Infrared spectra of alkali amphiboles of the glaucophane-riebeckite series and their relation to chemical composition

Abstract

Infrared absorption spectra in the 4000–330 cm⁻¹ range of the glaucophane-riebeckite series alkali amphiboles are presented with some discussions on their relationship to chemical composition.
The compositions of the (M(1)·M(1)·M(3)) sites cations were calculated according to the areal intensity relations of their OH stretching bands at 3700–3600 cm⁻¹, and these values roughly coincide with those obtained through chemical analyses. The ionic substitutions, (Fe²⁺ + Mn)-for–Mg in the M(1) and M(3) sites, and Fe³⁺-for–Al in the M(2) sites, cause a frequency decrease for most bands in the 1300–330 cm⁻¹ range. The ratios of Fe³⁺ and Al in M(2) sites can be estimated approximately by the peak intensity relationships of three kinds of bands: near 990 and 970 cm⁻¹ (Si–O stretchings); near 690 and 670 cm⁻¹ (mainly OH librations); near 640 cm⁻¹ (probably chain deformation). The nature of the bands near 465, 550, and 365 cm⁻¹ is also useful to estimate the degree of Fe²⁺-for-Al substitutions in the M(2) sites.