Comment on “Towards the reconciliation of viscosity change and CO₂-induced polymerization in silicate melts” by Yann Morizeta, Michael Paris, David Sifre, Ida Di Carlo, Sandra Ory, and Fabrice Gaillard [Chemical Geology 458 (2017) 38-47]

The recent paper by Morizet et al. (2017) has presented new ¹⁷O and ²⁹Si MAS NMR data on CO₂-bearing depolymerized aluminosilicate glasses to shed light on the effect of CO₂ dissolution on the structure and properties of silicate melts. These data gave convincing new evidence for the dissolution of CO₂ in silicate melts/glasses as free carbonate species, i.e. carbonate ions that are only bonded to network modifiers (e.g. Na, Ca, Mg), but not to the network formers (Si, Al), as well as direct evidence for the accompanying polymerization on the network structure. Together with glass transition temperature measurement, the study provided new insight into the effect of CO₂ on the viscosity of depolymerized silicate melts. Here I would like to point out a few errors in the paper, e.g. in the estimation method for the parameter related to the degree of depolymerization from ¹⁷O NMR data, in its expected correlation with CO₂ content, which led to an apparent observation that change in degree of polymerization is more than explainable by the formation of free carbonate species. The authors attempted to explain the latter by the change of Si Qⁿ speciation derived from ²⁹Si MAS NMR data, although these data are actually consistent with the formation of free carbonate species. Once the errors are corrected, their data can be shown to be quantitatively in agreement with the expected change of melt polymerization accompanying the formation of free carbonate species. It is hoped that the clarification of these errors here will reinforce the scientific value of the original, landmark work.