Sodalite-group minerals from Somma-Vesuvius volcano (Naples, Italy): a combined EPMA, SIMS and FTIR crystal chemical study

This work is focused on the crystal-chemical characterization of the cationic and anionic components of sodalite-group minerals (SGM), occurring in various igneous-tometamorphic 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 (at 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 haüyne. 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/haüynes, 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 with some constraints about their genesis. Present data can contribute to a more detailed comprehension of crystal chemistry of SGM, their geological environments of formation and possibly of technological characteristics of their synthetic analogues.