X-ray absorption spectroscopic investigation of the Ca and Mg environments in CO2-bearing silicate glasses

Abstract

Recent investigation showed that the CO2 solubility in silicate melts is strongly affected by the #Mg (MgO/MgO + CaO). CO2 solubility is decreasing with increasing #Mg implying that CO2 molecules dissolves in the vicinity of Ca atoms to form CO32– molecular groups. We have investigated several CO2-bearing (up to 17.2 wt%) silicate glasses using X-ray Absorption Spectroscopy (XAS) at the Mg and Ca K-edge in order to determine the structural environments of Mg2+ and Ca2+ cations. Analyses of the Mg XANES region show that the Mg environment is not affected by the presence of CO2 dissolved as CO32– groups regardless of the CO2 content. The Mg K-edge EXAFS simulations and XANES modelling show that the average MgO distance is close to 2 Å and the average Mg coordination is close to 6.On the contrary, the position of the Ca XANES peak main resonance related to first-coordination shell is negatively correlated with the CO2 content and shifts from 4051.4 to 4050.8 eV with CO2 increasing from 0 to 17.2 wt%. The Ca EXAFS simulations showed that CaO average distance is longer (~2.5 Å) than the MgO distance. Furthermore, we observed that the Ca coordination number is positively correlated with the CO2 content: Ca coordination number increases from 7 to 9 for CO2 content changing from 0 to 17.2 wt%.Mg and Ca XAS results suggest that CO32– groups are dissolved in the vicinity of Ca2+ cations and not in the surrounding of Mg2+ cations which environment remains unaffected by CO2 dissolution. The tighter Mg atomic environment as well as the lower coordination number for Mg cations as compared to Ca atomic environment could explain such behaviour. One implication of this result is the lower CO2 transport capacity for Mg-rich silicate melts such as komatiites or kimberlites.