Abstract: The formation of siliceous phases in montmorillonites at temperatures up to 1500°C has been studied by high temperature continuous x-ray diffraction techniques. Oriented clay aggregates on platinum slides are heated in a small platinum-wire resistor furnace placed on an x-ray spectrometer unit. The intensity of the main diffraction lines of each phase is noted versus the temperature. The nature and the temperature of formation of the different phases are closely related to the structure and composition of the sample. The important role of some cations in exchange position is emphasized.