Influence of the octahedral cationic-site occupancies on the framework vibrations of Li-free tourmalines, with implications for estimating temperature and oxygen fugacity in host rocks

Tourmalines, XY3Z6T6O18(BO3)3V3W, are excellent petrogenetic indicators as they capture the signature of the host-rock bulk composition. Raman spectra of tourmalines can be used as fingerprints for species identification and crystal-chemical analysis. While Li-bearing species are directly distinguishable by the shape of the OH-stretching vibrations, the discrimination of Mg- and Fe-dominant species can be hindered by the coexistence of at least two types of octahedrally coordinated Rn+ cations. Thirty Li-free tourmaline samples comprising 14 different species were studied by Raman spectroscopy and electron microprobe. All nine Fe3+-bearing samples were also analyzed by single-crystal X‑ray diffraction and Mössbauer spectroscopy. The Raman scattering analysis shows that Mg-dominant species can be immediately distinguished from Fe-dominant species by the shape of the vibrational modes at ~200–240 cm–1 arising from the YO6 vibrations. Trivalent Fe can be observed and quantified by shifts of the framework vibrations toward lower wavenumbers. The position of the main ZO6 vibrational mode (275–375 cm–1) can be used to determine the ZFe3+ content, while the YFe3+ content can be inferred from the position of the peak at ~315 cm–1. Fits to the data points indicate that the homovalent substitution of Fe3+ for Al3+ leads to a considerably larger downward shift of the ZO6 vibrational mode than the heterovalent substitution Mg2+ for Al3+. The intensity ratio of the two major YO6 vibrational modes (200–240 cm–1) of the fully characterized Fe3+-bearing samples reflects the amount of Y-site Mg and thus can be used to deduce the site-occupancy disorder of Mg over the Y and Z site for tourmaline species with Mg ≤2 apfu. By combining the information from framework and OH-stretching vibrations, Raman spectroscopy alone can be used as a micrometer-scale sensitive non-destructive method for the analysis of tourmaline crystal chemistry including trivalent Fe, which is the major tracer for oxygen fugacity and central for intersite geothermometry.