High-temperature impedance spectroscopy as a technique for monitoring the initial stages of phase transformations in minerals (exemplified by almandine from the Verkholovskaya garnet mine, Middle Urals)

Research subject and Methods. The electrical characteristics of an almandine sample from the Verkholovskaya garnet mine (Middle Urals, Russia) were studied using high-temperature impedance spectroscopy in both heating and cooling modes, over a temperature range of 200–900 °С and a frequency range of 1–106 Hz. For this method, electrodes made of platinum and lanthanum-strontium cobaltite were employed. The results were interpreted in combination with thermogravimetric data, X-ray diffraction XRD analyses and diffuse light scattering measurements for almandine powders in their initial state, after annealing at 750 °С and for model synthetic iron oxide Fe 2 O 3 . Results. In the cooling mode, a linear dependence was observed with a break in the temperature range of 600–625 °С with characteristic activation energies E a ⬚ 0.58 and 0.81 eV in the low-temperature (200–625 °С) and high-temperature (625–900 °С) regions, respectively. During the heating-cooling cycle an anomaly was noted at 750 °С, where the sample’s resistance remained constant or changed insignificantly with increasing temperature. Analysis of impedance spectroscopy data revealed the onset of decomposition of the almandine sample already at 750 °С. Previously, no changes in phase composition had been reported at this temperature. The initial stage of almandine destruction is accompanied by the formation of nanosized particles of iron oxide Fe 2 O 3 on its surface, which was confirmed by diffuse light scattering data. Traditional methods of detecting changes in phase composition (TG-DTA and X-ray phase analysis) indicate the appearance of the Fe 2 O 3 phase only at temperatures above 750 °С. This may be associated with their insufficient sensitivity and/or the specific morphology of the released Fe 2 O 3 phase. Conclusions. The impact of minor changes in the phase composition of compounds (initial stages of phase transformations) highlights the potential of impedance spectroscopy as a valuable tool for recording and investigating the early stages of thermal decomposition of both minerals and synthetic materials.