Infrared Study of the Hydroxyl Bands in Clinoamphiboles

Abstract

Sharp single peaks in the fundamental and first-overtone bands of the O-H stretching vibration in pure Mg²+ and Fe²+ amphiboles split into a maximum of four sharp peaks, corresponding to hydroxyl groups linked to 3 Fe, 2 Fe + Mg, 2 Mg + Fe, and 3 Mg, in mixed Fe²+-Mg²+ amphiboles. Within any one solid-solution series, the frequencies of these peaks can be correlated with the electronegativity of ions in the M_l and M₃ positions, and differences between series can be correlated with the size of ions in the M₄ position. The O-H vector lies approximately normal to z in the (010) plane. The distribution of Fe²+ and Mg²+ ions between the (M₁,M₃) and (M₂M₄) positions in the cummingtonite-grunerite series, and between the (M₁,M₃) and M₂ positions in the tremolite-ferroactinolite series, has also been estimated.