Sodalite-Group Minerals From the Somma-Vesuvius Volcano (Naples, Italy): A Combined EPMA, SIMS, and FTIR Crystal-Chemical Study

Abstract

This work is focused on the crystal-chemical characterization of the cationic and anionic components of sodalite-group minerals (SGM) occurring in various igneous-to-metamorphic rocks and ejecta from the alkaline-potassic Somma-Vesuvius volcano (southern Italy). A combination of different analytical methods, i.e. , XRD, SEM, EPMA, SIMS, FTIR, and μ-FTIR-FPA imaging, have been used. Minor and trace volatile elements (H, C, and F) have been quantified for the first time (to the authors’ knowledge) by SIMS, and in situ ion probe data were integrated with information derived from FTIR spectroscopy. The studied samples correspond to sodalite sensu stricto , nosean and hauyne. SIMS measurements for H, F, and C (quantified as H2O, F, and CO2 wt.%, respectively) show contents 0.02–5.0 wt.% H2O, 0.01–0.14 wt.% F, and 0.08–2.95 wt.% CO2. Within the single crystals, the F and, to a lesser extent, CO2 concentrations are homogeneous, whereas, in terms of H2O, they span from relatively homogenous to strongly zoned. Single-crystal FTIR spectra of SGM can be grouped into sodalites and noseans/hauynes, according to the occurrence of the 12CO2 absorption at 2340 cm–1. The absorption due to H2O and/or OH groups occurs as a very broad band extending from 3700 cm–1 to 3000 cm–1. In all samples FTIR data show the presence of CO32–. The collected data allow us to fully characterize SGM from Somma-Vesuvius, providing also some constraints regarding their genesis. Present data can contribute to a more detailed understanding of the crystal chemistry of SGM, their geological environments of formation, and possibly of technological characteristics of their synthetic analogues.