Ultrahigh-Field 25Mg NMR and DFT Study of Magnesium Borate Minerals

Abstract

A series of well-characterized magnesium borate minerals and synthetic analogues have been studied via ultrahigh-field 25Mg solid-state NMR spectroscopy. Correlations between 25Mg NMR parameters and the local structure at the magnesium site(s) are highlighted and discussed. First-principles density functional theory calculations of 25Mg NMR parameters carried out with the WIEN2k software package support our experimental 25Mg NMR data. Experimental 25Mg CQ values range from 0.7 ± 0.1 MHz in hungchaoite to 18.0 ± 0.5 MHz in boracite-type Mg3B7O13Br. To our knowledge, the latter CQ value is the largest reported 25Mg nuclear quadrupole coupling constant. In general, CQ values correlate positively with the degree of geometrical distortion at the Mg site, despite the diversity in nearest-neighbor ligands (O2–, OH–, H2O, F–, Cl–, and Br–) across the series of magnesium borates. Experimental δiso values range from 0.2 ± 0.5 ppm in hungchaoite to 23 ± 3 ppm in grandidierite, which are within the expected chemical ...