Electrical conductivity and thermal analyses studies of phase evolution in the illite – CaCO3 system

Mixture of a purified illitic clay (>90 wt% of illite, Northeastern Hungary) and CaCO3, in laboratory grade, was studied with respect to the high-temperature crystallization of anorthite and leucite mineral phases. The prepared samples were subjected to thermal analyses – differential scanning calorimetry (DSC) and thermodilatometry (DIL), and microstructure was characterized by scanning electron microscopy. The phase composition of the samples after firing was determined by the help of XRD measurement. Among anorthite and leucite (which were present as dominant mineral phases), gehlenite was found in the fired samples. Crystallization temperatures were determined from DSC peaks (940 °C and 1070 °C for gehlenite/anorthite and leucite crystallization, respectively). The samples exhibited a steep contraction as the low-viscosity glassy phase appeared and the anorthite/gehlenite started to crystallize. The crystallization temperatures were confirmed by the temperature dependence of the loss tangent, which exhibited two maxima at 940 °C and 1070 °C. Thus, during crystallization, the contribution of the ions to the conductivity was hindered by their cooperative migration to occupy new sites. •Thermal behavior of an illitic clay with calcite addition is studied and discussed.•Anorthite and gehlenite were identified as main mineral phases.•The effect of the crystallization on the AC conductivity is discussed.•The arranged displacements of ions led to a short-term increase in the AC conductivity.