Hydration and dehydration characteristics of synthetic water-swelling fluoromicas: comparisons between silicates and germanates

Solid solutions of fluoromicas WMg₂Li(Si_4−x Ge_xO₁₀)F₂ (W = K, Na, Li. 0 ≦ x ≦ 4), which were closely related to taeniolite, were synthesized and the effect of the isomorphic substitution Ge for Si and also the interlayer cations Li, Na and K respectively upon hydration and dehydration were studied by X-ray diffraction, infrared and thermal analysis. The Na- and Li-series exhibit swelling with water, whereas the K-series does not. The increase of tetrahedral germanium substitution reduces hydration and also lowers the dehydration temperature of hydrates in both the Na- and Li-series. The reasons for such tendencies are discussed. Repulsion between interlayer cations and tetrahedral cations seems to play the dominant role. K in opposition to the smaller interlayer cations Li and Na respectively fits in the interlayer sites of the mica structure and lowers repulsion. By increasing the substitution Ge for Si interionic distances become greater and repulsion also decreases.