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Although CO₂ is the second most abundant volatile component in volcanic gases, relatively little is known on the speciation of CO₂ in silicate melts. IR, Raman and NMR measurements have shown that in quenched silicate glasses CO₂ is dissolved as CO₂ molecules and distorted CO₃^2- groups. Apparently, speciation strongly depends on the degree of polymerization of the glass and on the type of cations present. Furthermore, in quenched albitic melts the CO₂/CO₃^2- ratio depends on temperature and pressure. However, there are no direct measurements of CO₂ speciation in silicate melts above the glass tranaition temperature.

We started a program to measure directly CO₂ speciation in albite glass and melt at high temperatures. Albite glasses with CO₂ concentrations from 0.1 to 1.0 wt% were synthezised in a piston cylinder apparatus at 1300°C and 10 kbar for 60 min. The PtRh sample capsules were embedded in Fe₂O₃ powder to maintain oxidizing conditions during the runs in order to prevent reduction of CO₂. After the experiments thin platelets were cut from the glass cylinders and polished on both sides.

In a first step a glass chip with 1 wt% CO₂ was heated on a 1-atm heating stage to 800°C. The absorption band of the asymmetric stretching vibration of molecular CO₂ at about 2350 cm^-1 was monitored in steps of 100°C using an FTIR spectrometer. Shape and intensity of this absorption band change strongly with increasing temperature (Fig. 3.5-10). At high temperature, the band becomes very broad with an asymmetric tail extending to low frequencies. Possibly, this tail represents some mobile type of CO₂ molecules inside the glass structure.
 
 

Fig. 3.5-10: FTIR spectra of CO2-bearing albite glass to 800°C. CO2 content is about 1 wt%.