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A suite of Fe-bearing natural and synthetic grossular-rich [(Ca,Fe) 3 (Al,Fe) 2 Si 3 O 12 ] and pyrope-rich [(Mg,Fe) 3 Al 2 Si 3 O 12 ] garnets were investigated using  27 Al and  29 Si 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 Fe 2+  and Fe 3+  and their site occupancies, even in grossular samples with very low concentrations of Fe. MAS NMR spectra were collected on Fe 2+ -bearing grossular- and pyrope-rich garnets with up to 25 mol% almandine component and on other Fe 3+ -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-Fe 2+  pyrope garnets by Stebbins and Kelsey (2009). Additional NMR peaks appear as the concentration of Fe 2+  increases, reflecting an increase in the average number of neighboring Fe2+ cations around AlO 6  and SiO 4  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 Fe 3+  on the MAS NMR spectra of garnet appears to be less pronounced, because it does not produce any observable paramagnetically shifted resonances.

Cation order-disorder in Fe-bearing pyrope and grossular garnets: A 27 Al and 29 Si MAS NMR and 57 Fe Mössbauer spectroscopy study