A suite of Fe-bearing natural and synthetic grossular-rich [(Ca,Fe)3(Al,Fe)2Si3O12] and pyrope-rich [(Mg,Fe)3Al2Si3O12] garnets were investigated using 27Al and 29Si MAS NMR and 57Fe Mössbauer spectroscopy. This was done to study the state of cation order-disorder in garnet solid solutions by analyzing paramagnetically shifted resonances in high-resolution NMR spectra. The Mössbauer spectra, along with electron microprobe results, give the concentrations of Fe2+ and Fe3+ and their site occupancies, even in grossular samples with very low concentrations of Fe. MAS NMR spectra were collected on Fe2+-bearing grossular- and pyrope-rich garnets with up to 25 mol% almandine component and on other Fe3+-bearing grossular samples with up to 9 mol% andradite component. Despite peak broadening and signal loss, structural information was even obtained from garnet with relatively high Fe contents (25 mol% almandine component). Paramagnetically shifted NMR peaks, related to the presence of Fe2+, were observed in grossular samples and are similar in nature to those reported previously for natural, relatively low-Fe2+ pyrope garnets by Stebbins and Kelsey (2009). Additional NMR peaks appear as the concentration of Fe2+ increases, reflecting an increase in the average number of neighboring Fe2+ cations around AlO6 and SiO4 groups. These newly observed peaks hold potential to provide information concerning the presence or absence of short-range ordering in certain Fe-bearing silicate garnets. The effect of Fe3+ on the MAS NMR spectra of garnet appears to be less pronounced, because it does not produce any observable paramagnetically shifted resonances.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.