X-Ray Study of Synthetic Alunite

Abstract

Alunite, KΑl3(SO4)2(OH)6, crystallizing in space group R3m, is a mineral identified in numerous sites in the world. It belongs to a family of structurally related substances described by a general formula AB3(SO4)2(OH)6, where A may become Na+, K+, H3O+, Nu+4 or Ag+ and B either Al3+ or Fe3+ Natural and synthetic alunites have been subjects of extensive stuctural investigations employing single-crystal and powder X-ray techniques, NMR and other methods [1-16]. One of the reasons of this interest is a possibility of industrial applications of alunite and related minerals: The composition of the natural alunite depends on the formation conditions and on the environment while that of a synthetic one is influenced by the way of synthesis. A number of recent papers [5, 6, 9, 13-16] deal with the problem of the composition of natural and synthetic alunite. The existing synthesis methods yield alunite of composition often differing from the stoichiometric one; the reported differences concern the deficiency in K and Al as well as the presence of excess water. In our work the term excess refers to various forms of water (e.g. H3O+, H2O) which may be present in alunite unit-cell except for stoichiometric OHions. Dne to differences in synthesis method and observed differences in composition, the published unit-cell dimensions vary within a range of about 0.05 Α