Naturally occurring potassium micas with a range of composition have been examined employing the analytical techniques of far infrared (IR) spectroscopy, X-ray powder diffraction (XRD), and major oxide and F analysis. Structure parameters predicted by geometric models, including torsional mode interlayer frequency (νi), d value (d001), and b cell parameter are compared with those determined from structural refinement of potassium micas of similar composition. Differences are likely the result of assumptions inherent in the geometric models, including the frequency of the interlayer mode as a proxy for the K inner-O bond distance and the constancy of the combined octahedral-tetrahedral sheet thickness. Study of the IR, XRD, and chemical parameters shows the following relationships: (1) Solid solution of OH and F in trioctahedral structures significantly affects torsional vibrational modes, with increasing frequency correlating with increasing F. (2) Heating-induced dehydroxylation of dioctahedral species and heating-induced oxidation of Fe2+ trioctahedral species provide strong evidence that OH-orientation controls the frequency of the interlayer torsional mode. (3) Multivariate statistical analysis of potassium-mica intralayer sheet composition and the frequency of the interlayer torsional mode reveals a strong correlation between the vibrational frequency of the K interlayer torsional mode and octahedral sheet composition. The following preliminary formula is presented, assuming 22 O equivalents per unit formula:
vi(cm1)=79.6+0.96Mg2+4.23Fe2++7.61Al3++9.09Fe3+4.65Li++2.20F.

The frequency of the interlayer torsional mode directly reflects the tetrahedral sheet environment. This relationship therefore supports previous suggestions, based on XRD data alone, that the sheet dimensions and distortions of phyllosilicates are controlled largely by the composition of the octahedral layer. (4) Multiple-mode behavior provides evidence for the presence of compositionally different octahedral cation sites within a given structure. However, absorption spectra alone do not provide information about ordering of the octahedral environments.

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