Thermal analysis as a tool for determining and defining spherical kaolinite

Samples containing spherical kaolinite, synthesized under hydrothermal conditions (T = 200 degrees C; t = 24 h, 192 h, 720 h) from gel with Si/Al = 0.84, were studied by differential thermal analysis/ thermogravimetry (DTA/TG) to provide a contribution to the mineralogical characterization of this unusual morphology. The data clearly show that dehydroxylation temperature of spherical kaolinite is lower than that of platy/lath kaolinite. It can also be used to detect the presence of spheres in the presence of the other morphologies. A rough estimation of its quantity can be obtained by TG data if spheres are present in appreciable concentrations. The results also confirm microscopic observations previously reported in the literature: spherical morphology is a discrete and metastable phase, and it is gradually dissolved along hydrothermal treatments of gels.