Dielectric properties of iron-bearing trioctahedral phyllosilicates

Abstract

 Measurements of the real part of conductivity (σ′) and dielectric function (ɛ′) were performed on large crystal flakes of biotite (Moen) and vermiculite (Benahavis), at variable frequencies (0.1 to 1000 kHz) and as a function of temperature (300 K<T<900 K). By heating, cooling and repeated heating experiments under inert atmospheric conditions effects involving water (H₂O) diffusion and electrical transport were separated. The effect of dehydration dominates the functional dependence of ɛ′ on T at frequencies below about 100 kHz, within the first heating run. Six dehydration steps for vermiculite Benahavis and two broad features for biotite Moen are observed, the water being transported effectively only in the interlayers. Electrical transport occurs along Fe paths within the octahedral layers and can be described by σ′=σ_dc+A·w^s. Values of s are between 0.45 and 0.8. σ_dc shows a temperature dependence according to exp(−E_a(T)/kT) with E_a(T) between 0.3 and 0.6 eV. E_a is suggested to be composed of a polaron plus disorder contribution.