Infrared Absorption of O-H Bonds in Some Micas and Other Phyllosilicates

Abstract
The frequencies, relative intensities and pleochroism of OH stretching bands in some micas and chlorites were studied, and the following conclusions are drawn Trioctahedral 2:1 and 2:1:1 phyllosilicates, where all octahedral positions are filled with Mg, will have an absorption band about 3700 cm−1. These OH-groups have their axes normal to the mineral’s cleavage. Substitution of Fe2+ for Mg results in a band about 3665 cm−1. Intensity ratios indicate that two Fe2+ most commonly substitute for two Mg in the same polyhedral group (pyramid). Decreasing octahedral occupancy in the phlogopite-lepidomelane group, with increasing substitution of R3+ for Mg, causes bands with lower frequencies. These are not very sensitive to the incident angle for the infrared beam. In Li-micas where less than 50 per cent of the octahedral positions are filled with Li, and the rest of the positions are filled with Al and Fe2+, it seems probable that OH-groups residing in a pyramid with two Li and one Al cause a band about 3580 cm−1. Combinations of two Al and one Li or one Li, one Al and one Fe2+ cause absorption about 3480 cm−1. Dioctahedral 2:1 phyllosilicates having Al or Fe3+ in octahedral positions cause absorption about 3620 cm−1.