Manganese oxides precipitated by bubbling air through 0.01 molar solutions of MnCl2, Mn(NO3)2, MnSO4, or Mn(ClO4)2 at a constantly maintained pH of 8.5 to 9.5 at temperatures of 25??C or higher consisted mainly of hausmannite, Mn3O4. At temperatures near 0??C, but with other conditions the same, the product is feitknechtite, ??MnOOH, except that if the initial solution is MnSO4 and the temperature is near 0??C the product is a mixture of manganite, ??MnOOH and groutite, ??MnOOH. All these oxides are metastable in aerated solution and alter by irreversible processes to more highly oxidized species during aging. A two-step nonequilibrium thermodynamic model predicts that the least stable species, ??MnOOH, should be most readily converted to MnO2. Some preparations of ??MnOOH aged in their native solution at 5??C attained a manganese oxidation state of +3.3 or more after 7 months. Hausmannite aged at 25??C altered to ??MnOOH. The latter is more stable than a or ??MnOOH, and manganese oxidation states above 3.0 were not reached in hausmannite precipitates during 4 months of aging. Initial precipitation of MnCO3 rather than a form of oxide is likely only where oxygen availability is very low. Composition of solutions and oxidation state and morphology of solids were determined during the aging process by chemical analyses, X-ray and electron diffraction and transmission electron micrographs. ?? 1983.
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Nonequilibrium models for predicting forms of precipitated manganese oxides