Original paper

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

Palke, Aaron C.; Geiger, Charles A.

European Journal of Mineralogy Volume 28 Number 1 (2016), p. 179 - 187

published: Mar 1, 2016

DOI: 10.1127/ejm/2015/0027-2507

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Abstract

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(Al0.915 V0.085)2Si3O12 and Mg3(Al0.98Cr0.02)2 Si3O12, and compared with the 29Si MAS NMR spectrum of a Fe3+ -bearing grossular garnet [Ca3 (Al0.956 Fe3+ 0.044)2Si3O 12]. 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 27 Al and 29 Si nuclei. The 27Al and29 Si 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 27 Al and 29 Si 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 ., Fe2+,3+, Cr3+, and V3+) in pyrope, is made in an attempt to explain the marked differences in their behavior in 27 Al and29 Si MAS NMR spectra.

Keywords

garnettransition metalsmas nmr spectroscopysilicate solid solutionsparamagnetic interactionspyrope