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.