The Thermal Decomposition of Synthetic and Natural Alunite: An Investigation by X-ray Diffraction, Electron Diffraction, and Electron Microscope Methods

Abstract

In the thermal decomposition of synthetic alunite, as studied by X-ray diffraction, dehydration at about 500� C. is accompanied by the formation of dehydrated alum and γ-alumina. At 700-750� C., α-alumina predominates together with hexagonal potassium sulphate. At 850� C. there is some evidence of the formation of " potassium aluminate ". The calcination at 850� C. of a synthetic mixture of alunite and natro-alunite in the ratio 3 : 1 gives glaserite as the water-soluble phase. The course of the thermal decomposition of natural alunite from Chandler, Western Australia, is similar except for important side reactions resulting from the presence of silica. The formation of an aluminium silicate (sillimanite or mullite), and of orthoclase or microcline was detected at temperatures as low as 750� C. Electron micrographs are shown of natural alunite at various stages of decomposition.