Na-23, Si-29, and Ga-71 MAS-NMR spectroscopy of synthetic gallian-fluor-amphiboles

A magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopic s tudy was done on the series of synthetic gallian-fluor amphiboles to identi fy the extent of Ga and Si ordering in the tetrahedral sites. Assignment of the peaks in the complex Si-29 MAS NMR spectra of pargasitic amphiboles is based on observations of the gradual change in peak intensity and position as well as by comparison with computer simulations of the relative intensi ties of the Si-29 MAS NMR spectra for the amphiboles along the compositiona l join. The Si-29 spectra agree best with models that allow Ga on both the T1 and T2 sites, which supports the previous cation distribution results ob tained by Rietveld refinement of powder X-ray diffraction data. It was not possible, however, to discriminate between cation distribution models that allowed completely random mixing of Ga and Si vs. the presence of Ga-O-Ga a voidance on the tetrahedral sites. Comparison of the very high speed Ga-71 MAS NMR spectra (35 kHz) with the distribution of Ga in octahedral and tetr ahedral sites shows that the ratio of tetrahedral to octahedral site occupa ncy is vastly overestimated from the NMR spectra due to the large quadrupol ar effects of the asymmetrical octahedral site. The Na-23 MAS NMR spectra o f tremolite and Ga-substituted pargasites show a shift to higher frequency and increasing peak width with Ga content that can be related to the reduct ion in magnetic shielding produced by the substitution of Ga for Si and Mg.