Determination of Cation Distribution in Tirodite-Dannemorite Series by Infrared and Mössbauer Spectroscopy

Abstract

Cation distribution in seven monoclinic amphiboles of tirodite-dannemo-rite series has been determined by infrared OH streching bands and ⁵⁷Fe Mössbauer spectra. The results indicate that Mg is concentrated in M2 site rather than in (M1, M3) sites but very few in M4 site, and that Fe²⁺ is contained more in (M1, M2, M3) sites than in M4 site. This tendency of Fe²⁺ dis ribution in the tirodite-dannemorite series is considerably different from that in Mg-Fe amphiboles, which is caused by the fact that Mn prefers M4 site rather than (M1, M2, M3) sites. Manganese ion was found to be distributed not only in M4 site but also in (M1, M2, M3) sites except in one sample, in which Mn content was low. The proportion of Mn in M4 site decreases with the increase of total Fe²⁺, which suggests that there is an intra-crystalline equilibrium of Mn and Fe²⁺ between (M1, M2, M3) sites and M4 site. The quadrupole splitting of Fe²⁺ in M4 site becomes smaller with increase of total Fe²⁺, and this tendency indicates that the distortion of M4 octahedron also increases with the increase of Fe²⁺.