Development of Electric Potentials in Clay Minerals and Alumina—Silica Systems

Abstract

DURING DC conductance measurements on 0.5 cm thick cylindrical pellets of kaolinite and halloysite at low pressures (10⁻³–10⁻⁴ torr) and high temperatures (500–1,100° C) using platinum electrodes, it was found that potential differences of −8 to +5 V developed across the samples. The magnitude and signs of the voltage peaks changed with temperature and were reproducible for any given clay (Fig. 1). On cooling and reheating the samples several times, two of the peaks were lost and the others reduced in intensity (Fig. 1), but when the mineralogical reactions were completed by heating to 1,400° C no further voltages were obtained. When the sample was held at constant temperature, the voltage showed no tendency to decay away. Reversing the cell leads changed the observed sign but not the magnitude of the voltage. Separate pellets of reagent grade silica and alumina gave no voltages. Voltages were observed with silica–alumina “sandwiches”, but these gave fewer voltage peaks (Fig. 2). Similar effects were observed with bayerite and BDH aluminium silicate (Fig. 2).