Ion exchange reactions between dehydroxylated micas and salt melts and the crystal chemistry of the interlayer cation in micas

The interlayer cations in dehydroxylated micas can be exchanged by reaction with salt melts (chlorides and bromides) at temperatures of 700 to 950 °C under atmospheric pressure within a few hours. Generally, during the reactions air and moisture must be excluded to prevent decomposition of the micas. By this method, dehydroxylated mica phases of the type M⁺Al₂[AlSi₃O₁₀O] with M = Li, Na, K, Rb, Cs, T1 were prepared as well as micas of the type M_½□_½Al₂[AlSi₃O₁₀O] with M = Ca, Sr, Ba. Additionally, a sodium mica was prepared with part of the Na⁺ ions replaced by La³⁺, probably according to the reaction 3 Na⁺ = La³⁺ + 2□. Most of these micas are compounds not described previously. For all micas except the Sr and Ba phases, complete sets of lattice constants are given. The lattice constants are almost exclusively determined by the ionic radius and not by the charge of the interlayer cation. The tetrahedral rotation angle can be varied by about 10° solely by the interlayer substitution. In micas with very small interlayer cations like Na⁺ or Li⁺, repulsive forces between the O atoms across the interlayer region cause an interlayer overshift, resulting in anomalously high basal spacings and a smaller β angle. The interlayer spacing of the lithium mica can expand by reversible uptake of water. The other dehydroxylated micas do not react with water at room temperature. The differences in the structure of micas with small and large interlayer cations explain many features of the geochemistry of these phases, such as the miscibility gap between paragonite and muscovite, the preferential uptake of potassium in micas and structurally related clay minerals, and the smaller thermodynamic stability range of paragonite compared with muscovite.