Investigations on the Crystal-Chemical Behavior of Transition-Metal-Bearing Aluminosilicate Garnet Solid Solutions Using 27Al and 29Si NMR Spectroscopy

Abstract:

The petrological importance of silicate garnet is derived from the presence of three distinct cation sites of varying size and coordination number. This allows for a wide range of trace, minor, and major element substitutions. However, a full and precise crystal-chemical understanding of the nature of transition metals in garnet is not at hand. Possible mechanisms of various charge-balanced substitutions (e.g. octahedral Ti⁴⁺ or tetrahedral Al³⁺) and the structural state of solid solutions (i.e. short- to long-range ordering) need study.

We report on ongoing efforts in these directions using ²⁷Al and ²⁹Si Magic-Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy. Early work on synthetic and natural Fe- and Mn-bearing pyrope- and grossular-rich garnets focused on the effect these paramagnetic transition metals have in measuring and interpreting NMR spectra. These results have been expanded with NMR measurements on synthetic pyrope-rich garnets containing other paramagnetic transition metals including Cr³⁺, V³⁺, Co²⁺, and Ni²⁺ as well as diamagnetic Ti⁴⁺. NMR peaks are severely broadened in the presence of even small concentrations of Cr³⁺, Mn²⁺, and Fe³⁺ leading to a loss of spectral resolution. On the other hand, the spectra of garnet containing V³⁺, Fe²⁺, Co²⁺, and Ni²⁺ have better resolution and show separate paramagnetically shifted NMR peaks. In some cases, crystal-chemical information can be obtained because of the large frequency separations between the NMR peaks that can be assigned to various local atomic configurations around Al and Si. Furthermore, the ²⁷Al NMR spectrum of a synthetic pyrope garnet with about 2% diamagnetic Ti⁴⁺ on the octahedral site showed the absence of any tetrahedral Al³⁺, which rules out the substitution mechanism ^VITi + ^IVAl = ^VIAl + ^IVSi in the solid solution.

Our NMR investigations on garnet are now being made at the exploratory level. We think that NMR spectra of diamagnetic garnet can provide information on a number of crystal-chemical properties. Spectra of garnet containing various paramagnetic transition elements can also, in some cases, give local structural information. With a better understanding of paramagnetic effects in NMR spectroscopy, this type of study can possibly be expanded to other geologically important paramagnetic minerals and phases.