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.