Structural roles of CO 2 and [CO 3 ] 2- in fully polymerized sodium aluminosilicate melts and glasses

Details Structural roles of CO 2 and [CO 3 ] 2- in fully polymerized sodium aluminosilicate melts and glasses Structural roles of CO 2 and [CO 3 ] 2- in fully polymerized sodium aluminosilicate melts and glasses

Feb 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59..683k&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 59, Issue 4, pp.683-698

Computer Science

4

Scientific paper

Ab initio, molecular orbital calculations of the structures, energetics, and vibrational spectra of six T 2 O 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 CO 2 is weakly bound to bridging oxygen atoms in T 2 O 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 G ° for T 2 O 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 2 O 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.

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