Cation distributions in magnesioriebeckites and riebeckites from infrared spectroscopy

Abstract

Infrared absorption spectra of the fundamental hydroxyl stretching frequencies of magnesioriebeckites and riebeckites of the amphibole series have been measured. Several sharp peaks and prominent inflections in the spectra were assigned to combinations of magnesium, ferrous and ferric ions in the three cation sites co-ordinated to the hydroxyl groups. From the ratios of the peak heights estimations of cation distributions, site preferences and clustering were obtained for these sites. Ferric ions are enriched in the M2 positions, but a small proportion also occurs in the M1 and M3 positions with relative enrichment in the M1 sites. The majority of ferrous ions occupy the M1 and M3 positions with relative enrichment in the M1 position. The results indicate that local charge balance controls the distribution of ferric ions in the structure of minerals formed at low temperatures. Minerals with high temperatures of formation were found to have a much more random cation distribution.