Short-Range Structure of Invert Glasses along the Pseudo-Binary Join MgSiO3-Mg2SiO4: Results from 29Si and 25Mg MAS NMR Spectroscopy

The short-range structure of invert glasses along the pseudobinary join MgSiO3-Mg2SiO4 has been studied using Si-29 and Mg-25 MAS NMR. spectroscopy. The results indicate a progressive compositional evolution in Q speciation that approximately follows a statistical distribution. The Mg2SiO4 glass shows an abrupt deviation from this trend with the presence of nearly 40% of the Si atoms as (Si2O7)(6-) dimers, i.e., Q(1) species. Mg2+ ions are present in predominantly octahedral coordination in all glasses. When taken together, these results indicate that glasses with MgO contents between 50 and 60 mol % are characterized by a structure consisting primarily of at least three types of Q species and MgO6 octahedra. On the other hand, the structure of glasses with >60 mol % MgO appears to consist of Q(0) and Q(1) species with structural connectivity being primarily provided by the MgO6 octahedra. The possible consequences of such compositional evolution of structure on the ability of glass formation in this system are discussed.