DIELECTRIC-PROPERTIES OF IRON-BEARING TRIOCTAHEDRAL PHYLLOSILICATES

Measurements of the real part of conductivity (sigma') and dielectric function (epsilon') were performed on large crystal flakes of biotite (Moen) and vermiculite (Benahavis), at variable frequencies (0.1 to 10 00 kHz) and as a function of temperature (300 K<T<900 K). By heating, cooling and repeated heating experiments under inert atmospheric condi tions effects involving water (H2O) diffusion and electrical transport were separated. The effect of dehydration dominates the functional de pendence of epsilon' on T at frequencies below about 100 kHz, within t he 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 describe d by sigma' = sigma(dc) + A.w(s). Values of s are between 0.45 and 0.8 . sigma(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.