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Published February 1995 | public
Journal Article

Structural roles of CO_2 and [CO_3]^(2-) in fully polymerized sodium aluminosilicate melts and glasses


Ab initio, molecular orbital calculations of the structures, energetics, and vibrational spectra of six T_2O_(br)-CO_2 clusters and five T-[CO_3]-T clusters (T = Al or Si, with and without Na^+ present) have been completed using a 3–21G^(∗∗) basis set and the Gaussian 92 program to evaluate possible configurations of CO_2 molecules and carbonate groups in melts and glasses on the NaAlO_2-SiO_2 join. Based on these calculations, we developed the following hypothesis for the solution mechanisms of CO_2 molecules and carbonate groups in fully polymerized melts and glasses on this join. Molecular CO2 is weakly bound to bridging oxygen atoms in T_2O_(br)-CO_2 clusters. Calculated energetics indicate that molecular CO_2 is more strongly bonded when associated with smaller angle T-O-T linkages, and thus may preferentially bond to linkages where at least one T = Al^(3+). [CO_3]^(2−) is most likely present in T-[CO_3]-T linkages. Si-[CO_3]-Al linkages probably form in melts toward the silica-rich end of the NaAlO_2-SiO_2 join; Si-[NaCO_3]-Al and/or Al-[CO_3]-Al become more significant with increasing NaAlO_2 content. The experimentally observed increase in [CO_3]^(2-)/CO_2 ratio accompanying higher NaAlO_2 compositions can be understood in terms of the predicted increasingly negative ΔG° for T_2O_(br)-CO_2 → T-[CO_3]-T reactions when Si-[NaCO_3]-Al and Al-[CO_3]-Al rather than Si-[CO_3]-Si and Si-[CO_3]-Al are the reaction products. In addition, a reaction pathway with a low activation energy was calculated for forming T-[CO_3]-T linkages from T_2O_(br)-CO_2 linkages. This model is consistent with available information on the vibrational and NMR spectra of C-bearing Na-aluminosilicate glasses, and on the relative proportions in these glasses of carbonate and molecular CO_2 and their dependence on pressure, temperature, and composition.

Additional Information

© 1995 Elsevier Science Ltd. Received 22 March 1994. Accepted 14 September 1994. Editorial handling: P. C. Hess. The authors acknowledge the support of NASA grant number NAGW 2320 and NSF grant EAR91-54186 (E. M. Stolper). J.D. Kubicki also acknowledges NSF grant EAR91-17946 (G. A. Blake). Computational facilities were provided by the Molecular Simulation Center (W. A. Goddard) of the Beckman Institute at Caltech and the Jet Propulsion Laboratory. Insightful reviews by A. B. Belonoshko and S.C. Kohn are also greatly appreciated.

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