Abstract
The alunite-natroalunite solid solution series were synthesized by precipitation from mixed solutions of K2SO4, Na2SO4, and Al2 (SO4)3 for 2 days' reaction at boiling temperature. The K ions in the initial solution were preferentially taken into structure during crystallization of the alunite, and amounts of products formed in this reaction decreased with decreasing K mol% of the solution. The calculated structural formulae on the basis of SO4=2 indicate that the synthetic alunites contain H3O ions in the A sites, which increased from 11 to 19 mol% with decreasing K mol%. In addition, the slight deficiency of Al ions suggests the presence of vacancies in the B sites. Structural changes of alunites were observed by the thermal treatment. At 530°C, formation of KAl (SO2)2 from alunite and Al2 (SO4)3 from natroalunite occurred simultaneously with the removal of OH ions. Removal of SO4 ions from the dehydroxylated phases led to formation of amorphous phases at about 803°C for alunite run and about 765°C for natroalunite run. The amorphous phases then recrystallized into K2SO4 and Al2O3 in the alunite run and Na2SO4 and Al2O3 in the natroalunite run. The thermal behavior suggests the existence of compositional immiscibility between alunite and natroalunite.
