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

Abstract

Recent investigation showed that the CO 2 solubility in silicate melts is strongly affected by the #Mg (MgO/MgO+CaO). CO 2 solubility is decreasing with increasing #Mg implying that CO 2 molecules dissolves in the vicinity of Ca atoms to form CO 3 2molecular groups. We have investigated several CO 2-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 Mg 2+ and Ca 2+ cations. Analyses of the Mg XANES region show that the Mg environment is not affected by the presence of CO 2 dissolved as CO 3 2groups regardless of the CO 2 content. The Mg K-edge EXAFS simulations and XANES modelling show that the average Mg-O 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 firstcoordination shell is negatively correlated with the CO 2 content and shifts from 4051.4 to 4050.8 eV with CO 2 increasing from 0 to 17.2 wt.%. The Ca EXAFS simulations showed that Ca-O average distance is longer (~2.5Å) than the Mg-O distance. Furthermore, we observed that the Ca coordination number is positively correlated with the CO 2 content: Ca coordination number increases from 7 to 9 for CO 2 content changing from 0 to 17.2 wt.%. Mg and Ca XAS results suggest that CO 3 2groups are dissolved in the vicinity of Ca 2+ cations and not in the surrounding of Mg 2+ cations which environment remains unaffected by CO 2 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 CO 2 transport capacity for Mg-rich silicate melts such as komatiites or kimberlites.

Iodine local environment in high pressure borosilicate glasses: An X-ray photoelectron spectroscopy and X-ray absorption spectroscopy investigation

“…Prior to the simulation, the EXAFS spectra were calibrated for the scattering amplitude and corrected for the edge position [56]. We simulated the spectrum obtained on CaI 2 with the CaI 2 crystalline model.…”

Titanium in GGBS-like calcium-magnesium-aluminosilicate glasses: Its role in the glass network, dissolution at alkaline pH and surface layer formation

“…The XANES spectrum of the slag core was composed of four main features, located at about 1306.1 eV for a pre-edge peak P, 1309.2 eV and 1314.2 eV for two main features A and B, and 1326.0 eV for a broader peak C. Previous studies on Mg-bearing minerals and silicate glasses have attributed these features to various Mg environments. [51][52][53][54][55] The pre-edge peak P was assigned to a highly asymmetric environment of the first coordination shell. Peak A was related to the coordination number of Mg, as its position shifts towards higher energies when the coordination number increases from [4] Mg (1307.2 eV) to [8] Mg (1309.7 eV) in various Mg-bearing minerals.…”

High S and high CO2 contents in haplokimberlite: An experimental and Raman spectroscopic study

“…One hypothesis for explaining the absence of S effect on CO2 solubility is that CO2 and S have two distinct dissolution mechanisms with different surrounding cations. Recent work (Morizet et al 2019) showed that CO3 2groups in depolymerized glasses are mostly dissolved in the surrounding of Ca 2+ cations (and not Mg 2+ ). Currently, the cationic affinity for SO4 2groups is unknown.…”