Cation distribution in Mg-Zn olivine solid solution: A ²⁹Si MAS NMR and first-principles calculation study

In order to clarify cation distributions in the M1 and M2 sites of (Mg,Zn)₂SiO₄ olivine solid solution, ²⁹Si MAS NMR spectroscopic measurement and first-principles NMR parameter calculation were conducted. The ²⁹Si MAS NMR spectra of (Mg_0.95Zn_0.05)₂SiO₄ and (Mg_0.90Zn_0.10)₂SiO₄ olivine samples reveled three new peaks at relative chemical shift differences of 0.2, 1.1 and 2.3 ppm from the main peak of forsterite (-61.8 ppm). These shifts can be attributed to changes in the second-nearest-neighbors of Si due to substitution of Mg by Zn. Based on first-principles calculations, these peaks can be assigned to the following three groups of local Si environments in the order of increasing shift from the main peak of forsterite: i) Si tetrahedra with one corner-shared Zn in M1 or M2 octahedron, ii) Si tetrahedra with one edge-shared Zn in M2 octahedron, and iii) Si tetrahedra with one edge-shared Zn in M1 octahedron. Since the last two peaks are well separated from the others, the relative abundances of Zn in the M1 and M2 sites can be quantified using these peaks. Preference of Zn for M1 site over M2 site was inferred from the observed peak intensities. The present study demonstrated the usefulness of ²⁹Si MAS NMR spectroscopy for quantitatively studying cation distributions in solid solutions.