Abstract
Cryptomelane was synthetised from several runs by methods whose reproductibility was proved. The best conditions of temperature and pH for the synthesis, the influence of the digestion time on the degree of crystallinity, and the role of the presence of Zn and K ions in the cryptomelane structure as a function of aging of the mineral in contact with the mother solution, were studied. By reduction of KMnO4 with HC1 or Zn/H2 SO4 , the highest degree of crystallinity was found at temperatures above 60ºC in an extended digestion time. The behavior of constituent ions as a function of digestion time indicates that the crystallinity of the cryptomelane is improved by increasing incorporation of K and Mn (IV) ions and decreasing of Mn (II) content, H2O and Zn ions. The probable mechanism of cryptomelane formation is thought to occur under conditions of low pH with K+ present, nucleation being induced mainly by H3O+.
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