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

Abstract

Pyrope-rich garnet is an important “sink” for trivalent transition metals such as Cr 3+ , V 3+ , and Fe 3+ in Earth’s upper mantle. In order to obtain a better crystal-chemical understanding of transition metals in garnet and especially pyrope solid solutions, 27 Al and 29 Si MAS NMR spectra were collected on synthetic crystals of composition Mg 3 (Al 0.915, V 0.085 ) 2 Si 3 O 12 and Mg 3 (Al 0.98, Cr 0.02 ) 2 Si 3 O 12 , and compared with the 29 Si MAS NMR spectrum of a Fe 3+ -bearing grossular garnet [Ca 3 (Al 0.956 Fe 3+ 0.044 ) 2 Si 3 O 12 ]. The purpose of the study is to explore the nature of paramagnetic resonances, or their lack thereof, arising from interactions of Cr 3+ , V 3+ , and Fe 3+ with 27 Al and 29 Si nuclei. The 27 Al and 29 Si MAS NMR spectra of Cr 3+ -bearing pyrope show significant resonance broadening, similar to that of Fe 3+ -bearing grossular, which hinders any crystal-chemical interpretation. The 27 Al and 29 Si MAS NMR spectra of V 3+ -bearing pyrope, on the other hand, show several distinct and narrow paramagnetically shifted NMR resonances, even at relatively high concentrations of V 3+ . The various NMR resonances can be assigned to different local atomic configurations around AlO 6 or SiO 4 groups having zero, one, two or three next-nearest octahedrally coordinated V 3+ neighbors. The intensities of the experimental resonances were measured and compared to those calculated for the hypothetical case of statistically random mixing of V 3+ cations to investigate Al-V 3+ 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+ , Cr 3+ , and V 3+ ) in pyrope, is made in an attempt to explain the marked differences in their behavior in 27 Al and 29 Si MAS NMR spectra.