Clustering of cations in amphiboles

Abstract

The OH⁻ stretching frequencies of clino-amphiboles are known to depend on the cations to which the OH⁻ is co-ordinated, and the intensities of the corresponding infra-red (ir) absorption bands have been used to obtain evidence as to the occupancy of the M1 and M3 sites. However, the possible effects on the method arising from clustering together of like cations, or of the mutual avoidance of like cations (anti-clustering), have not hitherto been explicitly analysed. A model is set up which permits these to be systematically explored. The results are exemplified for occupation of M1 and M3 by (Fe, Mg). Clustering of Fe increases the frequency of both Fe₃ and Mg₃ triads, though not equally, and the effect of anti-clustering is in the opposite sense. The effect on the frequencies of MgFe₂ and Mg₂Fe triads is much more complicated and the sense of the changes depends on both the overall Fe content and on the degree of clustering or anti-clustering.