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

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.