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 Mg(2)+ and Fe(2)+ 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 Fe(2)+-Mg(2)+ 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 M(3) positions, and differences between series can be correlated with the size of ions in the M(4) position. The O-H vector lies approximately normal to z in the (010) plane. The distribution of Fe(2)+ and Mg(2)+ ions between the (M(1),M(3)) and (M(2)M(4)) positions in the cummingtonite-grunerite series, and between the (M(1),M(3)) and M(2) positions in the tremolite-ferroactinolite series, has also been estimated.