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

Fourier-transform infrared spectra (FTIR) were recorded on a series of synt hetic amphiboles along the join Ca1.8Mg5.2Si8O22(OH)(2)-Ca-1.8(Mg4.2Al)(Si7 Al)O-22(OH)(2). The spectra were fitted by up to six component bands by opt imization and non-linear least-squares techniques. Al-27 MAS NMR and triple -quantum (3Q) MAS NMR spectra were recorded for the synthetic amphibole Ca- 1.8(Mg4.8Al0.4)(Si7.6Al0.4)O-22(OH)(2). The fitted FTIR and NMR spectra sho w that Al-VI occurs at the M2 site and at the M1 or M3 sites; by analogy wi th previous crystal-structure refinement results on natural amphiboles, Al- VI occupancy Of M2 and M3 is presumed. The fine structure present in the FT IR spectra indicates that they are also affected by NNN (next-nearest-neigh bor) interactions. There are two types of NNN arrangements: (1) SiSi or SiA l at adjacent T1T1 dimers; (2) permutation of Mg/Al over M2M2M3 sites. Disc ounting 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 ef fects; (2) the small number of bands due to NNN effects indicates that amph iboles show strong short-range order.