Stoichiometric end-member alunites of the general formula (A)Al3(SO4)2(OH)6, with A = Na+, K+, Rb+, H3O+, and NH4+ have been synthesized under hydrothermal conditions together with samples of the potassium-oxonium solid solution series over the entire compositional range. A basic gallium sulfate of the alunite type was also synthesized and compared with the oxonium alunite structure. These alunites were characterized by chemical methods, thermal analysis (TA), and powder X-ray diffraction (XRD).
The stages of thermal decomposition of these alunites show a common decomposition mechanism for all end-members (two main endothermic decomposition stages). Potassium and rubidium alunite show an additional exothermic step directly in front of the large second endothermic step. The thermogravimetric results confirmed the analytical results on the end-member alunites and reinforce these as synthetic stoichiometric alunites. Alunite, the pure potassium end-member, shows the highest decomposition temperature (490°C) while Na-alunite (480°C) and rubidium alunite (420°C) are less stable followed by ammonium alunite (350°C) and oxonium alunite (200°C). The thermal stability for the members of the solid solution series (H3O)1-XKXAl3(SO4)2(OH) 6 has also been deduced from the thermal analysis.
Alunites with different monovalent cations in A site (i. e. Na+, K+, Rb+, H3O+ and NH4+) are described and the unit-cell parameters for the rhombohedral space group R3̄m (# 166) with hexagonal cell dimensions a and c were determined. The c parameter increases with increasing effective ionic radius of cation in the A site, whereas the a parameter changes to a much lesser degree. The effects of substitution on the unit-cell parameters are rationalized in terms of the structural arrangements of alunite structure. The unit-cell parameters of the basic gallium sulfate have been determined and compared with the unit-cell parameters of oxonium alunite.