FAR INFRARED STUDY OF OCTAHEDRAL SHEET COMPOSITION ON THE K+-LAYER INTERACTIONS IN SYNTHETIC PHLOGOPITES

Far infrared (FIR) analysis of synthetic Mg-, Ni-, Co-, and Fe-phologopites coupled with structural data from X-ray diffraction reveals the K interlayer environments are directly related to octahedral sheet composition and geometry. The general phlogopite formula KM²⁺₃(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 (a). 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 a increases, inducing the collapse of the ditrigonal ring. In this case, the local anisotropy of the interlayer site increases resulting in every other six out of twelve K-O bonds becoming shorter and the in-plane K-O vibration band slightly shifts to higher wavenumbers. Synthetic phlogopites with octahedral substitutions by cations of larger ionic radii, (i.e., Co²⁺ and Fe²⁺) exhibit b parameter increases, where in the annite end-member case, a decreases to almost 0°. As a decreases, compensating cation sites become more hexagonal-like and the nearest K-O bond increase in length. The K-O vibration bands move towards much lower wavenumbers. FIR 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.