Order/disorder in high-pressure silicate minerals

In ongoing collaborations with colleagues in high-pressure labs, we are exploring the extent of cation disorder and the mechanisms of solid solution in a number of silicates that are important constituents of the Earth's mantle, or of crustal rocks that may be subducted into the mantle. High-field MAS NMR can now often resolve sites and quantify their occupancies even at relatively low concentrations, especially for such "sensitive" isotopes as 11B, 23Na, 27Al, and 31P. Shown below, for example, are 27Al NMR spectra (18.8 T field) of a roughly 3 mg sample of MgSiO3 perovskite containing a few % Al2O3, which clearly shows that Al can occur in both the A and B sites in this key mineral (collaboration with H. Kojitani and M. Akaogi). And, even at the level of < 5 wt %, 45Sc NMR can reveal the sites occupied by Sc3+ and how they differ between pyrope (Mg3Al2Si3O12) and grossular (Ca3Al2Si3O12) garnets (work with Roberta Oberti and Simona Quartieri).