Trivalent transition-metal cations and local structure in pyrope- and grossular-rich solid solutions investigated by 27Al and 29Si MAS NMR spectroscopy

Pyrope-rich garnet is an important “sink” for trivalent transition metals such as Cr³⁺, V³⁺, and Fe³⁺ in Earth’s upper mantle. In order to obtain a better crystal-chemical understanding of transition metals in garnet and especially pyrope solid solutions, ²⁷Al and ²⁹Si MAS NMR spectra were collected on synthetic crystals of composition Mg₃(Al_0.915V_0.085)₂Si₃O₁₂ and Mg₃(Al_0.98Cr_0.02)₂Si₃O₁₂, and compared with the ²⁹Si MAS NMR spectrum of a Fe³⁺-bearing grossular garnet [Ca₃(Al_0.956Fe³⁺_0.044)₂Si₃O₁₂]. The purpose of the study is to explore the nature of paramagnetic resonances, or their lack thereof, arising from interactions of Cr³⁺, V³⁺, and Fe³⁺ with ²⁷Al and ²⁹Si nuclei. The ²⁷Al and ²⁹Si MAS NMR spectra of Cr³⁺-bearing pyrope show significant resonance broadening, similar to that of Fe³⁺-bearing grossular, which hinders any crystal-chemical interpretation. The ²⁷Al and ²⁹Si MAS NMR spectra of V³⁺-bearing pyrope, on the other hand, show several distinct and narrow paramagnetically shifted NMR resonances, even at relatively high concentrations of V³⁺. The various NMR resonances can be assigned to different local atomic configurations around AlO₆ or SiO₄ groups having zero, one, two or three next-nearest octahedrally coordinated V³⁺ neighbors. The intensities of the experimental resonances were measured and compared to those calculated for the hypothetical case of statistically random mixing of V³⁺ cations to investigate Al-V³⁺ mixing behavior at the octahedral site. A simple analysis, based on the number of unpaired electrons for a given transition metal (i.e., Fe^2+,3+, Cr³⁺, and V³⁺) in pyrope, is made in an attempt to explain the marked differences in their behavior in ²⁷Al and ²⁹Si MAS NMR spectra.