CO₂ in minerals of the cancrinite-sodalite group: pitiglianoite

This paper presents spectroscopic evidence for the presence of CO2 molecules within the structural channels of a member of the cancrinite-sodalite group of feldspathoids. The study was carried out using single-crystal micro-FTIR spectroscopy on a sample of pitiglianoite from Monte Cavalluccio (Bonaccorsi & Orlandi, 1996). A sharp and prominent band is observed in the medium-infrared range at 2351 cm-1, which can be assigned to carbon dioxide molecules in the studied sample. Polarized-light FTIR spectra collected on a 112-μm-thick [001] section show maximum absorption with E ⊥ c, suggesting that the linear CO2 molecules are oriented perpendicular to the crystallographic c axis, as in beryl (Wood & Nassau, 1967) or cordierite (Armbruster & Bloss, 1980). In situ high-T FTIR data collection up to 550°C shows that the CO2 band decreases in intensity and broadens with increasing T; release of carbon dioxide from the structure starts at T > 250°C and is complete at ~ 450°C. The same kind of behaviour is observed for structural H2O; upon heating pitiglianoite becomes virtually anhydrous at T ~ 500°C. These data suggest that the extraframework volatile composition of cancrinite-type minerals is strongly T dependent, hence the analysis of pitiglianoite may provide geothermometric information on the genetic environment (typically late-stage volcanic) of these minerals