Distribution of iron cations in the crocidolite structure

Measurements have been made of the fundamental band of the O-H stretching frequency in three crocidolites of the alkali amphibole series. Several sharp peaks and prominent inflections are observed in the spectra, which correspond to hydroxyl groups linked to Fe²⁺, Mg²⁺ and Fe³⁺ ions in all possible distributions in the two M(1) and one M(3) positions of the amphibole structure. Peak intensities have been used to estimate site populations in each crocidolite. Fe³⁺ ions are enriched in M(2) positions, but a small proportion also occurs in M(1) and M(3) positions with relative enrichment in M(1) positions. The majority of Fe²⁺ ions occupy M(l) and M(3) positions with relative enrichment in M(1) positions. The results indicate that local charge balance controls the distribution of Fe³⁺ ions in the crocidolite structure.