sub-group 1, are the most abundant tourmaline species, and they have differentY-site compositions in the unit cell: dravite has more magnesium, while schorlhas 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 quickand easy measurements that provide chemical and structural information ontourmalines with a high spatial resolution, thus allowing analysis of smallgrains that could be found in sediments. In this work, we correlated the rela-tive Mg–Fe content (x=Mg/(Mg+Fe)) in different tourmaline samples fromthe dravite–schorl series (Na MgxFe1xðÞ3Al6Si6O18ðÞBO3ðÞ3OHðÞ3OH) withvariations in Raman spectrum parameters in order to find a model for quicktourmaline identification useful for provenance studies. The chemical compo-sitions of the analysed tourmalines are obtained by scanning electron micro-scope coupled with energy-dispersive spectroscopy (SEM-EDS). Ramanmeasurements with a portable spectrometer have also been performed in orderto evaluate our results for in situ applications.