Short-range order in synthetic aluminous tremolites: An infrared and triple-quantum MAS NMR study

Abstract : Fourier-transform infrared spectra (FTIR) were recorded on a series of synthetic amphiboles along the join Ca1.8Mg5.2Si8O22(OH)2–Ca1.8(Mg4.2Al)(Si7Al)O22(OH)2. The spectra were fitted by up to six component bands by optimization and non-linear least-squares techniques. 27Al MAS NMR and triple-quantum (3Q) MAS NMR spectra were recorded for the synthetic amphibole Ca1.8(Mg4.8Al0.4)(Si7.6Al0.4)O22(OH)2. The fitted FTIR and NMR spectra show that VIAl occurs at the M2 site and at the M1 or M3 sites; by analogy with previous crystal-structure refinement results on natural amphiboles, VIAl occupancy of M2 and M3 is presumed. The fine structure present in the FTIR spectra indicates that they are also affected by NNN (next-nearest-neighbor) interactions. There are two types of NNN arrangements: (1) SiSi or SiAl at adjacent T1T1 dimers; (2) permutation of Mg/Al over M2M2M3 sites. Discounting those arrangements unlikely on bond-valence grounds, there are two arrangements that give rise to five distinct bands in the infrared spectra. There are two principal conclusions: (1) infrared spectra of amphiboles in the principal OH-stretching region can be affected significantly by NNN effects; (2) the small number of bands due to NNN effects indicates that amphiboles show strong short-range order.