Far-infrared (FIR) analysis of synthetic Mg-, Ni-, Co-, and Fe-phlogopites coupled with structural data from X-ray diffraction revealed that the K interlayer environments are directly related to octahedral sheet composition and geometry. The general phlogopite formula, KM32+ (Si3Al)O10(OH)2, was varied according to octahedral compositions, where M2+ = Mg2+, Fe2+, Co2+, and Ni2+. Octahedral substitutions have a direct effect on the b lattice parameter, which is related to the tetrahedral-octahedral sheet misfit and manifested by change in the tetrahedral rotation angle (α). The ditrigonal interlayer cavity geometry and the potential for retention of the compensating cations therefore varies according to the ionic size and the types and oxidation state of octahedral cations. These structural features appear as frequency shifts on FIR spectra. When Mg2+ is replaced by a smaller cation, Ni2+, the b parameter decreases and the tetrahedral rotation angle, α, increases, inducing the collapse of the ditrigonal ring. When this happens, the local anisotropy of the interlayer site increases, resulting in every other six out of 12 K–O bonds becoming shorter and the in-plane K–O vibration band shifts slightly to greater wavenumbers. Synthetic phlogopites with octahedral substitutions by cations of larger ionic radii (i.e. Co2+ and Fe2+) exhibit b parameter increases, where in the case of the annite end-member, α decreases to almost 0°. As α decreases, compensating cation sites become more hexagonal like and the nearest K–O bond increases in length. The K–O vibration bands move toward much smaller wavenumbers. Far infrared offers the potential for a new approach to study the retention of interlayer cations in other phyllosilicates and the mechanisms by which they are altered, such as heating or by weathering reactions in the environment.