Magnesium coordination environments in glasses and minerals: New insight from high-field magnesium-25 MAS NMR

A comparison of Mg-25 magic angle spinning (MAS) NMR spectra of crystalline and glassy diopside (CaMgSi2O6) reveals that the chemical shift in the dis ordered phase corresponds to that of the mineral, suggesting that sixfold c oordination is essentially retained upon vitrification. Likewise, a crystal line leucite analogue (K2MgSi5O12) known to possess fourfold-coordinated Mg has the same peak position as the corresponding glass. In addition to bein g a sensitive probe of local structure by analogy with crystalline phases, these data may be understood in terms of the effect of competing cation fie ld strengths: competing network modifier cations with higher field strength induce higher magnesium coordination numbers, with a consequent increase i n bond length and decrease in chemical shift. This work demonstrates the ut ility of NMR at 14.1 Tesla for probing the coordination environment of Mg i n glasses and minerals, and suggests great potential for high-field NMR inv estigations of quadrupolar nuclides with low resonance frequencies in amorp hous systems.